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[Bug] Segmentation Fault receiving message on WatchOS with debugger attached #17926

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chamons opened this issue Nov 26, 2019 · 9 comments · Fixed by #18105
Open

[Bug] Segmentation Fault receiving message on WatchOS with debugger attached #17926

chamons opened this issue Nov 26, 2019 · 9 comments · Fixed by #18105

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@chamons
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@chamons chamons commented Nov 26, 2019

From @jschroeder-avi on Fri, 22 Nov 2019 19:55:03 GMT

I am developing an Apple Watch app in Xamarin to connect to the iOS side of my existing Xamarin.Forms app. This issue started appearing on the latest versions of Xcode and Xamarin.

When I run the watch project and send a message from the phone using WCSession.DefaultSession.SendMessage(...), the watch app crashes. This usually happens the second time a message is sent, although it varies. When the crash occurs, the debugger immediately exits and no relevant log is printed. It seems to crash in some internal logic, not on any line of my code. I sometimes (~5% of the time) get a crash file generated in "/Users/johns/Library/Logs/DiagnosticReports/", which is how I know the crash is due to a segmentation fault.

I created a sample project to reproduce the issue. The iOS app has a button that sends a message to the watch. Both projects set a WCSessionDelegate and activate the WCSession on startup. The behavior was reproduced on both an iMac and a Macbook. I made a forum post here https://forums.xamarin.com/discussion/174876/segmentation-fault-receiving-message-on-watchos-with-debugger-attached/p1?new=1 and attached the sample project and one of the crash logs.

Any help would be appreciated. Thanks.

My Environment:
=== Visual Studio Community 2019 for Mac ===

Version 8.3.10 (build 2)
Installation UUID: ca9efe8c-cd51-499b-83d0-27b4a235045b
GTK+ 2.24.23 (Raleigh theme)
Xamarin.Mac 5.16.1.24 (d16-3 / 08809f5b)

Package version: 604000208

=== Mono Framework MDK ===

Runtime:
Mono 6.4.0.208 (2019-06/07c23f2ca43) (64-bit)
Package version: 604000208

=== NuGet ===

Version: 5.3.0.6192

=== .NET Core SDK ===

SDK: /usr/local/share/dotnet/sdk/3.0.100/Sdks
SDK Versions:
3.0.100
3.0.100-preview8-013656
2.1.701
2.1.700
2.1.505
2.1.301
MSBuild SDKs: /Library/Frameworks/Mono.framework/Versions/6.4.0/lib/mono/msbuild/Current/bin/Sdks

=== .NET Core Runtime ===

Runtime: /usr/local/share/dotnet/dotnet
Runtime Versions:
3.0.0
3.0.0-preview8-28405-07
2.1.13
2.1.12
2.1.11
2.1.9
2.1.1

=== Xamarin.Profiler ===

Version: 1.6.12.26
Location: /Applications/Xamarin Profiler.app/Contents/MacOS/Xamarin Profiler

=== Updater ===

Version: 11

=== Xamarin.Android ===

Version: 10.0.6.2 (Visual Studio Community)
Commit: xamarin-android/d16-3/c407838
Android SDK: /Users/johns/Library/Developer/Xamarin/android-sdk-macosx
Supported Android versions:
8.1 (API level 27)

SDK Tools Version: 26.1.1
SDK Platform Tools Version: 29.0.4
SDK Build Tools Version: 27.0.3

Build Information:
Mono: 476d72b
Java.Interop: xamarin/java.interop@5836f58
LibZipSharp: grendello/LibZipSharp/d16-3@71f4a94
LibZip: nih-at/libzip@b95cf3f
ProGuard: xamarin/proguard@905836d
SQLite: xamarin/sqlite@8212a2d
Xamarin.Android Tools: xamarin/xamarin-android-tools@cb41333

=== Microsoft Mobile OpenJDK ===

Java SDK: /Users/johns/Library/Developer/Xamarin/jdk/microsoft_dist_openjdk_8.0.25
1.8.0-25
Android Designer EPL code available here:
https://github.com/xamarin/AndroidDesigner.EPL

=== Android SDK Manager ===

Version: 1.4.0.65
Hash: c33b107
Branch: remotes/origin/d16-3
Build date: 2019-11-19 20:33:22 UTC

=== Android Device Manager ===

Version: 1.2.0.116
Hash: d2b2af0
Branch: remotes/origin/d16-3
Build date: 2019-11-19 20:33:42 UTC

=== Xamarin Inspector ===

Version: 1.4.3
Hash: db27525
Branch: 1.4-release
Build date: Mon, 09 Jul 2018 21:20:18 GMT
Client compatibility: 1

=== Xamarin Designer ===

Version: 16.3.0.256
Hash: 8a223bfd7
Branch: remotes/origin/d16-3
Build date: 2019-11-01 21:02:02 UTC

=== Apple Developer Tools ===

Xcode 11.2.1 (15526.1)
Build 11B500

=== Xamarin.Mac ===

Version: 6.6.0.12 (Visual Studio Community)
Hash: e3c2b406d
Branch: xcode11.2
Build date: 2019-11-01 00:12:07-0400

=== Xamarin.iOS ===

Version: 13.6.0.12 (Visual Studio Community)
Hash: e3c2b406d
Branch: xcode11.2
Build date: 2019-11-01 00:12:08-0400

=== Build Information ===

Release ID: 803100002
Git revision: add3a4998a5cb5b081e0404e1fe13acfecb7801d
Build date: 2019-11-20 13:35:15+00
Build branch: release-8.3
Xamarin extensions: 79c69708816813b498283eeda7bcb4d464433a04

=== Operating System ===

Mac OS X 10.15.1
Darwin 19.0.0 Darwin Kernel Version 19.0.0
Thu Oct 17 16:17:15 PDT 2019
root:xnu-6153.41.3~29/RELEASE_X86_64 x86_64

Copied from original issue xamarin/xamarin-macios#7486

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@chamons chamons commented Nov 26, 2019

From @jschroeder-avi on Mon, 25 Nov 2019 16:32:53 GMT

As noted in my forum post, I'm getting a seemingly similar crash on startup in the Xamarin sample https://github.com/xamarin/ios-samples/tree/master/watchOS/SimpleWatchConnectivity . I also rewrote my test project in Xcode/Objective-c, and had no issues.

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@chamons chamons commented Nov 26, 2019

From @lewurm on Tue, 26 Nov 2019 18:18:13 GMT

Thread 7 Crashed:: tid_a10b  Dispatch queue: NSOperationQueue 0x802d29b0 (QOS: UTILITY)
0   com.mycompany.WCSessionTest.watchkitapp.watchkitextension	0x00378b41 mono_get_hazardous_pointer + 17 (hazard-pointer.c:208)
1   com.mycompany.WCSessionTest.watchkitapp.watchkitextension	0x0027088a mono_jit_info_table_find_internal + 58 (jit-info.c:295)
2   com.mycompany.WCSessionTest.watchkitapp.watchkitextension	0x001fcabd debugger_interrupt_critical + 77 (debugger-agent.c:2693)
3   com.mycompany.WCSessionTest.watchkitapp.watchkitextension	0x0038a457 mono_thread_info_safe_suspend_and_run + 743 (mono-threads.c:1343)
4   com.mycompany.WCSessionTest.watchkitapp.watchkitextension	0x001fc9d9 notify_thread + 89 (debugger-agent.c:2725)
5   com.mycompany.WCSessionTest.watchkitapp.watchkitextension	0x002b5bd9 mono_g_hash_table_foreach + 73 (mono-hash.c:310)
6   com.mycompany.WCSessionTest.watchkitapp.watchkitextension	0x001fad3c suspend_vm + 188 (debugger-agent.c:2822)
7   com.mycompany.WCSessionTest.watchkitapp.watchkitextension	0x001fee49 process_event + 3097 (debugger-agent.c:3873)
8   com.mycompany.WCSessionTest.watchkitapp.watchkitextension	0x001f8bd0 thread_startup + 496 (debugger-agent.c:4014)
9   com.mycompany.WCSessionTest.watchkitapp.watchkitextension	0x002cabe3 mono_profiler_raise_thread_started + 51 (profiler-events.h:103)
10  com.mycompany.WCSessionTest.watchkitapp.watchkitextension	0x002fe98d mono_thread_attach + 253 (threads.c:1115)
11  com.mycompany.WCSessionTest.watchkitapp.watchkitextension	0x003082fd mono_threads_attach_coop_internal + 125 (threads.c:6074)
12  com.mycompany.WCSessionTest.watchkitapp.watchkitextension	0x003083dc mono_threads_attach_coop + 44 (threads.c:6111)
13  com.mycompany.WCSessionTest.watchkitapp.watchkitextension	0x003a2789 xamarin_switch_gchandle + 185 (runtime.m:1851)
14  com.mycompany.WCSessionTest.watchkitapp.watchkitextension	0x003a55be xamarin_retain_trampoline + 78 (trampolines.m:695)
15  libobjc.A.dylib               	0x06aee520 objc_retain + 64
16  com.apple.WatchConnectivity   	0x011eccaa __66-[WCSession onqueue_handleDictionaryMessageRequest:withPairingID:]_block_invoke + 279
17  com.apple.Foundation          	0x051e7df7 __NSBLOCKOPERATION_IS_CALLING_OUT_TO_A_BLOCK__ + 12
18  com.apple.Foundation          	0x051e7cf4 -[NSBlockOperation main] + 88
19  com.apple.Foundation          	0x051eacee __NSOPERATION_IS_INVOKING_MAIN__ + 27

this is a problem in the managed debugger of mono. @chamons could you move this to mono/mono?

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@jschroeder-avi jschroeder-avi commented Dec 3, 2019

Xamarin_Issue.zip
Here is a new zip with more information about the bug. I made some changes to the sample project and included other sample projects as well. I also included additional logs and some test step information.

It seems like Xamarin watch development is completely blocked right now with all of these segmentation faults. It took me a long time to get reproduction steps and information on one way to trigger it. Hope this helps get closer to a solution.

@lewurm lewurm self-assigned this Dec 4, 2019
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@jschroeder-avi jschroeder-avi commented Dec 4, 2019

Interesting. When I run the "WCSessionTest" sample project (that I uploaded here) with the debugger attached to the phone project instead of the watch, I get tons of logs like this in the application output.

[WC] WCSession counterpart app not installed

Furthermore, going to the "Watch" app on the phone simulator - it looks like my watch project isn't fully installed. There is a loading circle next to the app and it is under "AVAILABLE APPS" instead of being in the section for installed apps. I'll attach a screenshot.
Screen Shot 2019-12-04 at 1 25 29 PM
Installing my app from Microsoft AppCenter doesn't have this behavior. From AppCenter, it appears to be installed normally.

I'll also upload my "Application Output" logs from my test on the phone simulator.
iPhone_sim_application_output.txt

For this test, I first opened the watch app. I then ran the iOS project through Visual Studio on the paired iPhone simulator, waited for boot, and hit the "Send message to watch" button a few times. As I have said before, the watch app does not crash when I test without the debugger on the watch project like this.

@lewurm lewurm added this to the 2019-10 (6.8.xx) milestone Dec 5, 2019
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@lewurm lewurm commented Dec 5, 2019

Thank you @jschroeder-avi, I can reproduce the issue.

The problem is that we pass NULL domain here:

(MonoDomain *)mono_thread_info_get_suspend_state (info)->unwind_data [MONO_UNWIND_DATA_DOMAIN],

need to figure out why.

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@DamianMehers DamianMehers commented Dec 8, 2019

Looks like the same issue that is causing xamarin/xamarin-macios#7275 (comment) ... same thing happens as soon as I make an HttpClient call in the simulator - debugger detaches.

lewurm added a commit to lewurm/mono that referenced this issue Dec 9, 2019
When the debugger tries to suspend all the VM threads, it can happen with cooperative-suspend that it tries to suspend a thread that has previously been attached, but then did a "light" detach (that only unsets the domain). With the domain set to `NULL`, looking up a JitInfo for a given `ip` will result into a
crash, but it isn't even necessarily needed for the purpose of suspending a thread.

More details: Consider the following:
```
(lldb) c
error: Process is running.  Use 'process interrupt' to pause execution.
Process 12832 stopped
* thread mono#9, name = 'tid_a31f', queue = 'NSOperationQueue 0x8069b670 (QOS: UTILITY)', stop reason = breakpoint 1.1
    frame #0: 0x00222870 WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical(info=0x81365400, user_data=0xb04dc718) at debugger-agent.c:2716:6
   2713         MonoDomain *domain = (MonoDomain *) mono_thread_info_get_suspend_state (info)->unwind_data [MONO_UNWIND_DATA_DOMAIN];
   2714         if (!domain) {
   2715                 /* not attached */
-> 2716                 ji = NULL;
   2717         } else {
   2718                 ji = mono_jit_info_table_find_internal ( domain, MONO_CONTEXT_GET_IP (&mono_thread_info_get_suspend_state (info)->ctx), TRUE, TRUE);
   2719         }
Target 0: (WatchWCSessionAppWatchOSExtension) stopped.
(lldb) bt
* thread mono#9, name = 'tid_a31f', queue = 'NSOperationQueue 0x8069b670 (QOS: UTILITY)', stop reason = breakpoint 1.1
  * frame #0: 0x00222870 WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical(info=0x81365400, user_data=0xb04dc718) at debugger-agent.c:2716:6
    frame #1: 0x004e177b WatchWCSessionAppWatchOSExtension`mono_thread_info_safe_suspend_and_run(id=0xb0767000, interrupt_kernel=0, callback=(WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical at debugger-agent.c:2708), user_data=0xb04dc718) at mono-threads.c:1358:19
    frame #2: 0x00222799 WatchWCSessionAppWatchOSExtension`notify_thread(key=0x03994508, value=0x81378c00, user_data=0x00000000) at debugger-agent.c:2747:2
    frame mono#3: 0x00355bd8 WatchWCSessionAppWatchOSExtension`mono_g_hash_table_foreach(hash=0x8007b4e0, func=(WatchWCSessionAppWatchOSExtension`notify_thread at debugger-agent.c:2733), user_data=0x00000000) at mono-hash.c:310:4
    frame mono#4: 0x0021e955 WatchWCSessionAppWatchOSExtension`suspend_vm at debugger-agent.c:2844:3
    frame mono#5: 0x00225ff0 WatchWCSessionAppWatchOSExtension`process_event(event=EVENT_KIND_THREAD_START, arg=0x039945d0, il_offset=0, ctx=0x00000000, events=0x805b28e0, suspend_policy=2) at debugger-agent.c:4012:3
    frame mono#6: 0x00227c7c WatchWCSessionAppWatchOSExtension`process_profiler_event(event=EVENT_KIND_THREAD_START, arg=0x039945d0) at debugger-agent.c:4072:2
    frame mono#7: 0x0021b174 WatchWCSessionAppWatchOSExtension`thread_startup(prof=0x00000000, tid=2957889536) at debugger-agent.c:4149:2
    frame mono#8: 0x0037912d WatchWCSessionAppWatchOSExtension`mono_profiler_raise_thread_started(tid=2957889536) at profiler-events.h:103:1
    frame mono#9: 0x003d53da WatchWCSessionAppWatchOSExtension`fire_attach_profiler_events(tid=0xb04dd000) at threads.c:1120:2
    frame mono#10: 0x003d4d83 WatchWCSessionAppWatchOSExtension`mono_thread_attach(domain=0x801750a0) at threads.c:1547:2
    frame mono#11: 0x003df1a1 WatchWCSessionAppWatchOSExtension`mono_threads_attach_coop_internal(domain=0x801750a0, cookie=0xb04dcc0c, stackdata=0xb04dcba8) at threads.c:6008:3
    frame mono#12: 0x003df287 WatchWCSessionAppWatchOSExtension`mono_threads_attach_coop(domain=0x00000000, dummy=0xb04dcc0c) at threads.c:6045:9
    frame mono#13: 0x005034b8 WatchWCSessionAppWatchOSExtension`::xamarin_switch_gchandle(self=0x80762c20, to_weak=false) at runtime.m:1805:2
    frame mono#14: 0x005065c1 WatchWCSessionAppWatchOSExtension`::xamarin_retain_trampoline(self=0x80762c20, sel="retain") at trampolines.m:693:2
    frame mono#15: 0x657ea520 libobjc.A.dylib`objc_retain + 64
    frame mono#16: 0x4b4d9caa WatchConnectivity`__66-[WCSession onqueue_handleDictionaryMessageRequest:withPairingID:]_block_invoke + 279
    frame mono#17: 0x453c7df7 Foundation`__NSBLOCKOPERATION_IS_CALLING_OUT_TO_A_BLOCK__ + 12
    frame mono#18: 0x453c7cf4 Foundation`-[NSBlockOperation main] + 88
    frame mono#19: 0x453cacee Foundation`__NSOPERATION_IS_INVOKING_MAIN__ + 27
    frame mono#20: 0x453c6ebd Foundation`-[NSOperation start] + 835
    frame mono#21: 0x453cb606 Foundation`__NSOPERATIONQUEUE_IS_STARTING_AN_OPERATION__ + 27
    frame mono#22: 0x453cb12e Foundation`__NSOQSchedule_f + 194
    frame mono#23: 0x453cb26e Foundation`____addOperations_block_invoke_4 + 20
    frame mono#24: 0x65de007b libdispatch.dylib`_dispatch_call_block_and_release + 15
    frame mono#25: 0x65de126f libdispatch.dylib`_dispatch_client_callout + 14
    frame mono#26: 0x65de3788 libdispatch.dylib`_dispatch_continuation_pop + 421
    frame mono#27: 0x65de2ee3 libdispatch.dylib`_dispatch_async_redirect_invoke + 818
    frame mono#28: 0x65df087d libdispatch.dylib`_dispatch_root_queue_drain + 354
    frame mono#29: 0x65df0ff3 libdispatch.dylib`_dispatch_worker_thread2 + 109
    frame mono#30: 0x66024fa0 libsystem_pthread.dylib`_pthread_wqthread + 208
    frame mono#31: 0x66024e44 libsystem_pthread.dylib`start_wqthread + 36
```

