Crash in background during onFlagSyncComplete

[pandavid87]

Describe the bug
Seeing a crash come in via Xcode Organizer related to the FlagSynchronizer. Looks like there is some callback that is dispatched on the main queue, which triggers a lock and resulted in the app being terminated by Watchdog.

To reproduce
No clear reproductions steps.

Expected behavior
App should not crash in the backgrond

Logs

Exception Type:  EXC_CRASH (SIGKILL)
Exception Codes: 0x0000000000000000, 0x0000000000000000
Exception Note:  EXC_CORPSE_NOTIFY
Termination Reason: FRONTBOARD 2343432205 
<RBSTerminateContext| domain:10 code:0x8BADF00D explanation:[application<com.fitbod>:89995] failed to terminate gracefully after 5.0s
ProcessVisibility: Background
ProcessState: Running
WatchdogEvent: process-exit
WatchdogVisibility: Background
WatchdogCPUStatistics: (
"Elapsed total CPU time (seconds): 7.860 (user 5.450, system 2.410), 18% CPU",
"Elapsed application CPU time (seconds): 0.051, 0% CPU"
) reportType:CrashLog maxTerminationResistance:Interactive>

Triggered by Thread:  0


Kernel Triage:
VM - Compressor failed a blocking pager_get
VM - Compressor failed a blocking pager_get
VM - Compressor failed a blocking pager_get
VM - Compressor failed a blocking pager_get
VM - Compressor failed a blocking pager_get


Thread 0 name:
Thread 0 Crashed:
0   libsystem_kernel.dylib        	0x00000001cd8b5178 __ulock_wait + 8
1   libdispatch.dylib             	0x0000000195a5941c _dlock_wait + 56 (lock.c:326)
2   libdispatch.dylib             	0x0000000195a591d0 _dispatch_thread_event_wait_slow + 56 (lock.c:366)
3   libdispatch.dylib             	0x0000000195a67f70 __DISPATCH_WAIT_FOR_QUEUE__ + 356 (lock.h:330)
4   libdispatch.dylib             	0x0000000195a67b28 _dispatch_sync_f_slow + 144 (queue.c:1774)
5   libswiftDispatch.dylib        	0x00000001af8cdcac implicit closure #2 in implicit closure #1 in OS_dispatch_queue.sync<A>(execute:) + 180
6   libswiftDispatch.dylib        	0x00000001af8ccd0c partial apply for implicit closure #2 in implicit closure #1 in OS_dispatch_queue.sync<A>(execute:) + 56 (<compiler-generated>:0)
7   libswiftDispatch.dylib        	0x00000001af8cd9ec OS_dispatch_queue._syncHelper<A>(fn:execute:rescue:) + 396 (Queue.swift:317)
8   libswiftDispatch.dylib        	0x00000001af8ccdbc OS_dispatch_queue.sync<A>(execute:) + 168 (Queue.swift:369)
9   fitbod                        	0x00000001058d4c64 featureFlags.get + 88 (FlagStore.swift:28)
10  fitbod                        	0x00000001058d4c64 featureFlags.get + 96 (<compiler-generated>:28)
11  fitbod                        	0x00000001058d4c64 LDClient.onFlagSyncComplete(result:) + 740 (LDClient.swift:679)
12  fitbod                        	0x0000000105940780 partial apply for closure #1 in FlagSynchronizer.reportSyncComplete(_:) + 32 (FlagSynchronizer.swift:266)
13  fitbod                        	0x00000001059410f0 thunk for @escaping @callee_guaranteed () -> () + 20 (<compiler-generated>:0)
14  libdispatch.dylib             	0x0000000195a56e68 _dispatch_call_block_and_release + 32 (init.c:1517)
15  libdispatch.dylib             	0x0000000195a58a2c _dispatch_client_callout + 20 (object.m:560)
16  libdispatch.dylib             	0x0000000195a66f48 _dispatch_main_queue_drain + 928 (inline_internal.h:2622)
17  libdispatch.dylib             	0x0000000195a66b98 _dispatch_main_queue_callback_4CF + 44 (queue.c:7770)
18  CoreFoundation                	0x0000000195daa2f0 __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 16 (CFRunLoop.c:1795)
19  CoreFoundation                	0x0000000195d641f4 __CFRunLoopRun + 2532 (CFRunLoop.c:3144)
20  CoreFoundation                	0x0000000195d776b8 CFRunLoopRunSpecific + 600 (CFRunLoop.c:3268)
21  GraphicsServices              	0x00000001b1e11374 GSEventRunModal + 164 (GSEvent.c:2200)
22  UIKitCore                     	0x00000001986dce88 -[UIApplication _run] + 1100 (UIApplication.m:3511)
23  UIKitCore                     	0x000000019845e5ec UIApplicationMain + 364 (UIApplication.m:5064)
24  fitbod                        	0x0000000104de969c main + 68 (FBALargeCardOnboardTableViewCell.swift:32)
25  dyld                          	0x0000000106281ce4 start + 520 (dyldMain.cpp:879)

SDK version
5.4.5

OS/platform
iOS 14+

Additional context
Based on the crash logs I'm seeing, we only started seeing this crash after we updated from LaunchDarkly SDK version 5.4.4 -> 5.4.5

[louis-launchdarkly]

Hello @pandavid87, sorry for the delay. We are looking into this and I have a couple of questions.

Does this happen frequently from your record? And does the device(s) that experienced this error have a good internet connection? Given the 5.4.5 change only updated the swift-eventsource library, we are suspecting there is an edge case for the fix we did that happens when the connection is not good.

[github-actions]

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