Going further, `info` is about this thread:
```
(lldb) p/x *(int *)(((char *) info->node.key) + 0xa0)
(int) $2 = 0x01243f93
(lldb) thread list
Process 12832 stopped
  thread #1: tid = 0x1243ee1, 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10, name = 'tid_303', queue = 'com.apple.main-thread'
  thread #2: tid = 0x1243ee6, 0x65f816e2 libsystem_kernel.dylib`__recvfrom + 10
  thread mono#3: tid = 0x1243ee7, 0x65f81aea libsystem_kernel.dylib`__psynch_cvwait + 10, name = 'SGen worker'
  thread mono#4: tid = 0x1243ee9, 0x65f7e3d2 libsystem_kernel.dylib`semaphore_wait_trap + 10, name = 'Finalizer'
  thread mono#5: tid = 0x1243eea, 0x65f816e2 libsystem_kernel.dylib`__recvfrom + 10, name = 'Debugger agent'
  thread mono#6: tid = 0x1243f1d, 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10, name = 'com.apple.uikit.eventfetch-thread'
  thread mono#8: tid = 0x1243f93, 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10, name = 'tid_6d0f', queue = 'NSOperationQueue 0x8069b300 (QOS: UTILITY)'
* thread mono#9: tid = 0x12443a9, 0x00222870 WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical(info=0x81365400, user_data=0xb04dc718) at debugger-agent.c:2716:6, name = 'tid_a31f', queue = 'NSOperationQueue 0x8069b670 (QOS: UTILITY)', stop reason = breakpoint 1.1
  thread mono#10: tid = 0x1244581, 0x65f7fd32 libsystem_kernel.dylib`__workq_kernreturn + 10
(lldb) thread select 8
* thread mono#8, name = 'tid_6d0f', queue = 'NSOperationQueue 0x8069b300 (QOS: UTILITY)'
    frame #0: 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10
libsystem_kernel.dylib`mach_msg_trap:
->  0x65f7e396 <+10>: retl
    0x65f7e397 <+11>: nop

libsystem_kernel.dylib`mach_msg_overwrite_trap:
    0x65f7e398 <+0>:  movl   $0xffffffe0, %eax         ; imm = 0xFFFFFFE0
    0x65f7e39d <+5>:  calll  0x65f85f44                ; _sysenter_trap
(lldb) bt
* thread mono#8, name = 'tid_6d0f', queue = 'NSOperationQueue 0x8069b300 (QOS: UTILITY)'
  * frame #0: 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10
    frame #1: 0x65f7e8ff libsystem_kernel.dylib`mach_msg + 47
    frame #2: 0x66079679 libxpc.dylib`_xpc_send_serializer + 104
    frame mono#3: 0x660794da libxpc.dylib`_xpc_pipe_simpleroutine + 80
    frame mono#4: 0x66079852 libxpc.dylib`xpc_pipe_simpleroutine + 43
    frame mono#5: 0x66043a8f libsystem_trace.dylib`___os_activity_stream_reflect_block_invoke_2 + 30
    frame mono#6: 0x65de126f libdispatch.dylib`_dispatch_client_callout + 14
    frame mono#7: 0x65de3d71 libdispatch.dylib`_dispatch_block_invoke_direct + 257
    frame mono#8: 0x65de3c62 libdispatch.dylib`dispatch_block_perform + 112
    frame mono#9: 0x6604349a libsystem_trace.dylib`_os_activity_stream_reflect + 725
    frame mono#10: 0x6604ef19 libsystem_trace.dylib`_os_log_impl_stream + 468
    frame mono#11: 0x6604e44d libsystem_trace.dylib`_os_log_impl_flatten_and_send + 6410
    frame mono#12: 0x6604cb3b libsystem_trace.dylib`_os_log + 137
    frame mono#13: 0x6604f4aa libsystem_trace.dylib`_os_log_impl + 31
    frame mono#14: 0x4b4eb4e9 WatchConnectivity`WCSerializePayloadDictionary + 393
    frame mono#15: 0x4b4d7c4d WatchConnectivity`-[WCSession onqueue_sendResponseDictionary:identifier:] + 195
    frame mono#16: 0x4b4da435 WatchConnectivity`__66-[WCSession onqueue_handleDictionaryMessageRequest:withPairingID:]_block_invoke.411 + 35
    frame mono#17: 0x453c7df7 Foundation`__NSBLOCKOPERATION_IS_CALLING_OUT_TO_A_BLOCK__ + 12
    frame mono#18: 0x453c7cf4 Foundation`-[NSBlockOperation main] + 88
    frame mono#19: 0x453cacee Foundation`__NSOPERATION_IS_INVOKING_MAIN__ + 27
    frame mono#20: 0x453c6ebd Foundation`-[NSOperation start] + 835
    frame mono#21: 0x453cb606 Foundation`__NSOPERATIONQUEUE_IS_STARTING_AN_OPERATION__ + 27
    frame mono#22: 0x453cb12e Foundation`__NSOQSchedule_f + 194
    frame mono#23: 0x453cb067 Foundation`____addOperations_block_invoke_2 + 20
    frame mono#24: 0x65dedf49 libdispatch.dylib`_dispatch_block_async_invoke2 + 77
    frame mono#25: 0x65de4461 libdispatch.dylib`_dispatch_block_async_invoke_and_release + 17
    frame mono#26: 0x65de126f libdispatch.dylib`_dispatch_client_callout + 14
    frame mono#27: 0x65de3788 libdispatch.dylib`_dispatch_continuation_pop + 421
    frame mono#28: 0x65de2ee3 libdispatch.dylib`_dispatch_async_redirect_invoke + 818
    frame mono#29: 0x65df087d libdispatch.dylib`_dispatch_root_queue_drain + 354
    frame mono#30: 0x65df0ff3 libdispatch.dylib`_dispatch_worker_thread2 + 109
    frame mono#31: 0x66024fa0 libsystem_pthread.dylib`_pthread_wqthread + 208
    frame mono#32: 0x66024e44 libsystem_pthread.dylib`start_wqthread + 36
```
which is a thread in a "light" detach state (aka. coop detach), where we only unset the domain:
https://github.com/mono/mono/blob/46381c2512350b62051ec92298cabea51925cf7f/mono/metadata/threads.c#L6084-L6111

Fixes mono#17926
lewurm added a commit to lewurm/mono that referenced this issue Dec 9, 2019
When the debugger tries to suspend all the VM threads, it can happen with cooperative-suspend that it tries to suspend a thread that has previously been attached, but then did a "light" detach (that only unsets the domain). With the domain set to `NULL`, looking up a JitInfo for a given `ip` will result into a crash, but it isn't even necessarily needed for the purpose of suspending a thread.

More details: Consider the following:
```
(lldb) c
error: Process is running.  Use 'process interrupt' to pause execution.
Process 12832 stopped
* thread mono#9, name = 'tid_a31f', queue = 'NSOperationQueue 0x8069b670 (QOS: UTILITY)', stop reason = breakpoint 1.1
    frame #0: 0x00222870 WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical(info=0x81365400, user_data=0xb04dc718) at debugger-agent.c:2716:6
   2713         MonoDomain *domain = (MonoDomain *) mono_thread_info_get_suspend_state (info)->unwind_data [MONO_UNWIND_DATA_DOMAIN];
   2714         if (!domain) {
   2715                 /* not attached */
-> 2716                 ji = NULL;
   2717         } else {
   2718                 ji = mono_jit_info_table_find_internal ( domain, MONO_CONTEXT_GET_IP (&mono_thread_info_get_suspend_state (info)->ctx), TRUE, TRUE);
   2719         }
Target 0: (WatchWCSessionAppWatchOSExtension) stopped.
(lldb) bt
* thread mono#9, name = 'tid_a31f', queue = 'NSOperationQueue 0x8069b670 (QOS: UTILITY)', stop reason = breakpoint 1.1
  * frame #0: 0x00222870 WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical(info=0x81365400, user_data=0xb04dc718) at debugger-agent.c:2716:6
    frame #1: 0x004e177b WatchWCSessionAppWatchOSExtension`mono_thread_info_safe_suspend_and_run(id=0xb0767000, interrupt_kernel=0, callback=(WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical at debugger-agent.c:2708), user_data=0xb04dc718) at mono-threads.c:1358:19
    frame #2: 0x00222799 WatchWCSessionAppWatchOSExtension`notify_thread(key=0x03994508, value=0x81378c00, user_data=0x00000000) at debugger-agent.c:2747:2
    frame mono#3: 0x00355bd8 WatchWCSessionAppWatchOSExtension`mono_g_hash_table_foreach(hash=0x8007b4e0, func=(WatchWCSessionAppWatchOSExtension`notify_thread at debugger-agent.c:2733), user_data=0x00000000) at mono-hash.c:310:4
    frame mono#4: 0x0021e955 WatchWCSessionAppWatchOSExtension`suspend_vm at debugger-agent.c:2844:3
    frame mono#5: 0x00225ff0 WatchWCSessionAppWatchOSExtension`process_event(event=EVENT_KIND_THREAD_START, arg=0x039945d0, il_offset=0, ctx=0x00000000, events=0x805b28e0, suspend_policy=2) at debugger-agent.c:4012:3
    frame mono#6: 0x00227c7c WatchWCSessionAppWatchOSExtension`process_profiler_event(event=EVENT_KIND_THREAD_START, arg=0x039945d0) at debugger-agent.c:4072:2
    frame mono#7: 0x0021b174 WatchWCSessionAppWatchOSExtension`thread_startup(prof=0x00000000, tid=2957889536) at debugger-agent.c:4149:2
    frame mono#8: 0x0037912d WatchWCSessionAppWatchOSExtension`mono_profiler_raise_thread_started(tid=2957889536) at profiler-events.h:103:1
    frame mono#9: 0x003d53da WatchWCSessionAppWatchOSExtension`fire_attach_profiler_events(tid=0xb04dd000) at threads.c:1120:2
    frame mono#10: 0x003d4d83 WatchWCSessionAppWatchOSExtension`mono_thread_attach(domain=0x801750a0) at threads.c:1547:2
    frame mono#11: 0x003df1a1 WatchWCSessionAppWatchOSExtension`mono_threads_attach_coop_internal(domain=0x801750a0, cookie=0xb04dcc0c, stackdata=0xb04dcba8) at threads.c:6008:3
    frame mono#12: 0x003df287 WatchWCSessionAppWatchOSExtension`mono_threads_attach_coop(domain=0x00000000, dummy=0xb04dcc0c) at threads.c:6045:9
    frame mono#13: 0x005034b8 WatchWCSessionAppWatchOSExtension`::xamarin_switch_gchandle(self=0x80762c20, to_weak=false) at runtime.m:1805:2
    frame mono#14: 0x005065c1 WatchWCSessionAppWatchOSExtension`::xamarin_retain_trampoline(self=0x80762c20, sel="retain") at trampolines.m:693:2
    frame mono#15: 0x657ea520 libobjc.A.dylib`objc_retain + 64
    frame mono#16: 0x4b4d9caa WatchConnectivity`__66-[WCSession onqueue_handleDictionaryMessageRequest:withPairingID:]_block_invoke + 279
    frame mono#17: 0x453c7df7 Foundation`__NSBLOCKOPERATION_IS_CALLING_OUT_TO_A_BLOCK__ + 12
    frame mono#18: 0x453c7cf4 Foundation`-[NSBlockOperation main] + 88
    frame mono#19: 0x453cacee Foundation`__NSOPERATION_IS_INVOKING_MAIN__ + 27
    frame mono#20: 0x453c6ebd Foundation`-[NSOperation start] + 835
    frame mono#21: 0x453cb606 Foundation`__NSOPERATIONQUEUE_IS_STARTING_AN_OPERATION__ + 27
    frame mono#22: 0x453cb12e Foundation`__NSOQSchedule_f + 194
    frame mono#23: 0x453cb26e Foundation`____addOperations_block_invoke_4 + 20
    frame mono#24: 0x65de007b libdispatch.dylib`_dispatch_call_block_and_release + 15
    frame mono#25: 0x65de126f libdispatch.dylib`_dispatch_client_callout + 14
    frame mono#26: 0x65de3788 libdispatch.dylib`_dispatch_continuation_pop + 421
    frame mono#27: 0x65de2ee3 libdispatch.dylib`_dispatch_async_redirect_invoke + 818
    frame mono#28: 0x65df087d libdispatch.dylib`_dispatch_root_queue_drain + 354
    frame mono#29: 0x65df0ff3 libdispatch.dylib`_dispatch_worker_thread2 + 109
    frame mono#30: 0x66024fa0 libsystem_pthread.dylib`_pthread_wqthread + 208
    frame mono#31: 0x66024e44 libsystem_pthread.dylib`start_wqthread + 36
```

Going further, `info` is about this thread:
```
(lldb) p/x *(int *)(((char *) info->node.key) + 0xa0)
(int) $2 = 0x01243f93
(lldb) thread list
Process 12832 stopped
  thread #1: tid = 0x1243ee1, 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10, name = 'tid_303', queue = 'com.apple.main-thread'
  thread #2: tid = 0x1243ee6, 0x65f816e2 libsystem_kernel.dylib`__recvfrom + 10
  thread mono#3: tid = 0x1243ee7, 0x65f81aea libsystem_kernel.dylib`__psynch_cvwait + 10, name = 'SGen worker'
  thread mono#4: tid = 0x1243ee9, 0x65f7e3d2 libsystem_kernel.dylib`semaphore_wait_trap + 10, name = 'Finalizer'
  thread mono#5: tid = 0x1243eea, 0x65f816e2 libsystem_kernel.dylib`__recvfrom + 10, name = 'Debugger agent'
  thread mono#6: tid = 0x1243f1d, 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10, name = 'com.apple.uikit.eventfetch-thread'
  thread mono#8: tid = 0x1243f93, 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10, name = 'tid_6d0f', queue = 'NSOperationQueue 0x8069b300 (QOS: UTILITY)'
* thread mono#9: tid = 0x12443a9, 0x00222870 WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical(info=0x81365400, user_data=0xb04dc718) at debugger-agent.c:2716:6, name = 'tid_a31f', queue = 'NSOperationQueue 0x8069b670 (QOS: UTILITY)', stop reason = breakpoint 1.1
  thread mono#10: tid = 0x1244581, 0x65f7fd32 libsystem_kernel.dylib`__workq_kernreturn + 10
(lldb) thread select 8
* thread mono#8, name = 'tid_6d0f', queue = 'NSOperationQueue 0x8069b300 (QOS: UTILITY)'
    frame #0: 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10
libsystem_kernel.dylib`mach_msg_trap:
->  0x65f7e396 <+10>: retl
    0x65f7e397 <+11>: nop

libsystem_kernel.dylib`mach_msg_overwrite_trap:
    0x65f7e398 <+0>:  movl   $0xffffffe0, %eax         ; imm = 0xFFFFFFE0
    0x65f7e39d <+5>:  calll  0x65f85f44                ; _sysenter_trap
(lldb) bt
* thread mono#8, name = 'tid_6d0f', queue = 'NSOperationQueue 0x8069b300 (QOS: UTILITY)'
  * frame #0: 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10
    frame #1: 0x65f7e8ff libsystem_kernel.dylib`mach_msg + 47
    frame #2: 0x66079679 libxpc.dylib`_xpc_send_serializer + 104
    frame mono#3: 0x660794da libxpc.dylib`_xpc_pipe_simpleroutine + 80
    frame mono#4: 0x66079852 libxpc.dylib`xpc_pipe_simpleroutine + 43
    frame mono#5: 0x66043a8f libsystem_trace.dylib`___os_activity_stream_reflect_block_invoke_2 + 30
    frame mono#6: 0x65de126f libdispatch.dylib`_dispatch_client_callout + 14
    frame mono#7: 0x65de3d71 libdispatch.dylib`_dispatch_block_invoke_direct + 257
    frame mono#8: 0x65de3c62 libdispatch.dylib`dispatch_block_perform + 112
    frame mono#9: 0x6604349a libsystem_trace.dylib`_os_activity_stream_reflect + 725
    frame mono#10: 0x6604ef19 libsystem_trace.dylib`_os_log_impl_stream + 468
    frame mono#11: 0x6604e44d libsystem_trace.dylib`_os_log_impl_flatten_and_send + 6410
    frame mono#12: 0x6604cb3b libsystem_trace.dylib`_os_log + 137
    frame mono#13: 0x6604f4aa libsystem_trace.dylib`_os_log_impl + 31
    frame mono#14: 0x4b4eb4e9 WatchConnectivity`WCSerializePayloadDictionary + 393
    frame mono#15: 0x4b4d7c4d WatchConnectivity`-[WCSession onqueue_sendResponseDictionary:identifier:] + 195
    frame mono#16: 0x4b4da435 WatchConnectivity`__66-[WCSession onqueue_handleDictionaryMessageRequest:withPairingID:]_block_invoke.411 + 35
    frame mono#17: 0x453c7df7 Foundation`__NSBLOCKOPERATION_IS_CALLING_OUT_TO_A_BLOCK__ + 12
    frame mono#18: 0x453c7cf4 Foundation`-[NSBlockOperation main] + 88
    frame mono#19: 0x453cacee Foundation`__NSOPERATION_IS_INVOKING_MAIN__ + 27
    frame mono#20: 0x453c6ebd Foundation`-[NSOperation start] + 835
    frame mono#21: 0x453cb606 Foundation`__NSOPERATIONQUEUE_IS_STARTING_AN_OPERATION__ + 27
    frame mono#22: 0x453cb12e Foundation`__NSOQSchedule_f + 194
    frame mono#23: 0x453cb067 Foundation`____addOperations_block_invoke_2 + 20
    frame mono#24: 0x65dedf49 libdispatch.dylib`_dispatch_block_async_invoke2 + 77
    frame mono#25: 0x65de4461 libdispatch.dylib`_dispatch_block_async_invoke_and_release + 17
    frame mono#26: 0x65de126f libdispatch.dylib`_dispatch_client_callout + 14
    frame mono#27: 0x65de3788 libdispatch.dylib`_dispatch_continuation_pop + 421
    frame mono#28: 0x65de2ee3 libdispatch.dylib`_dispatch_async_redirect_invoke + 818
    frame mono#29: 0x65df087d libdispatch.dylib`_dispatch_root_queue_drain + 354
    frame mono#30: 0x65df0ff3 libdispatch.dylib`_dispatch_worker_thread2 + 109
    frame mono#31: 0x66024fa0 libsystem_pthread.dylib`_pthread_wqthread + 208
    frame mono#32: 0x66024e44 libsystem_pthread.dylib`start_wqthread + 36
```
which is a thread in a "light" detach state (aka. coop detach), where we only unset the domain:
https://github.com/mono/mono/blob/4cefdcb7ce2d939ee78fb45d1b4913eb3bc064fd/mono/metadata/threads.c#L6084-L6111

Fixes mono#17926
lewurm added a commit that referenced this issue Dec 10, 2019
When the debugger tries to suspend all the VM threads, it can happen with cooperative-suspend that it tries to suspend a thread that has previously been attached, but then did a "light" detach (that only unsets the domain). With the domain set to `NULL`, looking up a JitInfo for a given `ip` will result into a crash, but it isn't even necessarily needed for the purpose of suspending a thread.

More details: Consider the following:
```
(lldb) c
error: Process is running.  Use 'process interrupt' to pause execution.
Process 12832 stopped
* thread #9, name = 'tid_a31f', queue = 'NSOperationQueue 0x8069b670 (QOS: UTILITY)', stop reason = breakpoint 1.1
    frame #0: 0x00222870 WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical(info=0x81365400, user_data=0xb04dc718) at debugger-agent.c:2716:6
   2713         MonoDomain *domain = (MonoDomain *) mono_thread_info_get_suspend_state (info)->unwind_data [MONO_UNWIND_DATA_DOMAIN];
   2714         if (!domain) {
   2715                 /* not attached */
-> 2716                 ji = NULL;
   2717         } else {
   2718                 ji = mono_jit_info_table_find_internal ( domain, MONO_CONTEXT_GET_IP (&mono_thread_info_get_suspend_state (info)->ctx), TRUE, TRUE);
   2719         }
Target 0: (WatchWCSessionAppWatchOSExtension) stopped.
(lldb) bt
* thread #9, name = 'tid_a31f', queue = 'NSOperationQueue 0x8069b670 (QOS: UTILITY)', stop reason = breakpoint 1.1
  * frame #0: 0x00222870 WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical(info=0x81365400, user_data=0xb04dc718) at debugger-agent.c:2716:6
    frame #1: 0x004e177b WatchWCSessionAppWatchOSExtension`mono_thread_info_safe_suspend_and_run(id=0xb0767000, interrupt_kernel=0, callback=(WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical at debugger-agent.c:2708), user_data=0xb04dc718) at mono-threads.c:1358:19
    frame #2: 0x00222799 WatchWCSessionAppWatchOSExtension`notify_thread(key=0x03994508, value=0x81378c00, user_data=0x00000000) at debugger-agent.c:2747:2
    frame #3: 0x00355bd8 WatchWCSessionAppWatchOSExtension`mono_g_hash_table_foreach(hash=0x8007b4e0, func=(WatchWCSessionAppWatchOSExtension`notify_thread at debugger-agent.c:2733), user_data=0x00000000) at mono-hash.c:310:4
    frame #4: 0x0021e955 WatchWCSessionAppWatchOSExtension`suspend_vm at debugger-agent.c:2844:3
    frame #5: 0x00225ff0 WatchWCSessionAppWatchOSExtension`process_event(event=EVENT_KIND_THREAD_START, arg=0x039945d0, il_offset=0, ctx=0x00000000, events=0x805b28e0, suspend_policy=2) at debugger-agent.c:4012:3
    frame #6: 0x00227c7c WatchWCSessionAppWatchOSExtension`process_profiler_event(event=EVENT_KIND_THREAD_START, arg=0x039945d0) at debugger-agent.c:4072:2
    frame #7: 0x0021b174 WatchWCSessionAppWatchOSExtension`thread_startup(prof=0x00000000, tid=2957889536) at debugger-agent.c:4149:2
    frame #8: 0x0037912d WatchWCSessionAppWatchOSExtension`mono_profiler_raise_thread_started(tid=2957889536) at profiler-events.h:103:1
    frame #9: 0x003d53da WatchWCSessionAppWatchOSExtension`fire_attach_profiler_events(tid=0xb04dd000) at threads.c:1120:2
    frame #10: 0x003d4d83 WatchWCSessionAppWatchOSExtension`mono_thread_attach(domain=0x801750a0) at threads.c:1547:2
    frame #11: 0x003df1a1 WatchWCSessionAppWatchOSExtension`mono_threads_attach_coop_internal(domain=0x801750a0, cookie=0xb04dcc0c, stackdata=0xb04dcba8) at threads.c:6008:3
    frame #12: 0x003df287 WatchWCSessionAppWatchOSExtension`mono_threads_attach_coop(domain=0x00000000, dummy=0xb04dcc0c) at threads.c:6045:9
    frame #13: 0x005034b8 WatchWCSessionAppWatchOSExtension`::xamarin_switch_gchandle(self=0x80762c20, to_weak=false) at runtime.m:1805:2
    frame #14: 0x005065c1 WatchWCSessionAppWatchOSExtension`::xamarin_retain_trampoline(self=0x80762c20, sel="retain") at trampolines.m:693:2
    frame #15: 0x657ea520 libobjc.A.dylib`objc_retain + 64
    frame #16: 0x4b4d9caa WatchConnectivity`__66-[WCSession onqueue_handleDictionaryMessageRequest:withPairingID:]_block_invoke + 279
    frame #17: 0x453c7df7 Foundation`__NSBLOCKOPERATION_IS_CALLING_OUT_TO_A_BLOCK__ + 12
    frame #18: 0x453c7cf4 Foundation`-[NSBlockOperation main] + 88
    frame #19: 0x453cacee Foundation`__NSOPERATION_IS_INVOKING_MAIN__ + 27
    frame #20: 0x453c6ebd Foundation`-[NSOperation start] + 835
    frame #21: 0x453cb606 Foundation`__NSOPERATIONQUEUE_IS_STARTING_AN_OPERATION__ + 27
    frame #22: 0x453cb12e Foundation`__NSOQSchedule_f + 194
    frame #23: 0x453cb26e Foundation`____addOperations_block_invoke_4 + 20
    frame #24: 0x65de007b libdispatch.dylib`_dispatch_call_block_and_release + 15
    frame #25: 0x65de126f libdispatch.dylib`_dispatch_client_callout + 14
    frame #26: 0x65de3788 libdispatch.dylib`_dispatch_continuation_pop + 421
    frame #27: 0x65de2ee3 libdispatch.dylib`_dispatch_async_redirect_invoke + 818
    frame #28: 0x65df087d libdispatch.dylib`_dispatch_root_queue_drain + 354
    frame #29: 0x65df0ff3 libdispatch.dylib`_dispatch_worker_thread2 + 109
    frame #30: 0x66024fa0 libsystem_pthread.dylib`_pthread_wqthread + 208
    frame #31: 0x66024e44 libsystem_pthread.dylib`start_wqthread + 36
```

Going further, `info` is about this thread:
```
(lldb) p/x *(int *)(((char *) info->node.key) + 0xa0)
(int) $2 = 0x01243f93
(lldb) thread list
Process 12832 stopped
  thread #1: tid = 0x1243ee1, 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10, name = 'tid_303', queue = 'com.apple.main-thread'
  thread #2: tid = 0x1243ee6, 0x65f816e2 libsystem_kernel.dylib`__recvfrom + 10
  thread #3: tid = 0x1243ee7, 0x65f81aea libsystem_kernel.dylib`__psynch_cvwait + 10, name = 'SGen worker'
  thread #4: tid = 0x1243ee9, 0x65f7e3d2 libsystem_kernel.dylib`semaphore_wait_trap + 10, name = 'Finalizer'
  thread #5: tid = 0x1243eea, 0x65f816e2 libsystem_kernel.dylib`__recvfrom + 10, name = 'Debugger agent'
  thread #6: tid = 0x1243f1d, 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10, name = 'com.apple.uikit.eventfetch-thread'
  thread #8: tid = 0x1243f93, 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10, name = 'tid_6d0f', queue = 'NSOperationQueue 0x8069b300 (QOS: UTILITY)'
* thread #9: tid = 0x12443a9, 0x00222870 WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical(info=0x81365400, user_data=0xb04dc718) at debugger-agent.c:2716:6, name = 'tid_a31f', queue = 'NSOperationQueue 0x8069b670 (QOS: UTILITY)', stop reason = breakpoint 1.1
  thread #10: tid = 0x1244581, 0x65f7fd32 libsystem_kernel.dylib`__workq_kernreturn + 10
(lldb) thread select 8
* thread #8, name = 'tid_6d0f', queue = 'NSOperationQueue 0x8069b300 (QOS: UTILITY)'
    frame #0: 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10
libsystem_kernel.dylib`mach_msg_trap:
->  0x65f7e396 <+10>: retl
    0x65f7e397 <+11>: nop

libsystem_kernel.dylib`mach_msg_overwrite_trap:
    0x65f7e398 <+0>:  movl   $0xffffffe0, %eax         ; imm = 0xFFFFFFE0
    0x65f7e39d <+5>:  calll  0x65f85f44                ; _sysenter_trap
(lldb) bt
* thread #8, name = 'tid_6d0f', queue = 'NSOperationQueue 0x8069b300 (QOS: UTILITY)'
  * frame #0: 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10
    frame #1: 0x65f7e8ff libsystem_kernel.dylib`mach_msg + 47
    frame #2: 0x66079679 libxpc.dylib`_xpc_send_serializer + 104
    frame #3: 0x660794da libxpc.dylib`_xpc_pipe_simpleroutine + 80
    frame #4: 0x66079852 libxpc.dylib`xpc_pipe_simpleroutine + 43
    frame #5: 0x66043a8f libsystem_trace.dylib`___os_activity_stream_reflect_block_invoke_2 + 30
    frame #6: 0x65de126f libdispatch.dylib`_dispatch_client_callout + 14
    frame #7: 0x65de3d71 libdispatch.dylib`_dispatch_block_invoke_direct + 257
    frame #8: 0x65de3c62 libdispatch.dylib`dispatch_block_perform + 112
    frame #9: 0x6604349a libsystem_trace.dylib`_os_activity_stream_reflect + 725
    frame #10: 0x6604ef19 libsystem_trace.dylib`_os_log_impl_stream + 468
    frame #11: 0x6604e44d libsystem_trace.dylib`_os_log_impl_flatten_and_send + 6410
    frame #12: 0x6604cb3b libsystem_trace.dylib`_os_log + 137
    frame #13: 0x6604f4aa libsystem_trace.dylib`_os_log_impl + 31
    frame #14: 0x4b4eb4e9 WatchConnectivity`WCSerializePayloadDictionary + 393
    frame #15: 0x4b4d7c4d WatchConnectivity`-[WCSession onqueue_sendResponseDictionary:identifier:] + 195
    frame #16: 0x4b4da435 WatchConnectivity`__66-[WCSession onqueue_handleDictionaryMessageRequest:withPairingID:]_block_invoke.411 + 35
    frame #17: 0x453c7df7 Foundation`__NSBLOCKOPERATION_IS_CALLING_OUT_TO_A_BLOCK__ + 12
    frame #18: 0x453c7cf4 Foundation`-[NSBlockOperation main] + 88
    frame #19: 0x453cacee Foundation`__NSOPERATION_IS_INVOKING_MAIN__ + 27
    frame #20: 0x453c6ebd Foundation`-[NSOperation start] + 835
    frame #21: 0x453cb606 Foundation`__NSOPERATIONQUEUE_IS_STARTING_AN_OPERATION__ + 27
    frame #22: 0x453cb12e Foundation`__NSOQSchedule_f + 194
    frame #23: 0x453cb067 Foundation`____addOperations_block_invoke_2 + 20
    frame #24: 0x65dedf49 libdispatch.dylib`_dispatch_block_async_invoke2 + 77
    frame #25: 0x65de4461 libdispatch.dylib`_dispatch_block_async_invoke_and_release + 17
    frame #26: 0x65de126f libdispatch.dylib`_dispatch_client_callout + 14
    frame #27: 0x65de3788 libdispatch.dylib`_dispatch_continuation_pop + 421
    frame #28: 0x65de2ee3 libdispatch.dylib`_dispatch_async_redirect_invoke + 818
    frame #29: 0x65df087d libdispatch.dylib`_dispatch_root_queue_drain + 354
    frame #30: 0x65df0ff3 libdispatch.dylib`_dispatch_worker_thread2 + 109
    frame #31: 0x66024fa0 libsystem_pthread.dylib`_pthread_wqthread + 208
    frame #32: 0x66024e44 libsystem_pthread.dylib`start_wqthread + 36
```
which is a thread in a "light" detach state (aka. coop detach), where we only unset the domain:
https://github.com/mono/mono/blob/4cefdcb7ce2d939ee78fb45d1b4913eb3bc064fd/mono/metadata/threads.c#L6084-L6111

Fixes #17926
@lewurm lewurm reopened this Dec 10, 2019
@lewurm

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@lewurm lewurm commented Dec 10, 2019

let's keep it open until the backports have landed.

monojenkins added a commit to monojenkins/mono that referenced this issue Dec 10, 2019
When the debugger tries to suspend all the VM threads, it can happen with cooperative-suspend that it tries to suspend a thread that has previously been attached, but then did a "light" detach (that only unsets the domain). With the domain set to `NULL`, looking up a JitInfo for a given `ip` will result into a crash, but it isn't even necessarily needed for the purpose of suspending a thread.

More details: Consider the following:
```
(lldb) c
error: Process is running.  Use 'process interrupt' to pause execution.
Process 12832 stopped
* thread mono#9, name = 'tid_a31f', queue = 'NSOperationQueue 0x8069b670 (QOS: UTILITY)', stop reason = breakpoint 1.1
    frame #0: 0x00222870 WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical(info=0x81365400, user_data=0xb04dc718) at debugger-agent.c:2716:6
   2713         MonoDomain *domain = (MonoDomain *) mono_thread_info_get_suspend_state (info)->unwind_data [MONO_UNWIND_DATA_DOMAIN];
   2714         if (!domain) {
   2715                 /* not attached */
-> 2716                 ji = NULL;
   2717         } else {
   2718                 ji = mono_jit_info_table_find_internal ( domain, MONO_CONTEXT_GET_IP (&mono_thread_info_get_suspend_state (info)->ctx), TRUE, TRUE);
   2719         }
Target 0: (WatchWCSessionAppWatchOSExtension) stopped.
(lldb) bt
* thread mono#9, name = 'tid_a31f', queue = 'NSOperationQueue 0x8069b670 (QOS: UTILITY)', stop reason = breakpoint 1.1
  * frame #0: 0x00222870 WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical(info=0x81365400, user_data=0xb04dc718) at debugger-agent.c:2716:6
    frame mono#1: 0x004e177b WatchWCSessionAppWatchOSExtension`mono_thread_info_safe_suspend_and_run(id=0xb0767000, interrupt_kernel=0, callback=(WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical at debugger-agent.c:2708), user_data=0xb04dc718) at mono-threads.c:1358:19
    frame mono#2: 0x00222799 WatchWCSessionAppWatchOSExtension`notify_thread(key=0x03994508, value=0x81378c00, user_data=0x00000000) at debugger-agent.c:2747:2
    frame mono#3: 0x00355bd8 WatchWCSessionAppWatchOSExtension`mono_g_hash_table_foreach(hash=0x8007b4e0, func=(WatchWCSessionAppWatchOSExtension`notify_thread at debugger-agent.c:2733), user_data=0x00000000) at mono-hash.c:310:4
    frame mono#4: 0x0021e955 WatchWCSessionAppWatchOSExtension`suspend_vm at debugger-agent.c:2844:3
    frame mono#5: 0x00225ff0 WatchWCSessionAppWatchOSExtension`process_event(event=EVENT_KIND_THREAD_START, arg=0x039945d0, il_offset=0, ctx=0x00000000, events=0x805b28e0, suspend_policy=2) at debugger-agent.c:4012:3
    frame mono#6: 0x00227c7c WatchWCSessionAppWatchOSExtension`process_profiler_event(event=EVENT_KIND_THREAD_START, arg=0x039945d0) at debugger-agent.c:4072:2
    frame mono#7: 0x0021b174 WatchWCSessionAppWatchOSExtension`thread_startup(prof=0x00000000, tid=2957889536) at debugger-agent.c:4149:2
    frame mono#8: 0x0037912d WatchWCSessionAppWatchOSExtension`mono_profiler_raise_thread_started(tid=2957889536) at profiler-events.h:103:1
    frame mono#9: 0x003d53da WatchWCSessionAppWatchOSExtension`fire_attach_profiler_events(tid=0xb04dd000) at threads.c:1120:2
    frame mono#10: 0x003d4d83 WatchWCSessionAppWatchOSExtension`mono_thread_attach(domain=0x801750a0) at threads.c:1547:2
    frame mono#11: 0x003df1a1 WatchWCSessionAppWatchOSExtension`mono_threads_attach_coop_internal(domain=0x801750a0, cookie=0xb04dcc0c, stackdata=0xb04dcba8) at threads.c:6008:3
    frame mono#12: 0x003df287 WatchWCSessionAppWatchOSExtension`mono_threads_attach_coop(domain=0x00000000, dummy=0xb04dcc0c) at threads.c:6045:9
    frame mono#13: 0x005034b8 WatchWCSessionAppWatchOSExtension`::xamarin_switch_gchandle(self=0x80762c20, to_weak=false) at runtime.m:1805:2
    frame mono#14: 0x005065c1 WatchWCSessionAppWatchOSExtension`::xamarin_retain_trampoline(self=0x80762c20, sel="retain") at trampolines.m:693:2
    frame mono#15: 0x657ea520 libobjc.A.dylib`objc_retain + 64
    frame mono#16: 0x4b4d9caa WatchConnectivity`__66-[WCSession onqueue_handleDictionaryMessageRequest:withPairingID:]_block_invoke + 279
    frame mono#17: 0x453c7df7 Foundation`__NSBLOCKOPERATION_IS_CALLING_OUT_TO_A_BLOCK__ + 12
    frame mono#18: 0x453c7cf4 Foundation`-[NSBlockOperation main] + 88
    frame mono#19: 0x453cacee Foundation`__NSOPERATION_IS_INVOKING_MAIN__ + 27
    frame mono#20: 0x453c6ebd Foundation`-[NSOperation start] + 835
    frame mono#21: 0x453cb606 Foundation`__NSOPERATIONQUEUE_IS_STARTING_AN_OPERATION__ + 27
    frame mono#22: 0x453cb12e Foundation`__NSOQSchedule_f + 194
    frame mono#23: 0x453cb26e Foundation`____addOperations_block_invoke_4 + 20
    frame mono#24: 0x65de007b libdispatch.dylib`_dispatch_call_block_and_release + 15
    frame mono#25: 0x65de126f libdispatch.dylib`_dispatch_client_callout + 14
    frame mono#26: 0x65de3788 libdispatch.dylib`_dispatch_continuation_pop + 421
    frame mono#27: 0x65de2ee3 libdispatch.dylib`_dispatch_async_redirect_invoke + 818
    frame mono#28: 0x65df087d libdispatch.dylib`_dispatch_root_queue_drain + 354
    frame mono#29: 0x65df0ff3 libdispatch.dylib`_dispatch_worker_thread2 + 109
    frame mono#30: 0x66024fa0 libsystem_pthread.dylib`_pthread_wqthread + 208
    frame mono#31: 0x66024e44 libsystem_pthread.dylib`start_wqthread + 36
```

Going further, `info` is about this thread:
```
(lldb) p/x *(int *)(((char *) info->node.key) + 0xa0)
(int) $2 = 0x01243f93
(lldb) thread list
Process 12832 stopped
  thread mono#1: tid = 0x1243ee1, 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10, name = 'tid_303', queue = 'com.apple.main-thread'
  thread mono#2: tid = 0x1243ee6, 0x65f816e2 libsystem_kernel.dylib`__recvfrom + 10
  thread mono#3: tid = 0x1243ee7, 0x65f81aea libsystem_kernel.dylib`__psynch_cvwait + 10, name = 'SGen worker'
  thread mono#4: tid = 0x1243ee9, 0x65f7e3d2 libsystem_kernel.dylib`semaphore_wait_trap + 10, name = 'Finalizer'
  thread mono#5: tid = 0x1243eea, 0x65f816e2 libsystem_kernel.dylib`__recvfrom + 10, name = 'Debugger agent'
  thread mono#6: tid = 0x1243f1d, 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10, name = 'com.apple.uikit.eventfetch-thread'
  thread mono#8: tid = 0x1243f93, 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10, name = 'tid_6d0f', queue = 'NSOperationQueue 0x8069b300 (QOS: UTILITY)'
* thread mono#9: tid = 0x12443a9, 0x00222870 WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical(info=0x81365400, user_data=0xb04dc718) at debugger-agent.c:2716:6, name = 'tid_a31f', queue = 'NSOperationQueue 0x8069b670 (QOS: UTILITY)', stop reason = breakpoint 1.1
  thread mono#10: tid = 0x1244581, 0x65f7fd32 libsystem_kernel.dylib`__workq_kernreturn + 10
(lldb) thread select 8
* thread mono#8, name = 'tid_6d0f', queue = 'NSOperationQueue 0x8069b300 (QOS: UTILITY)'
    frame #0: 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10
libsystem_kernel.dylib`mach_msg_trap:
->  0x65f7e396 <+10>: retl
    0x65f7e397 <+11>: nop

libsystem_kernel.dylib`mach_msg_overwrite_trap:
    0x65f7e398 <+0>:  movl   $0xffffffe0, %eax         ; imm = 0xFFFFFFE0
    0x65f7e39d <+5>:  calll  0x65f85f44                ; _sysenter_trap
(lldb) bt
* thread mono#8, name = 'tid_6d0f', queue = 'NSOperationQueue 0x8069b300 (QOS: UTILITY)'
  * frame #0: 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10
    frame mono#1: 0x65f7e8ff libsystem_kernel.dylib`mach_msg + 47
    frame mono#2: 0x66079679 libxpc.dylib`_xpc_send_serializer + 104
    frame mono#3: 0x660794da libxpc.dylib`_xpc_pipe_simpleroutine + 80
    frame mono#4: 0x66079852 libxpc.dylib`xpc_pipe_simpleroutine + 43
    frame mono#5: 0x66043a8f libsystem_trace.dylib`___os_activity_stream_reflect_block_invoke_2 + 30
    frame mono#6: 0x65de126f libdispatch.dylib`_dispatch_client_callout + 14
    frame mono#7: 0x65de3d71 libdispatch.dylib`_dispatch_block_invoke_direct + 257
    frame mono#8: 0x65de3c62 libdispatch.dylib`dispatch_block_perform + 112
    frame mono#9: 0x6604349a libsystem_trace.dylib`_os_activity_stream_reflect + 725
    frame mono#10: 0x6604ef19 libsystem_trace.dylib`_os_log_impl_stream + 468
    frame mono#11: 0x6604e44d libsystem_trace.dylib`_os_log_impl_flatten_and_send + 6410
    frame mono#12: 0x6604cb3b libsystem_trace.dylib`_os_log + 137
    frame mono#13: 0x6604f4aa libsystem_trace.dylib`_os_log_impl + 31
    frame mono#14: 0x4b4eb4e9 WatchConnectivity`WCSerializePayloadDictionary + 393
    frame mono#15: 0x4b4d7c4d WatchConnectivity`-[WCSession onqueue_sendResponseDictionary:identifier:] + 195
    frame mono#16: 0x4b4da435 WatchConnectivity`__66-[WCSession onqueue_handleDictionaryMessageRequest:withPairingID:]_block_invoke.411 + 35
    frame mono#17: 0x453c7df7 Foundation`__NSBLOCKOPERATION_IS_CALLING_OUT_TO_A_BLOCK__ + 12
    frame mono#18: 0x453c7cf4 Foundation`-[NSBlockOperation main] + 88
    frame mono#19: 0x453cacee Foundation`__NSOPERATION_IS_INVOKING_MAIN__ + 27
    frame mono#20: 0x453c6ebd Foundation`-[NSOperation start] + 835
    frame mono#21: 0x453cb606 Foundation`__NSOPERATIONQUEUE_IS_STARTING_AN_OPERATION__ + 27
    frame mono#22: 0x453cb12e Foundation`__NSOQSchedule_f + 194
    frame mono#23: 0x453cb067 Foundation`____addOperations_block_invoke_2 + 20
    frame mono#24: 0x65dedf49 libdispatch.dylib`_dispatch_block_async_invoke2 + 77
    frame mono#25: 0x65de4461 libdispatch.dylib`_dispatch_block_async_invoke_and_release + 17
    frame mono#26: 0x65de126f libdispatch.dylib`_dispatch_client_callout + 14
    frame mono#27: 0x65de3788 libdispatch.dylib`_dispatch_continuation_pop + 421
    frame mono#28: 0x65de2ee3 libdispatch.dylib`_dispatch_async_redirect_invoke + 818
    frame mono#29: 0x65df087d libdispatch.dylib`_dispatch_root_queue_drain + 354
    frame mono#30: 0x65df0ff3 libdispatch.dylib`_dispatch_worker_thread2 + 109
    frame mono#31: 0x66024fa0 libsystem_pthread.dylib`_pthread_wqthread + 208
    frame mono#32: 0x66024e44 libsystem_pthread.dylib`start_wqthread + 36
```
which is a thread in a "light" detach state (aka. coop detach), where we only unset the domain:
https://github.com/mono/mono/blob/4cefdcb7ce2d939ee78fb45d1b4913eb3bc064fd/mono/metadata/threads.c#L6084-L6111

Fixes mono#17926
monojenkins added a commit to monojenkins/mono that referenced this issue Dec 10, 2019
When the debugger tries to suspend all the VM threads, it can happen with cooperative-suspend that it tries to suspend a thread that has previously been attached, but then did a "light" detach (that only unsets the domain). With the domain set to `NULL`, looking up a JitInfo for a given `ip` will result into a crash, but it isn't even necessarily needed for the purpose of suspending a thread.

More details: Consider the following:
```
(lldb) c
error: Process is running.  Use 'process interrupt' to pause execution.
Process 12832 stopped
* thread mono#9, name = 'tid_a31f', queue = 'NSOperationQueue 0x8069b670 (QOS: UTILITY)', stop reason = breakpoint 1.1
    frame #0: 0x00222870 WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical(info=0x81365400, user_data=0xb04dc718) at debugger-agent.c:2716:6
   2713         MonoDomain *domain = (MonoDomain *) mono_thread_info_get_suspend_state (info)->unwind_data [MONO_UNWIND_DATA_DOMAIN];
   2714         if (!domain) {
   2715                 /* not attached */
-> 2716                 ji = NULL;
   2717         } else {
   2718                 ji = mono_jit_info_table_find_internal ( domain, MONO_CONTEXT_GET_IP (&mono_thread_info_get_suspend_state (info)->ctx), TRUE, TRUE);
   2719         }
Target 0: (WatchWCSessionAppWatchOSExtension) stopped.
(lldb) bt
* thread mono#9, name = 'tid_a31f', queue = 'NSOperationQueue 0x8069b670 (QOS: UTILITY)', stop reason = breakpoint 1.1
  * frame #0: 0x00222870 WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical(info=0x81365400, user_data=0xb04dc718) at debugger-agent.c:2716:6
    frame mono#1: 0x004e177b WatchWCSessionAppWatchOSExtension`mono_thread_info_safe_suspend_and_run(id=0xb0767000, interrupt_kernel=0, callback=(WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical at debugger-agent.c:2708), user_data=0xb04dc718) at mono-threads.c:1358:19
    frame mono#2: 0x00222799 WatchWCSessionAppWatchOSExtension`notify_thread(key=0x03994508, value=0x81378c00, user_data=0x00000000) at debugger-agent.c:2747:2
    frame mono#3: 0x00355bd8 WatchWCSessionAppWatchOSExtension`mono_g_hash_table_foreach(hash=0x8007b4e0, func=(WatchWCSessionAppWatchOSExtension`notify_thread at debugger-agent.c:2733), user_data=0x00000000) at mono-hash.c:310:4
    frame mono#4: 0x0021e955 WatchWCSessionAppWatchOSExtension`suspend_vm at debugger-agent.c:2844:3
    frame mono#5: 0x00225ff0 WatchWCSessionAppWatchOSExtension`process_event(event=EVENT_KIND_THREAD_START, arg=0x039945d0, il_offset=0, ctx=0x00000000, events=0x805b28e0, suspend_policy=2) at debugger-agent.c:4012:3
    frame mono#6: 0x00227c7c WatchWCSessionAppWatchOSExtension`process_profiler_event(event=EVENT_KIND_THREAD_START, arg=0x039945d0) at debugger-agent.c:4072:2
    frame mono#7: 0x0021b174 WatchWCSessionAppWatchOSExtension`thread_startup(prof=0x00000000, tid=2957889536) at debugger-agent.c:4149:2
    frame mono#8: 0x0037912d WatchWCSessionAppWatchOSExtension`mono_profiler_raise_thread_started(tid=2957889536) at profiler-events.h:103:1
    frame mono#9: 0x003d53da WatchWCSessionAppWatchOSExtension`fire_attach_profiler_events(tid=0xb04dd000) at threads.c:1120:2
    frame mono#10: 0x003d4d83 WatchWCSessionAppWatchOSExtension`mono_thread_attach(domain=0x801750a0) at threads.c:1547:2
    frame mono#11: 0x003df1a1 WatchWCSessionAppWatchOSExtension`mono_threads_attach_coop_internal(domain=0x801750a0, cookie=0xb04dcc0c, stackdata=0xb04dcba8) at threads.c:6008:3
    frame mono#12: 0x003df287 WatchWCSessionAppWatchOSExtension`mono_threads_attach_coop(domain=0x00000000, dummy=0xb04dcc0c) at threads.c:6045:9
    frame mono#13: 0x005034b8 WatchWCSessionAppWatchOSExtension`::xamarin_switch_gchandle(self=0x80762c20, to_weak=false) at runtime.m:1805:2
    frame mono#14: 0x005065c1 WatchWCSessionAppWatchOSExtension`::xamarin_retain_trampoline(self=0x80762c20, sel="retain") at trampolines.m:693:2
    frame mono#15: 0x657ea520 libobjc.A.dylib`objc_retain + 64
    frame mono#16: 0x4b4d9caa WatchConnectivity`__66-[WCSession onqueue_handleDictionaryMessageRequest:withPairingID:]_block_invoke + 279
    frame mono#17: 0x453c7df7 Foundation`__NSBLOCKOPERATION_IS_CALLING_OUT_TO_A_BLOCK__ + 12
    frame mono#18: 0x453c7cf4 Foundation`-[NSBlockOperation main] + 88
    frame mono#19: 0x453cacee Foundation`__NSOPERATION_IS_INVOKING_MAIN__ + 27
    frame mono#20: 0x453c6ebd Foundation`-[NSOperation start] + 835
    frame mono#21: 0x453cb606 Foundation`__NSOPERATIONQUEUE_IS_STARTING_AN_OPERATION__ + 27
    frame mono#22: 0x453cb12e Foundation`__NSOQSchedule_f + 194
    frame mono#23: 0x453cb26e Foundation`____addOperations_block_invoke_4 + 20
    frame mono#24: 0x65de007b libdispatch.dylib`_dispatch_call_block_and_release + 15
    frame mono#25: 0x65de126f libdispatch.dylib`_dispatch_client_callout + 14
    frame mono#26: 0x65de3788 libdispatch.dylib`_dispatch_continuation_pop + 421
    frame mono#27: 0x65de2ee3 libdispatch.dylib`_dispatch_async_redirect_invoke + 818
    frame mono#28: 0x65df087d libdispatch.dylib`_dispatch_root_queue_drain + 354
    frame mono#29: 0x65df0ff3 libdispatch.dylib`_dispatch_worker_thread2 + 109
    frame mono#30: 0x66024fa0 libsystem_pthread.dylib`_pthread_wqthread + 208
    frame mono#31: 0x66024e44 libsystem_pthread.dylib`start_wqthread + 36
```

Going further, `info` is about this thread:
```
(lldb) p/x *(int *)(((char *) info->node.key) + 0xa0)
(int) $2 = 0x01243f93
(lldb) thread list
Process 12832 stopped
  thread mono#1: tid = 0x1243ee1, 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10, name = 'tid_303', queue = 'com.apple.main-thread'
  thread mono#2: tid = 0x1243ee6, 0x65f816e2 libsystem_kernel.dylib`__recvfrom + 10
  thread mono#3: tid = 0x1243ee7, 0x65f81aea libsystem_kernel.dylib`__psynch_cvwait + 10, name = 'SGen worker'
  thread mono#4: tid = 0x1243ee9, 0x65f7e3d2 libsystem_kernel.dylib`semaphore_wait_trap + 10, name = 'Finalizer'
  thread mono#5: tid = 0x1243eea, 0x65f816e2 libsystem_kernel.dylib`__recvfrom + 10, name = 'Debugger agent'
  thread mono#6: tid = 0x1243f1d, 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10, name = 'com.apple.uikit.eventfetch-thread'
  thread mono#8: tid = 0x1243f93, 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10, name = 'tid_6d0f', queue = 'NSOperationQueue 0x8069b300 (QOS: UTILITY)'
* thread mono#9: tid = 0x12443a9, 0x00222870 WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical(info=0x81365400, user_data=0xb04dc718) at debugger-agent.c:2716:6, name = 'tid_a31f', queue = 'NSOperationQueue 0x8069b670 (QOS: UTILITY)', stop reason = breakpoint 1.1
  thread mono#10: tid = 0x1244581, 0x65f7fd32 libsystem_kernel.dylib`__workq_kernreturn + 10
(lldb) thread select 8
* thread mono#8, name = 'tid_6d0f', queue = 'NSOperationQueue 0x8069b300 (QOS: UTILITY)'
    frame #0: 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10
libsystem_kernel.dylib`mach_msg_trap:
->  0x65f7e396 <+10>: retl
    0x65f7e397 <+11>: nop

libsystem_kernel.dylib`mach_msg_overwrite_trap:
    0x65f7e398 <+0>:  movl   $0xffffffe0, %eax         ; imm = 0xFFFFFFE0
    0x65f7e39d <+5>:  calll  0x65f85f44                ; _sysenter_trap
(lldb) bt
* thread mono#8, name = 'tid_6d0f', queue = 'NSOperationQueue 0x8069b300 (QOS: UTILITY)'
  * frame #0: 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10
    frame mono#1: 0x65f7e8ff libsystem_kernel.dylib`mach_msg + 47
    frame mono#2: 0x66079679 libxpc.dylib`_xpc_send_serializer + 104
    frame mono#3: 0x660794da libxpc.dylib`_xpc_pipe_simpleroutine + 80
    frame mono#4: 0x66079852 libxpc.dylib`xpc_pipe_simpleroutine + 43
    frame mono#5: 0x66043a8f libsystem_trace.dylib`___os_activity_stream_reflect_block_invoke_2 + 30
    frame mono#6: 0x65de126f libdispatch.dylib`_dispatch_client_callout + 14
    frame mono#7: 0x65de3d71 libdispatch.dylib`_dispatch_block_invoke_direct + 257
    frame mono#8: 0x65de3c62 libdispatch.dylib`dispatch_block_perform + 112
    frame mono#9: 0x6604349a libsystem_trace.dylib`_os_activity_stream_reflect + 725
    frame mono#10: 0x6604ef19 libsystem_trace.dylib`_os_log_impl_stream + 468
    frame mono#11: 0x6604e44d libsystem_trace.dylib`_os_log_impl_flatten_and_send + 6410
    frame mono#12: 0x6604cb3b libsystem_trace.dylib`_os_log + 137
    frame mono#13: 0x6604f4aa libsystem_trace.dylib`_os_log_impl + 31
    frame mono#14: 0x4b4eb4e9 WatchConnectivity`WCSerializePayloadDictionary + 393
    frame mono#15: 0x4b4d7c4d WatchConnectivity`-[WCSession onqueue_sendResponseDictionary:identifier:] + 195
    frame mono#16: 0x4b4da435 WatchConnectivity`__66-[WCSession onqueue_handleDictionaryMessageRequest:withPairingID:]_block_invoke.411 + 35
    frame mono#17: 0x453c7df7 Foundation`__NSBLOCKOPERATION_IS_CALLING_OUT_TO_A_BLOCK__ + 12
    frame mono#18: 0x453c7cf4 Foundation`-[NSBlockOperation main] + 88
    frame mono#19: 0x453cacee Foundation`__NSOPERATION_IS_INVOKING_MAIN__ + 27
    frame mono#20: 0x453c6ebd Foundation`-[NSOperation start] + 835
    frame mono#21: 0x453cb606 Foundation`__NSOPERATIONQUEUE_IS_STARTING_AN_OPERATION__ + 27
    frame mono#22: 0x453cb12e Foundation`__NSOQSchedule_f + 194
    frame mono#23: 0x453cb067 Foundation`____addOperations_block_invoke_2 + 20
    frame mono#24: 0x65dedf49 libdispatch.dylib`_dispatch_block_async_invoke2 + 77
    frame mono#25: 0x65de4461 libdispatch.dylib`_dispatch_block_async_invoke_and_release + 17
    frame mono#26: 0x65de126f libdispatch.dylib`_dispatch_client_callout + 14
    frame mono#27: 0x65de3788 libdispatch.dylib`_dispatch_continuation_pop + 421
    frame mono#28: 0x65de2ee3 libdispatch.dylib`_dispatch_async_redirect_invoke + 818
    frame mono#29: 0x65df087d libdispatch.dylib`_dispatch_root_queue_drain + 354
    frame mono#30: 0x65df0ff3 libdispatch.dylib`_dispatch_worker_thread2 + 109
    frame mono#31: 0x66024fa0 libsystem_pthread.dylib`_pthread_wqthread + 208
    frame mono#32: 0x66024e44 libsystem_pthread.dylib`start_wqthread + 36
```
which is a thread in a "light" detach state (aka. coop detach), where we only unset the domain:
https://github.com/mono/mono/blob/4cefdcb7ce2d939ee78fb45d1b4913eb3bc064fd/mono/metadata/threads.c#L6084-L6111

Fixes mono#17926
@lewurm

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@lewurm lewurm commented Dec 10, 2019

@chamons is that a candidate for 2019-08 too? (if it's not too late for it yet?)

monojenkins added a commit that referenced this issue Dec 10, 2019
[2019-12] [debugger] skip suspend for unattached threads

When the debugger tries to suspend all the VM threads, it can happen with cooperative-suspend that it tries to suspend a thread that has previously been attached, but then did a "light" detach (that only unsets the domain). With the domain set to `NULL`, looking up a JitInfo for a given `ip` will result into a crash, but it isn't even necessarily needed for the purpose of suspending a thread.

More details: Consider the following:
```
(lldb) c
error: Process is running.  Use 'process interrupt' to pause execution.
Process 12832 stopped
* thread #9, name = 'tid_a31f', queue = 'NSOperationQueue 0x8069b670 (QOS: UTILITY)', stop reason = breakpoint 1.1
    frame #0: 0x00222870 WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical(info=0x81365400, user_data=0xb04dc718) at debugger-agent.c:2716:6
   2713         MonoDomain *domain = (MonoDomain *) mono_thread_info_get_suspend_state (info)->unwind_data [MONO_UNWIND_DATA_DOMAIN];
   2714         if (!domain) {
   2715                 /* not attached */
-> 2716                 ji = NULL;
   2717         } else {
   2718                 ji = mono_jit_info_table_find_internal ( domain, MONO_CONTEXT_GET_IP (&mono_thread_info_get_suspend_state (info)->ctx), TRUE, TRUE);
   2719         }
Target 0: (WatchWCSessionAppWatchOSExtension) stopped.
(lldb) bt
* thread #9, name = 'tid_a31f', queue = 'NSOperationQueue 0x8069b670 (QOS: UTILITY)', stop reason = breakpoint 1.1
  * frame #0: 0x00222870 WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical(info=0x81365400, user_data=0xb04dc718) at debugger-agent.c:2716:6
    frame #1: 0x004e177b WatchWCSessionAppWatchOSExtension`mono_thread_info_safe_suspend_and_run(id=0xb0767000, interrupt_kernel=0, callback=(WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical at debugger-agent.c:2708), user_data=0xb04dc718) at mono-threads.c:1358:19
    frame #2: 0x00222799 WatchWCSessionAppWatchOSExtension`notify_thread(key=0x03994508, value=0x81378c00, user_data=0x00000000) at debugger-agent.c:2747:2
    frame #3: 0x00355bd8 WatchWCSessionAppWatchOSExtension`mono_g_hash_table_foreach(hash=0x8007b4e0, func=(WatchWCSessionAppWatchOSExtension`notify_thread at debugger-agent.c:2733), user_data=0x00000000) at mono-hash.c:310:4
    frame #4: 0x0021e955 WatchWCSessionAppWatchOSExtension`suspend_vm at debugger-agent.c:2844:3
    frame #5: 0x00225ff0 WatchWCSessionAppWatchOSExtension`process_event(event=EVENT_KIND_THREAD_START, arg=0x039945d0, il_offset=0, ctx=0x00000000, events=0x805b28e0, suspend_policy=2) at debugger-agent.c:4012:3
    frame #6: 0x00227c7c WatchWCSessionAppWatchOSExtension`process_profiler_event(event=EVENT_KIND_THREAD_START, arg=0x039945d0) at debugger-agent.c:4072:2
    frame #7: 0x0021b174 WatchWCSessionAppWatchOSExtension`thread_startup(prof=0x00000000, tid=2957889536) at debugger-agent.c:4149:2
    frame #8: 0x0037912d WatchWCSessionAppWatchOSExtension`mono_profiler_raise_thread_started(tid=2957889536) at profiler-events.h:103:1
    frame #9: 0x003d53da WatchWCSessionAppWatchOSExtension`fire_attach_profiler_events(tid=0xb04dd000) at threads.c:1120:2
    frame #10: 0x003d4d83 WatchWCSessionAppWatchOSExtension`mono_thread_attach(domain=0x801750a0) at threads.c:1547:2
    frame #11: 0x003df1a1 WatchWCSessionAppWatchOSExtension`mono_threads_attach_coop_internal(domain=0x801750a0, cookie=0xb04dcc0c, stackdata=0xb04dcba8) at threads.c:6008:3
    frame #12: 0x003df287 WatchWCSessionAppWatchOSExtension`mono_threads_attach_coop(domain=0x00000000, dummy=0xb04dcc0c) at threads.c:6045:9
    frame #13: 0x005034b8 WatchWCSessionAppWatchOSExtension`::xamarin_switch_gchandle(self=0x80762c20, to_weak=false) at runtime.m:1805:2
    frame #14: 0x005065c1 WatchWCSessionAppWatchOSExtension`::xamarin_retain_trampoline(self=0x80762c20, sel="retain") at trampolines.m:693:2
    frame #15: 0x657ea520 libobjc.A.dylib`objc_retain + 64
    frame #16: 0x4b4d9caa WatchConnectivity`__66-[WCSession onqueue_handleDictionaryMessageRequest:withPairingID:]_block_invoke + 279
    frame #17: 0x453c7df7 Foundation`__NSBLOCKOPERATION_IS_CALLING_OUT_TO_A_BLOCK__ + 12
    frame #18: 0x453c7cf4 Foundation`-[NSBlockOperation main] + 88
    frame #19: 0x453cacee Foundation`__NSOPERATION_IS_INVOKING_MAIN__ + 27
    frame #20: 0x453c6ebd Foundation`-[NSOperation start] + 835
    frame #21: 0x453cb606 Foundation`__NSOPERATIONQUEUE_IS_STARTING_AN_OPERATION__ + 27
    frame #22: 0x453cb12e Foundation`__NSOQSchedule_f + 194
    frame #23: 0x453cb26e Foundation`____addOperations_block_invoke_4 + 20
    frame #24: 0x65de007b libdispatch.dylib`_dispatch_call_block_and_release + 15
    frame #25: 0x65de126f libdispatch.dylib`_dispatch_client_callout + 14
    frame #26: 0x65de3788 libdispatch.dylib`_dispatch_continuation_pop + 421
    frame #27: 0x65de2ee3 libdispatch.dylib`_dispatch_async_redirect_invoke + 818
    frame #28: 0x65df087d libdispatch.dylib`_dispatch_root_queue_drain + 354
    frame #29: 0x65df0ff3 libdispatch.dylib`_dispatch_worker_thread2 + 109
    frame #30: 0x66024fa0 libsystem_pthread.dylib`_pthread_wqthread + 208
    frame #31: 0x66024e44 libsystem_pthread.dylib`start_wqthread + 36
```

Going further, `info` is about this thread:
```
(lldb) p/x *(int *)(((char *) info->node.key) + 0xa0)
(int) $2 = 0x01243f93
(lldb) thread list
Process 12832 stopped
  thread #1: tid = 0x1243ee1, 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10, name = 'tid_303', queue = 'com.apple.main-thread'
  thread #2: tid = 0x1243ee6, 0x65f816e2 libsystem_kernel.dylib`__recvfrom + 10
  thread #3: tid = 0x1243ee7, 0x65f81aea libsystem_kernel.dylib`__psynch_cvwait + 10, name = 'SGen worker'
  thread #4: tid = 0x1243ee9, 0x65f7e3d2 libsystem_kernel.dylib`semaphore_wait_trap + 10, name = 'Finalizer'
  thread #5: tid = 0x1243eea, 0x65f816e2 libsystem_kernel.dylib`__recvfrom + 10, name = 'Debugger agent'
  thread #6: tid = 0x1243f1d, 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10, name = 'com.apple.uikit.eventfetch-thread'
  thread #8: tid = 0x1243f93, 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10, name = 'tid_6d0f', queue = 'NSOperationQueue 0x8069b300 (QOS: UTILITY)'
* thread #9: tid = 0x12443a9, 0x00222870 WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical(info=0x81365400, user_data=0xb04dc718) at debugger-agent.c:2716:6, name = 'tid_a31f', queue = 'NSOperationQueue 0x8069b670 (QOS: UTILITY)', stop reason = breakpoint 1.1
  thread #10: tid = 0x1244581, 0x65f7fd32 libsystem_kernel.dylib`__workq_kernreturn + 10
(lldb) thread select 8
* thread #8, name = 'tid_6d0f', queue = 'NSOperationQueue 0x8069b300 (QOS: UTILITY)'
    frame #0: 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10
libsystem_kernel.dylib`mach_msg_trap:
->  0x65f7e396 <+10>: retl
    0x65f7e397 <+11>: nop

libsystem_kernel.dylib`mach_msg_overwrite_trap:
    0x65f7e398 <+0>:  movl   $0xffffffe0, %eax         ; imm = 0xFFFFFFE0
    0x65f7e39d <+5>:  calll  0x65f85f44                ; _sysenter_trap
(lldb) bt
* thread #8, name = 'tid_6d0f', queue = 'NSOperationQueue 0x8069b300 (QOS: UTILITY)'
  * frame #0: 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10
    frame #1: 0x65f7e8ff libsystem_kernel.dylib`mach_msg + 47
    frame #2: 0x66079679 libxpc.dylib`_xpc_send_serializer + 104
    frame #3: 0x660794da libxpc.dylib`_xpc_pipe_simpleroutine + 80
    frame #4: 0x66079852 libxpc.dylib`xpc_pipe_simpleroutine + 43
    frame #5: 0x66043a8f libsystem_trace.dylib`___os_activity_stream_reflect_block_invoke_2 + 30
    frame #6: 0x65de126f libdispatch.dylib`_dispatch_client_callout + 14
    frame #7: 0x65de3d71 libdispatch.dylib`_dispatch_block_invoke_direct + 257
    frame #8: 0x65de3c62 libdispatch.dylib`dispatch_block_perform + 112
    frame #9: 0x6604349a libsystem_trace.dylib`_os_activity_stream_reflect + 725
    frame #10: 0x6604ef19 libsystem_trace.dylib`_os_log_impl_stream + 468
    frame #11: 0x6604e44d libsystem_trace.dylib`_os_log_impl_flatten_and_send + 6410
    frame #12: 0x6604cb3b libsystem_trace.dylib`_os_log + 137
    frame #13: 0x6604f4aa libsystem_trace.dylib`_os_log_impl + 31
    frame #14: 0x4b4eb4e9 WatchConnectivity`WCSerializePayloadDictionary + 393
    frame #15: 0x4b4d7c4d WatchConnectivity`-[WCSession onqueue_sendResponseDictionary:identifier:] + 195
    frame #16: 0x4b4da435 WatchConnectivity`__66-[WCSession onqueue_handleDictionaryMessageRequest:withPairingID:]_block_invoke.411 + 35
    frame #17: 0x453c7df7 Foundation`__NSBLOCKOPERATION_IS_CALLING_OUT_TO_A_BLOCK__ + 12
    frame #18: 0x453c7cf4 Foundation`-[NSBlockOperation main] + 88
    frame #19: 0x453cacee Foundation`__NSOPERATION_IS_INVOKING_MAIN__ + 27
    frame #20: 0x453c6ebd Foundation`-[NSOperation start] + 835
    frame #21: 0x453cb606 Foundation`__NSOPERATIONQUEUE_IS_STARTING_AN_OPERATION__ + 27
    frame #22: 0x453cb12e Foundation`__NSOQSchedule_f + 194
    frame #23: 0x453cb067 Foundation`____addOperations_block_invoke_2 + 20
    frame #24: 0x65dedf49 libdispatch.dylib`_dispatch_block_async_invoke2 + 77
    frame #25: 0x65de4461 libdispatch.dylib`_dispatch_block_async_invoke_and_release + 17
    frame #26: 0x65de126f libdispatch.dylib`_dispatch_client_callout + 14
    frame #27: 0x65de3788 libdispatch.dylib`_dispatch_continuation_pop + 421
    frame #28: 0x65de2ee3 libdispatch.dylib`_dispatch_async_redirect_invoke + 818
    frame #29: 0x65df087d libdispatch.dylib`_dispatch_root_queue_drain + 354
    frame #30: 0x65df0ff3 libdispatch.dylib`_dispatch_worker_thread2 + 109
    frame #31: 0x66024fa0 libsystem_pthread.dylib`_pthread_wqthread + 208
    frame #32: 0x66024e44 libsystem_pthread.dylib`start_wqthread + 36
```
which is a thread in a "light" detach state (aka. coop detach), where we only unset the domain:
https://github.com/mono/mono/blob/4cefdcb7ce2d939ee78fb45d1b4913eb3bc064fd/mono/metadata/threads.c#L6084-L6111

Fixes #17926



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Backport of #18105.

/cc @lewurm
lambdageek added a commit that referenced this issue Dec 10, 2019
When the debugger tries to suspend all the VM threads, it can happen with cooperative-suspend that it tries to suspend a thread that has previously been attached, but then did a "light" detach (that only unsets the domain). With the domain set to `NULL`, looking up a JitInfo for a given `ip` will result into a crash, but it isn't even necessarily needed for the purpose of suspending a thread.

More details: Consider the following:
```
(lldb) c
error: Process is running.  Use 'process interrupt' to pause execution.
Process 12832 stopped
* thread #9, name = 'tid_a31f', queue = 'NSOperationQueue 0x8069b670 (QOS: UTILITY)', stop reason = breakpoint 1.1
    frame #0: 0x00222870 WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical(info=0x81365400, user_data=0xb04dc718) at debugger-agent.c:2716:6
   2713         MonoDomain *domain = (MonoDomain *) mono_thread_info_get_suspend_state (info)->unwind_data [MONO_UNWIND_DATA_DOMAIN];
   2714         if (!domain) {
   2715                 /* not attached */
-> 2716                 ji = NULL;
   2717         } else {
   2718                 ji = mono_jit_info_table_find_internal ( domain, MONO_CONTEXT_GET_IP (&mono_thread_info_get_suspend_state (info)->ctx), TRUE, TRUE);
   2719         }
Target 0: (WatchWCSessionAppWatchOSExtension) stopped.
(lldb) bt
* thread #9, name = 'tid_a31f', queue = 'NSOperationQueue 0x8069b670 (QOS: UTILITY)', stop reason = breakpoint 1.1
  * frame #0: 0x00222870 WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical(info=0x81365400, user_data=0xb04dc718) at debugger-agent.c:2716:6
    frame #1: 0x004e177b WatchWCSessionAppWatchOSExtension`mono_thread_info_safe_suspend_and_run(id=0xb0767000, interrupt_kernel=0, callback=(WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical at debugger-agent.c:2708), user_data=0xb04dc718) at mono-threads.c:1358:19
    frame #2: 0x00222799 WatchWCSessionAppWatchOSExtension`notify_thread(key=0x03994508, value=0x81378c00, user_data=0x00000000) at debugger-agent.c:2747:2
    frame #3: 0x00355bd8 WatchWCSessionAppWatchOSExtension`mono_g_hash_table_foreach(hash=0x8007b4e0, func=(WatchWCSessionAppWatchOSExtension`notify_thread at debugger-agent.c:2733), user_data=0x00000000) at mono-hash.c:310:4
    frame #4: 0x0021e955 WatchWCSessionAppWatchOSExtension`suspend_vm at debugger-agent.c:2844:3
    frame #5: 0x00225ff0 WatchWCSessionAppWatchOSExtension`process_event(event=EVENT_KIND_THREAD_START, arg=0x039945d0, il_offset=0, ctx=0x00000000, events=0x805b28e0, suspend_policy=2) at debugger-agent.c:4012:3
    frame #6: 0x00227c7c WatchWCSessionAppWatchOSExtension`process_profiler_event(event=EVENT_KIND_THREAD_START, arg=0x039945d0) at debugger-agent.c:4072:2
    frame #7: 0x0021b174 WatchWCSessionAppWatchOSExtension`thread_startup(prof=0x00000000, tid=2957889536) at debugger-agent.c:4149:2
    frame #8: 0x0037912d WatchWCSessionAppWatchOSExtension`mono_profiler_raise_thread_started(tid=2957889536) at profiler-events.h:103:1
    frame #9: 0x003d53da WatchWCSessionAppWatchOSExtension`fire_attach_profiler_events(tid=0xb04dd000) at threads.c:1120:2
    frame #10: 0x003d4d83 WatchWCSessionAppWatchOSExtension`mono_thread_attach(domain=0x801750a0) at threads.c:1547:2
    frame #11: 0x003df1a1 WatchWCSessionAppWatchOSExtension`mono_threads_attach_coop_internal(domain=0x801750a0, cookie=0xb04dcc0c, stackdata=0xb04dcba8) at threads.c:6008:3
    frame #12: 0x003df287 WatchWCSessionAppWatchOSExtension`mono_threads_attach_coop(domain=0x00000000, dummy=0xb04dcc0c) at threads.c:6045:9
    frame #13: 0x005034b8 WatchWCSessionAppWatchOSExtension`::xamarin_switch_gchandle(self=0x80762c20, to_weak=false) at runtime.m:1805:2
    frame #14: 0x005065c1 WatchWCSessionAppWatchOSExtension`::xamarin_retain_trampoline(self=0x80762c20, sel="retain") at trampolines.m:693:2
    frame #15: 0x657ea520 libobjc.A.dylib`objc_retain + 64
    frame #16: 0x4b4d9caa WatchConnectivity`__66-[WCSession onqueue_handleDictionaryMessageRequest:withPairingID:]_block_invoke + 279
    frame #17: 0x453c7df7 Foundation`__NSBLOCKOPERATION_IS_CALLING_OUT_TO_A_BLOCK__ + 12
    frame #18: 0x453c7cf4 Foundation`-[NSBlockOperation main] + 88
    frame #19: 0x453cacee Foundation`__NSOPERATION_IS_INVOKING_MAIN__ + 27
    frame #20: 0x453c6ebd Foundation`-[NSOperation start] + 835
    frame #21: 0x453cb606 Foundation`__NSOPERATIONQUEUE_IS_STARTING_AN_OPERATION__ + 27
    frame #22: 0x453cb12e Foundation`__NSOQSchedule_f + 194
    frame #23: 0x453cb26e Foundation`____addOperations_block_invoke_4 + 20
    frame #24: 0x65de007b libdispatch.dylib`_dispatch_call_block_and_release + 15
    frame #25: 0x65de126f libdispatch.dylib`_dispatch_client_callout + 14
    frame #26: 0x65de3788 libdispatch.dylib`_dispatch_continuation_pop + 421
    frame #27: 0x65de2ee3 libdispatch.dylib`_dispatch_async_redirect_invoke + 818
    frame #28: 0x65df087d libdispatch.dylib`_dispatch_root_queue_drain + 354
    frame #29: 0x65df0ff3 libdispatch.dylib`_dispatch_worker_thread2 + 109
    frame #30: 0x66024fa0 libsystem_pthread.dylib`_pthread_wqthread + 208
    frame #31: 0x66024e44 libsystem_pthread.dylib`start_wqthread + 36
```

Going further, `info` is about this thread:
```
(lldb) p/x *(int *)(((char *) info->node.key) + 0xa0)
(int) $2 = 0x01243f93
(lldb) thread list
Process 12832 stopped
  thread #1: tid = 0x1243ee1, 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10, name = 'tid_303', queue = 'com.apple.main-thread'
  thread #2: tid = 0x1243ee6, 0x65f816e2 libsystem_kernel.dylib`__recvfrom + 10
  thread #3: tid = 0x1243ee7, 0x65f81aea libsystem_kernel.dylib`__psynch_cvwait + 10, name = 'SGen worker'
  thread #4: tid = 0x1243ee9, 0x65f7e3d2 libsystem_kernel.dylib`semaphore_wait_trap + 10, name = 'Finalizer'
  thread #5: tid = 0x1243eea, 0x65f816e2 libsystem_kernel.dylib`__recvfrom + 10, name = 'Debugger agent'
  thread #6: tid = 0x1243f1d, 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10, name = 'com.apple.uikit.eventfetch-thread'
  thread #8: tid = 0x1243f93, 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10, name = 'tid_6d0f', queue = 'NSOperationQueue 0x8069b300 (QOS: UTILITY)'
* thread #9: tid = 0x12443a9, 0x00222870 WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical(info=0x81365400, user_data=0xb04dc718) at debugger-agent.c:2716:6, name = 'tid_a31f', queue = 'NSOperationQueue 0x8069b670 (QOS: UTILITY)', stop reason = breakpoint 1.1
  thread #10: tid = 0x1244581, 0x65f7fd32 libsystem_kernel.dylib`__workq_kernreturn + 10
(lldb) thread select 8
* thread #8, name = 'tid_6d0f', queue = 'NSOperationQueue 0x8069b300 (QOS: UTILITY)'
    frame #0: 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10
libsystem_kernel.dylib`mach_msg_trap:
->  0x65f7e396 <+10>: retl
    0x65f7e397 <+11>: nop

libsystem_kernel.dylib`mach_msg_overwrite_trap:
    0x65f7e398 <+0>:  movl   $0xffffffe0, %eax         ; imm = 0xFFFFFFE0
    0x65f7e39d <+5>:  calll  0x65f85f44                ; _sysenter_trap
(lldb) bt
* thread #8, name = 'tid_6d0f', queue = 'NSOperationQueue 0x8069b300 (QOS: UTILITY)'
  * frame #0: 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10
    frame #1: 0x65f7e8ff libsystem_kernel.dylib`mach_msg + 47
    frame #2: 0x66079679 libxpc.dylib`_xpc_send_serializer + 104
    frame #3: 0x660794da libxpc.dylib`_xpc_pipe_simpleroutine + 80
    frame #4: 0x66079852 libxpc.dylib`xpc_pipe_simpleroutine + 43
    frame #5: 0x66043a8f libsystem_trace.dylib`___os_activity_stream_reflect_block_invoke_2 + 30
    frame #6: 0x65de126f libdispatch.dylib`_dispatch_client_callout + 14
    frame #7: 0x65de3d71 libdispatch.dylib`_dispatch_block_invoke_direct + 257
    frame #8: 0x65de3c62 libdispatch.dylib`dispatch_block_perform + 112
    frame #9: 0x6604349a libsystem_trace.dylib`_os_activity_stream_reflect + 725
    frame #10: 0x6604ef19 libsystem_trace.dylib`_os_log_impl_stream + 468
    frame #11: 0x6604e44d libsystem_trace.dylib`_os_log_impl_flatten_and_send + 6410
    frame #12: 0x6604cb3b libsystem_trace.dylib`_os_log + 137
    frame #13: 0x6604f4aa libsystem_trace.dylib`_os_log_impl + 31
    frame #14: 0x4b4eb4e9 WatchConnectivity`WCSerializePayloadDictionary + 393
    frame #15: 0x4b4d7c4d WatchConnectivity`-[WCSession onqueue_sendResponseDictionary:identifier:] + 195
    frame #16: 0x4b4da435 WatchConnectivity`__66-[WCSession onqueue_handleDictionaryMessageRequest:withPairingID:]_block_invoke.411 + 35
    frame #17: 0x453c7df7 Foundation`__NSBLOCKOPERATION_IS_CALLING_OUT_TO_A_BLOCK__ + 12
    frame #18: 0x453c7cf4 Foundation`-[NSBlockOperation main] + 88
    frame #19: 0x453cacee Foundation`__NSOPERATION_IS_INVOKING_MAIN__ + 27
    frame #20: 0x453c6ebd Foundation`-[NSOperation start] + 835
    frame #21: 0x453cb606 Foundation`__NSOPERATIONQUEUE_IS_STARTING_AN_OPERATION__ + 27
    frame #22: 0x453cb12e Foundation`__NSOQSchedule_f + 194
    frame #23: 0x453cb067 Foundation`____addOperations_block_invoke_2 + 20
    frame #24: 0x65dedf49 libdispatch.dylib`_dispatch_block_async_invoke2 + 77
    frame #25: 0x65de4461 libdispatch.dylib`_dispatch_block_async_invoke_and_release + 17
    frame #26: 0x65de126f libdispatch.dylib`_dispatch_client_callout + 14
    frame #27: 0x65de3788 libdispatch.dylib`_dispatch_continuation_pop + 421
    frame #28: 0x65de2ee3 libdispatch.dylib`_dispatch_async_redirect_invoke + 818
    frame #29: 0x65df087d libdispatch.dylib`_dispatch_root_queue_drain + 354
    frame #30: 0x65df0ff3 libdispatch.dylib`_dispatch_worker_thread2 + 109
    frame #31: 0x66024fa0 libsystem_pthread.dylib`_pthread_wqthread + 208
    frame #32: 0x66024e44 libsystem_pthread.dylib`start_wqthread + 36
```
which is a thread in a "light" detach state (aka. coop detach), where we only unset the domain:
https://github.com/mono/mono/blob/4cefdcb7ce2d939ee78fb45d1b4913eb3bc064fd/mono/metadata/threads.c#L6084-L6111

Fixes #17926
monojenkins added a commit to monojenkins/mono that referenced this issue Dec 11, 2019
When the debugger tries to suspend all the VM threads, it can happen with cooperative-suspend that it tries to suspend a thread that has previously been attached, but then did a "light" detach (that only unsets the domain). With the domain set to `NULL`, looking up a JitInfo for a given `ip` will result into a crash, but it isn't even necessarily needed for the purpose of suspending a thread.

More details: Consider the following:
```
(lldb) c
error: Process is running.  Use 'process interrupt' to pause execution.
Process 12832 stopped
* thread mono#9, name = 'tid_a31f', queue = 'NSOperationQueue 0x8069b670 (QOS: UTILITY)', stop reason = breakpoint 1.1
    frame #0: 0x00222870 WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical(info=0x81365400, user_data=0xb04dc718) at debugger-agent.c:2716:6
   2713         MonoDomain *domain = (MonoDomain *) mono_thread_info_get_suspend_state (info)->unwind_data [MONO_UNWIND_DATA_DOMAIN];
   2714         if (!domain) {
   2715                 /* not attached */
-> 2716                 ji = NULL;
   2717         } else {
   2718                 ji = mono_jit_info_table_find_internal ( domain, MONO_CONTEXT_GET_IP (&mono_thread_info_get_suspend_state (info)->ctx), TRUE, TRUE);
   2719         }
Target 0: (WatchWCSessionAppWatchOSExtension) stopped.
(lldb) bt
* thread mono#9, name = 'tid_a31f', queue = 'NSOperationQueue 0x8069b670 (QOS: UTILITY)', stop reason = breakpoint 1.1
  * frame #0: 0x00222870 WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical(info=0x81365400, user_data=0xb04dc718) at debugger-agent.c:2716:6
    frame mono#1: 0x004e177b WatchWCSessionAppWatchOSExtension`mono_thread_info_safe_suspend_and_run(id=0xb0767000, interrupt_kernel=0, callback=(WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical at debugger-agent.c:2708), user_data=0xb04dc718) at mono-threads.c:1358:19
    frame mono#2: 0x00222799 WatchWCSessionAppWatchOSExtension`notify_thread(key=0x03994508, value=0x81378c00, user_data=0x00000000) at debugger-agent.c:2747:2
    frame mono#3: 0x00355bd8 WatchWCSessionAppWatchOSExtension`mono_g_hash_table_foreach(hash=0x8007b4e0, func=(WatchWCSessionAppWatchOSExtension`notify_thread at debugger-agent.c:2733), user_data=0x00000000) at mono-hash.c:310:4
    frame mono#4: 0x0021e955 WatchWCSessionAppWatchOSExtension`suspend_vm at debugger-agent.c:2844:3
    frame mono#5: 0x00225ff0 WatchWCSessionAppWatchOSExtension`process_event(event=EVENT_KIND_THREAD_START, arg=0x039945d0, il_offset=0, ctx=0x00000000, events=0x805b28e0, suspend_policy=2) at debugger-agent.c:4012:3
    frame mono#6: 0x00227c7c WatchWCSessionAppWatchOSExtension`process_profiler_event(event=EVENT_KIND_THREAD_START, arg=0x039945d0) at debugger-agent.c:4072:2
    frame mono#7: 0x0021b174 WatchWCSessionAppWatchOSExtension`thread_startup(prof=0x00000000, tid=2957889536) at debugger-agent.c:4149:2
    frame mono#8: 0x0037912d WatchWCSessionAppWatchOSExtension`mono_profiler_raise_thread_started(tid=2957889536) at profiler-events.h:103:1
    frame mono#9: 0x003d53da WatchWCSessionAppWatchOSExtension`fire_attach_profiler_events(tid=0xb04dd000) at threads.c:1120:2
    frame mono#10: 0x003d4d83 WatchWCSessionAppWatchOSExtension`mono_thread_attach(domain=0x801750a0) at threads.c:1547:2
    frame mono#11: 0x003df1a1 WatchWCSessionAppWatchOSExtension`mono_threads_attach_coop_internal(domain=0x801750a0, cookie=0xb04dcc0c, stackdata=0xb04dcba8) at threads.c:6008:3
    frame mono#12: 0x003df287 WatchWCSessionAppWatchOSExtension`mono_threads_attach_coop(domain=0x00000000, dummy=0xb04dcc0c) at threads.c:6045:9
    frame mono#13: 0x005034b8 WatchWCSessionAppWatchOSExtension`::xamarin_switch_gchandle(self=0x80762c20, to_weak=false) at runtime.m:1805:2
    frame mono#14: 0x005065c1 WatchWCSessionAppWatchOSExtension`::xamarin_retain_trampoline(self=0x80762c20, sel="retain") at trampolines.m:693:2
    frame mono#15: 0x657ea520 libobjc.A.dylib`objc_retain + 64
    frame mono#16: 0x4b4d9caa WatchConnectivity`__66-[WCSession onqueue_handleDictionaryMessageRequest:withPairingID:]_block_invoke + 279
    frame mono#17: 0x453c7df7 Foundation`__NSBLOCKOPERATION_IS_CALLING_OUT_TO_A_BLOCK__ + 12
    frame mono#18: 0x453c7cf4 Foundation`-[NSBlockOperation main] + 88
    frame mono#19: 0x453cacee Foundation`__NSOPERATION_IS_INVOKING_MAIN__ + 27
    frame mono#20: 0x453c6ebd Foundation`-[NSOperation start] + 835
    frame mono#21: 0x453cb606 Foundation`__NSOPERATIONQUEUE_IS_STARTING_AN_OPERATION__ + 27
    frame mono#22: 0x453cb12e Foundation`__NSOQSchedule_f + 194
    frame mono#23: 0x453cb26e Foundation`____addOperations_block_invoke_4 + 20
    frame mono#24: 0x65de007b libdispatch.dylib`_dispatch_call_block_and_release + 15
    frame mono#25: 0x65de126f libdispatch.dylib`_dispatch_client_callout + 14
    frame mono#26: 0x65de3788 libdispatch.dylib`_dispatch_continuation_pop + 421
    frame mono#27: 0x65de2ee3 libdispatch.dylib`_dispatch_async_redirect_invoke + 818
    frame mono#28: 0x65df087d libdispatch.dylib`_dispatch_root_queue_drain + 354
    frame mono#29: 0x65df0ff3 libdispatch.dylib`_dispatch_worker_thread2 + 109
    frame mono#30: 0x66024fa0 libsystem_pthread.dylib`_pthread_wqthread + 208
    frame mono#31: 0x66024e44 libsystem_pthread.dylib`start_wqthread + 36
```

Going further, `info` is about this thread:
```
(lldb) p/x *(int *)(((char *) info->node.key) + 0xa0)
(int) $2 = 0x01243f93
(lldb) thread list
Process 12832 stopped
  thread mono#1: tid = 0x1243ee1, 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10, name = 'tid_303', queue = 'com.apple.main-thread'
  thread mono#2: tid = 0x1243ee6, 0x65f816e2 libsystem_kernel.dylib`__recvfrom + 10
  thread mono#3: tid = 0x1243ee7, 0x65f81aea libsystem_kernel.dylib`__psynch_cvwait + 10, name = 'SGen worker'
  thread mono#4: tid = 0x1243ee9, 0x65f7e3d2 libsystem_kernel.dylib`semaphore_wait_trap + 10, name = 'Finalizer'
  thread mono#5: tid = 0x1243eea, 0x65f816e2 libsystem_kernel.dylib`__recvfrom + 10, name = 'Debugger agent'
  thread mono#6: tid = 0x1243f1d, 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10, name = 'com.apple.uikit.eventfetch-thread'
  thread mono#8: tid = 0x1243f93, 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10, name = 'tid_6d0f', queue = 'NSOperationQueue 0x8069b300 (QOS: UTILITY)'
* thread mono#9: tid = 0x12443a9, 0x00222870 WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical(info=0x81365400, user_data=0xb04dc718) at debugger-agent.c:2716:6, name = 'tid_a31f', queue = 'NSOperationQueue 0x8069b670 (QOS: UTILITY)', stop reason = breakpoint 1.1
  thread mono#10: tid = 0x1244581, 0x65f7fd32 libsystem_kernel.dylib`__workq_kernreturn + 10
(lldb) thread select 8
* thread mono#8, name = 'tid_6d0f', queue = 'NSOperationQueue 0x8069b300 (QOS: UTILITY)'
    frame #0: 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10
libsystem_kernel.dylib`mach_msg_trap:
->  0x65f7e396 <+10>: retl
    0x65f7e397 <+11>: nop

libsystem_kernel.dylib`mach_msg_overwrite_trap:
    0x65f7e398 <+0>:  movl   $0xffffffe0, %eax         ; imm = 0xFFFFFFE0
    0x65f7e39d <+5>:  calll  0x65f85f44                ; _sysenter_trap
(lldb) bt
* thread mono#8, name = 'tid_6d0f', queue = 'NSOperationQueue 0x8069b300 (QOS: UTILITY)'
  * frame #0: 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10
    frame mono#1: 0x65f7e8ff libsystem_kernel.dylib`mach_msg + 47
    frame mono#2: 0x66079679 libxpc.dylib`_xpc_send_serializer + 104
    frame mono#3: 0x660794da libxpc.dylib`_xpc_pipe_simpleroutine + 80
    frame mono#4: 0x66079852 libxpc.dylib`xpc_pipe_simpleroutine + 43
    frame mono#5: 0x66043a8f libsystem_trace.dylib`___os_activity_stream_reflect_block_invoke_2 + 30
    frame mono#6: 0x65de126f libdispatch.dylib`_dispatch_client_callout + 14
    frame mono#7: 0x65de3d71 libdispatch.dylib`_dispatch_block_invoke_direct + 257
    frame mono#8: 0x65de3c62 libdispatch.dylib`dispatch_block_perform + 112
    frame mono#9: 0x6604349a libsystem_trace.dylib`_os_activity_stream_reflect + 725
    frame mono#10: 0x6604ef19 libsystem_trace.dylib`_os_log_impl_stream + 468
    frame mono#11: 0x6604e44d libsystem_trace.dylib`_os_log_impl_flatten_and_send + 6410
    frame mono#12: 0x6604cb3b libsystem_trace.dylib`_os_log + 137
    frame mono#13: 0x6604f4aa libsystem_trace.dylib`_os_log_impl + 31
    frame mono#14: 0x4b4eb4e9 WatchConnectivity`WCSerializePayloadDictionary + 393
    frame mono#15: 0x4b4d7c4d WatchConnectivity`-[WCSession onqueue_sendResponseDictionary:identifier:] + 195
    frame mono#16: 0x4b4da435 WatchConnectivity`__66-[WCSession onqueue_handleDictionaryMessageRequest:withPairingID:]_block_invoke.411 + 35
    frame mono#17: 0x453c7df7 Foundation`__NSBLOCKOPERATION_IS_CALLING_OUT_TO_A_BLOCK__ + 12
    frame mono#18: 0x453c7cf4 Foundation`-[NSBlockOperation main] + 88
    frame mono#19: 0x453cacee Foundation`__NSOPERATION_IS_INVOKING_MAIN__ + 27
    frame mono#20: 0x453c6ebd Foundation`-[NSOperation start] + 835
    frame mono#21: 0x453cb606 Foundation`__NSOPERATIONQUEUE_IS_STARTING_AN_OPERATION__ + 27
    frame mono#22: 0x453cb12e Foundation`__NSOQSchedule_f + 194
    frame mono#23: 0x453cb067 Foundation`____addOperations_block_invoke_2 + 20
    frame mono#24: 0x65dedf49 libdispatch.dylib`_dispatch_block_async_invoke2 + 77
    frame mono#25: 0x65de4461 libdispatch.dylib`_dispatch_block_async_invoke_and_release + 17
    frame mono#26: 0x65de126f libdispatch.dylib`_dispatch_client_callout + 14
    frame mono#27: 0x65de3788 libdispatch.dylib`_dispatch_continuation_pop + 421
    frame mono#28: 0x65de2ee3 libdispatch.dylib`_dispatch_async_redirect_invoke + 818
    frame mono#29: 0x65df087d libdispatch.dylib`_dispatch_root_queue_drain + 354
    frame mono#30: 0x65df0ff3 libdispatch.dylib`_dispatch_worker_thread2 + 109
    frame mono#31: 0x66024fa0 libsystem_pthread.dylib`_pthread_wqthread + 208
    frame mono#32: 0x66024e44 libsystem_pthread.dylib`start_wqthread + 36
```
which is a thread in a "light" detach state (aka. coop detach), where we only unset the domain:
https://github.com/mono/mono/blob/4cefdcb7ce2d939ee78fb45d1b4913eb3bc064fd/mono/metadata/threads.c#L6084-L6111

Fixes mono#17926
@lewurm

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@lewurm lewurm commented Dec 11, 2019

@chamons is that a candidate for 2019-08 too? (if it's not too late for it yet?)

got the ACK on Slack: https://xamarinhq.slack.com/archives/C03CFKFGL/p1576091318010200

monojenkins added a commit that referenced this issue Dec 12, 2019
[2019-08] [debugger] skip suspend for unattached threads

When the debugger tries to suspend all the VM threads, it can happen with cooperative-suspend that it tries to suspend a thread that has previously been attached, but then did a "light" detach (that only unsets the domain). With the domain set to `NULL`, looking up a JitInfo for a given `ip` will result into a crash, but it isn't even necessarily needed for the purpose of suspending a thread.

More details: Consider the following:
```
(lldb) c
error: Process is running.  Use 'process interrupt' to pause execution.
Process 12832 stopped
* thread #9, name = 'tid_a31f', queue = 'NSOperationQueue 0x8069b670 (QOS: UTILITY)', stop reason = breakpoint 1.1
    frame #0: 0x00222870 WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical(info=0x81365400, user_data=0xb04dc718) at debugger-agent.c:2716:6
   2713         MonoDomain *domain = (MonoDomain *) mono_thread_info_get_suspend_state (info)->unwind_data [MONO_UNWIND_DATA_DOMAIN];
   2714         if (!domain) {
   2715                 /* not attached */
-> 2716                 ji = NULL;
   2717         } else {
   2718                 ji = mono_jit_info_table_find_internal ( domain, MONO_CONTEXT_GET_IP (&mono_thread_info_get_suspend_state (info)->ctx), TRUE, TRUE);
   2719         }
Target 0: (WatchWCSessionAppWatchOSExtension) stopped.
(lldb) bt
* thread #9, name = 'tid_a31f', queue = 'NSOperationQueue 0x8069b670 (QOS: UTILITY)', stop reason = breakpoint 1.1
  * frame #0: 0x00222870 WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical(info=0x81365400, user_data=0xb04dc718) at debugger-agent.c:2716:6
    frame #1: 0x004e177b WatchWCSessionAppWatchOSExtension`mono_thread_info_safe_suspend_and_run(id=0xb0767000, interrupt_kernel=0, callback=(WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical at debugger-agent.c:2708), user_data=0xb04dc718) at mono-threads.c:1358:19
    frame #2: 0x00222799 WatchWCSessionAppWatchOSExtension`notify_thread(key=0x03994508, value=0x81378c00, user_data=0x00000000) at debugger-agent.c:2747:2
    frame #3: 0x00355bd8 WatchWCSessionAppWatchOSExtension`mono_g_hash_table_foreach(hash=0x8007b4e0, func=(WatchWCSessionAppWatchOSExtension`notify_thread at debugger-agent.c:2733), user_data=0x00000000) at mono-hash.c:310:4
    frame #4: 0x0021e955 WatchWCSessionAppWatchOSExtension`suspend_vm at debugger-agent.c:2844:3
    frame #5: 0x00225ff0 WatchWCSessionAppWatchOSExtension`process_event(event=EVENT_KIND_THREAD_START, arg=0x039945d0, il_offset=0, ctx=0x00000000, events=0x805b28e0, suspend_policy=2) at debugger-agent.c:4012:3
    frame #6: 0x00227c7c WatchWCSessionAppWatchOSExtension`process_profiler_event(event=EVENT_KIND_THREAD_START, arg=0x039945d0) at debugger-agent.c:4072:2
    frame #7: 0x0021b174 WatchWCSessionAppWatchOSExtension`thread_startup(prof=0x00000000, tid=2957889536) at debugger-agent.c:4149:2
    frame #8: 0x0037912d WatchWCSessionAppWatchOSExtension`mono_profiler_raise_thread_started(tid=2957889536) at profiler-events.h:103:1
    frame #9: 0x003d53da WatchWCSessionAppWatchOSExtension`fire_attach_profiler_events(tid=0xb04dd000) at threads.c:1120:2
    frame #10: 0x003d4d83 WatchWCSessionAppWatchOSExtension`mono_thread_attach(domain=0x801750a0) at threads.c:1547:2
    frame #11: 0x003df1a1 WatchWCSessionAppWatchOSExtension`mono_threads_attach_coop_internal(domain=0x801750a0, cookie=0xb04dcc0c, stackdata=0xb04dcba8) at threads.c:6008:3
    frame #12: 0x003df287 WatchWCSessionAppWatchOSExtension`mono_threads_attach_coop(domain=0x00000000, dummy=0xb04dcc0c) at threads.c:6045:9
    frame #13: 0x005034b8 WatchWCSessionAppWatchOSExtension`::xamarin_switch_gchandle(self=0x80762c20, to_weak=false) at runtime.m:1805:2
    frame #14: 0x005065c1 WatchWCSessionAppWatchOSExtension`::xamarin_retain_trampoline(self=0x80762c20, sel="retain") at trampolines.m:693:2
    frame #15: 0x657ea520 libobjc.A.dylib`objc_retain + 64
    frame #16: 0x4b4d9caa WatchConnectivity`__66-[WCSession onqueue_handleDictionaryMessageRequest:withPairingID:]_block_invoke + 279
    frame #17: 0x453c7df7 Foundation`__NSBLOCKOPERATION_IS_CALLING_OUT_TO_A_BLOCK__ + 12
    frame #18: 0x453c7cf4 Foundation`-[NSBlockOperation main] + 88
    frame #19: 0x453cacee Foundation`__NSOPERATION_IS_INVOKING_MAIN__ + 27
    frame #20: 0x453c6ebd Foundation`-[NSOperation start] + 835
    frame #21: 0x453cb606 Foundation`__NSOPERATIONQUEUE_IS_STARTING_AN_OPERATION__ + 27
    frame #22: 0x453cb12e Foundation`__NSOQSchedule_f + 194
    frame #23: 0x453cb26e Foundation`____addOperations_block_invoke_4 + 20
    frame #24: 0x65de007b libdispatch.dylib`_dispatch_call_block_and_release + 15
    frame #25: 0x65de126f libdispatch.dylib`_dispatch_client_callout + 14
    frame #26: 0x65de3788 libdispatch.dylib`_dispatch_continuation_pop + 421
    frame #27: 0x65de2ee3 libdispatch.dylib`_dispatch_async_redirect_invoke + 818
    frame #28: 0x65df087d libdispatch.dylib`_dispatch_root_queue_drain + 354
    frame #29: 0x65df0ff3 libdispatch.dylib`_dispatch_worker_thread2 + 109
    frame #30: 0x66024fa0 libsystem_pthread.dylib`_pthread_wqthread + 208
    frame #31: 0x66024e44 libsystem_pthread.dylib`start_wqthread + 36
```

Going further, `info` is about this thread:
```
(lldb) p/x *(int *)(((char *) info->node.key) + 0xa0)
(int) $2 = 0x01243f93
(lldb) thread list
Process 12832 stopped
  thread #1: tid = 0x1243ee1, 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10, name = 'tid_303', queue = 'com.apple.main-thread'
  thread #2: tid = 0x1243ee6, 0x65f816e2 libsystem_kernel.dylib`__recvfrom + 10
  thread #3: tid = 0x1243ee7, 0x65f81aea libsystem_kernel.dylib`__psynch_cvwait + 10, name = 'SGen worker'
  thread #4: tid = 0x1243ee9, 0x65f7e3d2 libsystem_kernel.dylib`semaphore_wait_trap + 10, name = 'Finalizer'
  thread #5: tid = 0x1243eea, 0x65f816e2 libsystem_kernel.dylib`__recvfrom + 10, name = 'Debugger agent'
  thread #6: tid = 0x1243f1d, 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10, name = 'com.apple.uikit.eventfetch-thread'
  thread #8: tid = 0x1243f93, 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10, name = 'tid_6d0f', queue = 'NSOperationQueue 0x8069b300 (QOS: UTILITY)'
* thread #9: tid = 0x12443a9, 0x00222870 WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical(info=0x81365400, user_data=0xb04dc718) at debugger-agent.c:2716:6, name = 'tid_a31f', queue = 'NSOperationQueue 0x8069b670 (QOS: UTILITY)', stop reason = breakpoint 1.1
  thread #10: tid = 0x1244581, 0x65f7fd32 libsystem_kernel.dylib`__workq_kernreturn + 10
(lldb) thread select 8
* thread #8, name = 'tid_6d0f', queue = 'NSOperationQueue 0x8069b300 (QOS: UTILITY)'
    frame #0: 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10
libsystem_kernel.dylib`mach_msg_trap:
->  0x65f7e396 <+10>: retl
    0x65f7e397 <+11>: nop

libsystem_kernel.dylib`mach_msg_overwrite_trap:
    0x65f7e398 <+0>:  movl   $0xffffffe0, %eax         ; imm = 0xFFFFFFE0
    0x65f7e39d <+5>:  calll  0x65f85f44                ; _sysenter_trap
(lldb) bt
* thread #8, name = 'tid_6d0f', queue = 'NSOperationQueue 0x8069b300 (QOS: UTILITY)'
  * frame #0: 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10
    frame #1: 0x65f7e8ff libsystem_kernel.dylib`mach_msg + 47
    frame #2: 0x66079679 libxpc.dylib`_xpc_send_serializer + 104
    frame #3: 0x660794da libxpc.dylib`_xpc_pipe_simpleroutine + 80
    frame #4: 0x66079852 libxpc.dylib`xpc_pipe_simpleroutine + 43
    frame #5: 0x66043a8f libsystem_trace.dylib`___os_activity_stream_reflect_block_invoke_2 + 30
    frame #6: 0x65de126f libdispatch.dylib`_dispatch_client_callout + 14
    frame #7: 0x65de3d71 libdispatch.dylib`_dispatch_block_invoke_direct + 257
    frame #8: 0x65de3c62 libdispatch.dylib`dispatch_block_perform + 112
    frame #9: 0x6604349a libsystem_trace.dylib`_os_activity_stream_reflect + 725
    frame #10: 0x6604ef19 libsystem_trace.dylib`_os_log_impl_stream + 468
    frame #11: 0x6604e44d libsystem_trace.dylib`_os_log_impl_flatten_and_send + 6410
    frame #12: 0x6604cb3b libsystem_trace.dylib`_os_log + 137
    frame #13: 0x6604f4aa libsystem_trace.dylib`_os_log_impl + 31
    frame #14: 0x4b4eb4e9 WatchConnectivity`WCSerializePayloadDictionary + 393
    frame #15: 0x4b4d7c4d WatchConnectivity`-[WCSession onqueue_sendResponseDictionary:identifier:] + 195
    frame #16: 0x4b4da435 WatchConnectivity`__66-[WCSession onqueue_handleDictionaryMessageRequest:withPairingID:]_block_invoke.411 + 35
    frame #17: 0x453c7df7 Foundation`__NSBLOCKOPERATION_IS_CALLING_OUT_TO_A_BLOCK__ + 12
    frame #18: 0x453c7cf4 Foundation`-[NSBlockOperation main] + 88
    frame #19: 0x453cacee Foundation`__NSOPERATION_IS_INVOKING_MAIN__ + 27
    frame #20: 0x453c6ebd Foundation`-[NSOperation start] + 835
    frame #21: 0x453cb606 Foundation`__NSOPERATIONQUEUE_IS_STARTING_AN_OPERATION__ + 27
    frame #22: 0x453cb12e Foundation`__NSOQSchedule_f + 194
    frame #23: 0x453cb067 Foundation`____addOperations_block_invoke_2 + 20
    frame #24: 0x65dedf49 libdispatch.dylib`_dispatch_block_async_invoke2 + 77
    frame #25: 0x65de4461 libdispatch.dylib`_dispatch_block_async_invoke_and_release + 17
    frame #26: 0x65de126f libdispatch.dylib`_dispatch_client_callout + 14
    frame #27: 0x65de3788 libdispatch.dylib`_dispatch_continuation_pop + 421
    frame #28: 0x65de2ee3 libdispatch.dylib`_dispatch_async_redirect_invoke + 818
    frame #29: 0x65df087d libdispatch.dylib`_dispatch_root_queue_drain + 354
    frame #30: 0x65df0ff3 libdispatch.dylib`_dispatch_worker_thread2 + 109
    frame #31: 0x66024fa0 libsystem_pthread.dylib`_pthread_wqthread + 208
    frame #32: 0x66024e44 libsystem_pthread.dylib`start_wqthread + 36
```
which is a thread in a "light" detach state (aka. coop detach), where we only unset the domain:
https://github.com/mono/mono/blob/4cefdcb7ce2d939ee78fb45d1b4913eb3bc064fd/mono/metadata/threads.c#L6084-L6111

Fixes #17926



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Backport of #18105.

/cc @lewurm
jonpryor added a commit to jonpryor/xamarin-android that referenced this issue Dec 12, 2019
Changes: mono/mono@18920a8...9bcfa60

Context: mono/mono#17869
Context: mono/mono#17878
Context: mono/mono#17926

  * mono/mono@9bcfa60: [2019-10] Bump msbuild to track mono-2019-10 (#18132)
  * mono/mono@e6cd9a1: [2019-10] [mini] Fix delegate trampoline virtual call via delgate Invoke (#18115)
  * mono/mono@7c95e4b: [debugger] skip suspend for unattached threads (#18111)
  * mono/mono@77147e7: [2019-10] Bump msbuild+roslyn to track mono-2019-10 (#18093)
  * mono/mono@d7337e5: Bump to mono/corefx@f3815a9 (#18104)
  * mono/mono@6ac1197: [2019-10] [threads] Add back mono_threads_attach_tools_thread as a public API (#18075)
  * mono/mono@b687667: Bump corefx to pick up mono/corefx#370 (#18068)
  * mono/mono@da605d8: Remove DNS lookups of the local hostname in tests (#18059)
  * mono/mono@a6e867f: Bump msbuild to track mono-2019-10 (#18045)
  * mono/mono@3387bd4: Bump bockbuild to get mono/bockbuild#136
  * mono/mono@5dd74a6: Fix side effect.
  * mono/mono@a36f07c: Fix DIM in a generic interface. Fixes #17869
  * mono/mono@08e5a97: Bump msbuild to track mono-2019-10 (#18014)
  * mono/mono@56a583e: [2019-10] [profiler] disable inlining when coverage profiler is used and fix allocation sizes (#17991)
  * mono/mono@914ffea: [sgen] Don't trigger collections during allocation of thread objects (#17997)
jonpryor added a commit to jonpryor/xamarin-android that referenced this issue Dec 13, 2019
Changes: mono/mono@bef1e63...e3084ec

Context: mono/mono#17926

  * mono/mono@e3084ec: [debugger] skip suspend for unattached threads (#18138)
  * mono/mono@da5b1c6: [2019-08] Enable GSS on android (#18116)
jonpryor added a commit to xamarin/xamarin-android that referenced this issue Dec 16, 2019
Changes: mono/mono@18920a8...9bcfa60

Context: mono/mono#17869
Context: mono/mono#17878
Context: mono/mono#17926

  * mono/mono@9bcfa60: [2019-10] Bump msbuild to track mono-2019-10 (#18132)
  * mono/mono@e6cd9a1: [2019-10] [mini] Fix delegate trampoline virtual call via delgate Invoke (#18115)
  * mono/mono@7c95e4b: [debugger] skip suspend for unattached threads (#18111)
  * mono/mono@77147e7: [2019-10] Bump msbuild+roslyn to track mono-2019-10 (#18093)
  * mono/mono@d7337e5: Bump to mono/corefx@f3815a9 (#18104)
  * mono/mono@6ac1197: [2019-10] [threads] Add back mono_threads_attach_tools_thread as a public API (#18075)
  * mono/mono@b687667: Bump corefx to pick up mono/corefx#370 (#18068)
  * mono/mono@da605d8: Remove DNS lookups of the local hostname in tests (#18059)
  * mono/mono@a6e867f: Bump msbuild to track mono-2019-10 (#18045)
  * mono/mono@3387bd4: Bump bockbuild to get mono/bockbuild#136
  * mono/mono@5dd74a6: Fix side effect.
  * mono/mono@a36f07c: Fix DIM in a generic interface. Fixes #17869
  * mono/mono@08e5a97: Bump msbuild to track mono-2019-10 (#18014)
  * mono/mono@56a583e: [2019-10] [profiler] disable inlining when coverage profiler is used and fix allocation sizes (#17991)
  * mono/mono@914ffea: [sgen] Don't trigger collections during allocation of thread objects (#17997)
jonpryor added a commit to xamarin/xamarin-android that referenced this issue Jan 8, 2020
Changes: mono/mono@bef1e63...e3084ec

Context: mono/mono#17926

  * mono/mono@e3084ec: [debugger] skip suspend for unattached threads (#18138)
  * mono/mono@da5b1c6: [2019-08] Enable GSS on android (#18116)
ManickaP pushed a commit to ManickaP/runtime that referenced this issue Jan 20, 2020
When the debugger tries to suspend all the VM threads, it can happen with cooperative-suspend that it tries to suspend a thread that has previously been attached, but then did a "light" detach (that only unsets the domain). With the domain set to `NULL`, looking up a JitInfo for a given `ip` will result into a crash, but it isn't even necessarily needed for the purpose of suspending a thread.

More details: Consider the following:
```
(lldb) c
error: Process is running.  Use 'process interrupt' to pause execution.
Process 12832 stopped
* thread mono/mono#9, name = 'tid_a31f', queue = 'NSOperationQueue 0x8069b670 (QOS: UTILITY)', stop reason = breakpoint 1.1
    frame mono/mono#0: 0x00222870 WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical(info=0x81365400, user_data=0xb04dc718) at debugger-agent.c:2716:6
   2713         MonoDomain *domain = (MonoDomain *) mono_thread_info_get_suspend_state (info)->unwind_data [MONO_UNWIND_DATA_DOMAIN];
   2714         if (!domain) {
   2715                 /* not attached */
-> 2716                 ji = NULL;
   2717         } else {
   2718                 ji = mono_jit_info_table_find_internal ( domain, MONO_CONTEXT_GET_IP (&mono_thread_info_get_suspend_state (info)->ctx), TRUE, TRUE);
   2719         }
Target 0: (WatchWCSessionAppWatchOSExtension) stopped.
(lldb) bt
* thread mono/mono#9, name = 'tid_a31f', queue = 'NSOperationQueue 0x8069b670 (QOS: UTILITY)', stop reason = breakpoint 1.1
  * frame mono/mono#0: 0x00222870 WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical(info=0x81365400, user_data=0xb04dc718) at debugger-agent.c:2716:6
    frame mono/mono#1: 0x004e177b WatchWCSessionAppWatchOSExtension`mono_thread_info_safe_suspend_and_run(id=0xb0767000, interrupt_kernel=0, callback=(WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical at debugger-agent.c:2708), user_data=0xb04dc718) at mono-threads.c:1358:19
    frame mono/mono#2: 0x00222799 WatchWCSessionAppWatchOSExtension`notify_thread(key=0x03994508, value=0x81378c00, user_data=0x00000000) at debugger-agent.c:2747:2
    frame mono/mono#3: 0x00355bd8 WatchWCSessionAppWatchOSExtension`mono_g_hash_table_foreach(hash=0x8007b4e0, func=(WatchWCSessionAppWatchOSExtension`notify_thread at debugger-agent.c:2733), user_data=0x00000000) at mono-hash.c:310:4
    frame mono/mono#4: 0x0021e955 WatchWCSessionAppWatchOSExtension`suspend_vm at debugger-agent.c:2844:3
    frame mono/mono#5: 0x00225ff0 WatchWCSessionAppWatchOSExtension`process_event(event=EVENT_KIND_THREAD_START, arg=0x039945d0, il_offset=0, ctx=0x00000000, events=0x805b28e0, suspend_policy=2) at debugger-agent.c:4012:3
    frame mono/mono#6: 0x00227c7c WatchWCSessionAppWatchOSExtension`process_profiler_event(event=EVENT_KIND_THREAD_START, arg=0x039945d0) at debugger-agent.c:4072:2
    frame mono/mono#7: 0x0021b174 WatchWCSessionAppWatchOSExtension`thread_startup(prof=0x00000000, tid=2957889536) at debugger-agent.c:4149:2
    frame mono/mono#8: 0x0037912d WatchWCSessionAppWatchOSExtension`mono_profiler_raise_thread_started(tid=2957889536) at profiler-events.h:103:1
    frame mono/mono#9: 0x003d53da WatchWCSessionAppWatchOSExtension`fire_attach_profiler_events(tid=0xb04dd000) at threads.c:1120:2
    frame mono/mono#10: 0x003d4d83 WatchWCSessionAppWatchOSExtension`mono_thread_attach(domain=0x801750a0) at threads.c:1547:2
    frame mono/mono#11: 0x003df1a1 WatchWCSessionAppWatchOSExtension`mono_threads_attach_coop_internal(domain=0x801750a0, cookie=0xb04dcc0c, stackdata=0xb04dcba8) at threads.c:6008:3
    frame mono/mono#12: 0x003df287 WatchWCSessionAppWatchOSExtension`mono_threads_attach_coop(domain=0x00000000, dummy=0xb04dcc0c) at threads.c:6045:9
    frame mono/mono#13: 0x005034b8 WatchWCSessionAppWatchOSExtension`::xamarin_switch_gchandle(self=0x80762c20, to_weak=false) at runtime.m:1805:2
    frame mono/mono#14: 0x005065c1 WatchWCSessionAppWatchOSExtension`::xamarin_retain_trampoline(self=0x80762c20, sel="retain") at trampolines.m:693:2
    frame mono/mono#15: 0x657ea520 libobjc.A.dylib`objc_retain + 64
    frame mono/mono#16: 0x4b4d9caa WatchConnectivity`__66-[WCSession onqueue_handleDictionaryMessageRequest:withPairingID:]_block_invoke + 279
    frame mono/mono#17: 0x453c7df7 Foundation`__NSBLOCKOPERATION_IS_CALLING_OUT_TO_A_BLOCK__ + 12
    frame mono/mono#18: 0x453c7cf4 Foundation`-[NSBlockOperation main] + 88
    frame mono/mono#19: 0x453cacee Foundation`__NSOPERATION_IS_INVOKING_MAIN__ + 27
    frame mono/mono#20: 0x453c6ebd Foundation`-[NSOperation start] + 835
    frame mono/mono#21: 0x453cb606 Foundation`__NSOPERATIONQUEUE_IS_STARTING_AN_OPERATION__ + 27
    frame mono/mono#22: 0x453cb12e Foundation`__NSOQSchedule_f + 194
    frame mono/mono#23: 0x453cb26e Foundation`____addOperations_block_invoke_4 + 20
    frame mono/mono#24: 0x65de007b libdispatch.dylib`_dispatch_call_block_and_release + 15
    frame mono/mono#25: 0x65de126f libdispatch.dylib`_dispatch_client_callout + 14
    frame mono/mono#26: 0x65de3788 libdispatch.dylib`_dispatch_continuation_pop + 421
    frame mono/mono#27: 0x65de2ee3 libdispatch.dylib`_dispatch_async_redirect_invoke + 818
    frame mono/mono#28: 0x65df087d libdispatch.dylib`_dispatch_root_queue_drain + 354
    frame mono/mono#29: 0x65df0ff3 libdispatch.dylib`_dispatch_worker_thread2 + 109
    frame mono/mono#30: 0x66024fa0 libsystem_pthread.dylib`_pthread_wqthread + 208
    frame mono/mono#31: 0x66024e44 libsystem_pthread.dylib`start_wqthread + 36
```

Going further, `info` is about this thread:
```
(lldb) p/x *(int *)(((char *) info->node.key) + 0xa0)
(int) $2 = 0x01243f93
(lldb) thread list
Process 12832 stopped
  thread mono/mono#1: tid = 0x1243ee1, 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10, name = 'tid_303', queue = 'com.apple.main-thread'
  thread mono/mono#2: tid = 0x1243ee6, 0x65f816e2 libsystem_kernel.dylib`__recvfrom + 10
  thread mono/mono#3: tid = 0x1243ee7, 0x65f81aea libsystem_kernel.dylib`__psynch_cvwait + 10, name = 'SGen worker'
  thread mono/mono#4: tid = 0x1243ee9, 0x65f7e3d2 libsystem_kernel.dylib`semaphore_wait_trap + 10, name = 'Finalizer'
  thread mono/mono#5: tid = 0x1243eea, 0x65f816e2 libsystem_kernel.dylib`__recvfrom + 10, name = 'Debugger agent'
  thread mono/mono#6: tid = 0x1243f1d, 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10, name = 'com.apple.uikit.eventfetch-thread'
  thread mono/mono#8: tid = 0x1243f93, 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10, name = 'tid_6d0f', queue = 'NSOperationQueue 0x8069b300 (QOS: UTILITY)'
* thread mono/mono#9: tid = 0x12443a9, 0x00222870 WatchWCSessionAppWatchOSExtension`debugger_interrupt_critical(info=0x81365400, user_data=0xb04dc718) at debugger-agent.c:2716:6, name = 'tid_a31f', queue = 'NSOperationQueue 0x8069b670 (QOS: UTILITY)', stop reason = breakpoint 1.1
  thread mono/mono#10: tid = 0x1244581, 0x65f7fd32 libsystem_kernel.dylib`__workq_kernreturn + 10
(lldb) thread select 8
* thread mono/mono#8, name = 'tid_6d0f', queue = 'NSOperationQueue 0x8069b300 (QOS: UTILITY)'
    frame mono/mono#0: 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10
libsystem_kernel.dylib`mach_msg_trap:
->  0x65f7e396 <+10>: retl
    0x65f7e397 <+11>: nop

libsystem_kernel.dylib`mach_msg_overwrite_trap:
    0x65f7e398 <+0>:  movl   $0xffffffe0, %eax         ; imm = 0xFFFFFFE0
    0x65f7e39d <+5>:  calll  0x65f85f44                ; _sysenter_trap
(lldb) bt
* thread mono/mono#8, name = 'tid_6d0f', queue = 'NSOperationQueue 0x8069b300 (QOS: UTILITY)'
  * frame mono/mono#0: 0x65f7e396 libsystem_kernel.dylib`mach_msg_trap + 10
    frame mono/mono#1: 0x65f7e8ff libsystem_kernel.dylib`mach_msg + 47
    frame mono/mono#2: 0x66079679 libxpc.dylib`_xpc_send_serializer + 104
    frame mono/mono#3: 0x660794da libxpc.dylib`_xpc_pipe_simpleroutine + 80
    frame mono/mono#4: 0x66079852 libxpc.dylib`xpc_pipe_simpleroutine + 43
    frame mono/mono#5: 0x66043a8f libsystem_trace.dylib`___os_activity_stream_reflect_block_invoke_2 + 30
    frame mono/mono#6: 0x65de126f libdispatch.dylib`_dispatch_client_callout + 14
    frame mono/mono#7: 0x65de3d71 libdispatch.dylib`_dispatch_block_invoke_direct + 257
    frame mono/mono#8: 0x65de3c62 libdispatch.dylib`dispatch_block_perform + 112
    frame mono/mono#9: 0x6604349a libsystem_trace.dylib`_os_activity_stream_reflect + 725
    frame mono/mono#10: 0x6604ef19 libsystem_trace.dylib`_os_log_impl_stream + 468
    frame mono/mono#11: 0x6604e44d libsystem_trace.dylib`_os_log_impl_flatten_and_send + 6410
    frame mono/mono#12: 0x6604cb3b libsystem_trace.dylib`_os_log + 137
    frame mono/mono#13: 0x6604f4aa libsystem_trace.dylib`_os_log_impl + 31
    frame mono/mono#14: 0x4b4eb4e9 WatchConnectivity`WCSerializePayloadDictionary + 393
    frame mono/mono#15: 0x4b4d7c4d WatchConnectivity`-[WCSession onqueue_sendResponseDictionary:identifier:] + 195
    frame mono/mono#16: 0x4b4da435 WatchConnectivity`__66-[WCSession onqueue_handleDictionaryMessageRequest:withPairingID:]_block_invoke.411 + 35
    frame mono/mono#17: 0x453c7df7 Foundation`__NSBLOCKOPERATION_IS_CALLING_OUT_TO_A_BLOCK__ + 12
    frame mono/mono#18: 0x453c7cf4 Foundation`-[NSBlockOperation main] + 88
    frame mono/mono#19: 0x453cacee Foundation`__NSOPERATION_IS_INVOKING_MAIN__ + 27
    frame mono/mono#20: 0x453c6ebd Foundation`-[NSOperation start] + 835
    frame mono/mono#21: 0x453cb606 Foundation`__NSOPERATIONQUEUE_IS_STARTING_AN_OPERATION__ + 27
    frame mono/mono#22: 0x453cb12e Foundation`__NSOQSchedule_f + 194
    frame mono/mono#23: 0x453cb067 Foundation`____addOperations_block_invoke_2 + 20
    frame mono/mono#24: 0x65dedf49 libdispatch.dylib`_dispatch_block_async_invoke2 + 77
    frame mono/mono#25: 0x65de4461 libdispatch.dylib`_dispatch_block_async_invoke_and_release + 17
    frame mono/mono#26: 0x65de126f libdispatch.dylib`_dispatch_client_callout + 14
    frame mono/mono#27: 0x65de3788 libdispatch.dylib`_dispatch_continuation_pop + 421
    frame mono/mono#28: 0x65de2ee3 libdispatch.dylib`_dispatch_async_redirect_invoke + 818
    frame mono/mono#29: 0x65df087d libdispatch.dylib`_dispatch_root_queue_drain + 354
    frame mono/mono#30: 0x65df0ff3 libdispatch.dylib`_dispatch_worker_thread2 + 109
    frame mono/mono#31: 0x66024fa0 libsystem_pthread.dylib`_pthread_wqthread + 208
    frame mono/mono#32: 0x66024e44 libsystem_pthread.dylib`start_wqthread + 36
```
which is a thread in a "light" detach state (aka. coop detach), where we only unset the domain:
https://github.com/mono/mono/blob/mono/mono@4cefdcb7ce2d939ee78fb45d1b4913eb3bc064fd/mono/metadata/threads.c#L6084-L6111

Fixes mono/mono#17926

Commit migrated from mono/mono@80b1e10
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