runtime: make crashing more useful on Windows

[bhiggins]

What did you do?

Triggered an access violation, crashing the program.

What did you expect to see?

We configured user-mode dumps and were hoping to get a dump (either a full dump or a minidump). See: https://msdn.microsoft.com/en-us/library/windows/desktop/bb787181(v=vs.85).aspx

What did you see instead?

No dump.

Two suggestions:

1. In runtime/signal_windows.go, change lastcontinuehandler to return _EXCEPTION_CONTINUE_SEARCH if docrash is true (instead of calling crash and exit(2)-ing). This will allow Windows's crash handling to run instead of being suppressed.
2. In runtime/signal_windows.go, change crash() to call RaiseException instead of being a no-op.

We can submit a PR if this is agreeable.

[bradfitz]

/cc @alexbrainman

[ianlancetaylor]

Try setting the environment variable GOTRACEBACK to "crash".

[ianlancetaylor]

I should mention that GOTRACEBACK is documented at https://golang.org/pkg/runtime/ .

[bhiggins]

@ianlancetaylor, thanks, I didn't know about GOTRACEBACK, but it doesn't help here as "crashes in an operating system-specific manner" for Windows is currently a no-op with a TODO comment. See runtime/signal_windows.go (and my second suggestion above):

func crash() {
	// TODO: This routine should do whatever is needed
	// to make the Windows program abort/crash as it
	// would if Go was not intercepting signals.
	// On Unix the routine would remove the custom signal
	// handler and then raise a signal (like SIGABRT).
	// Something like that should happen here.
	// It's okay to leave this empty for now: if crash returns
	// the ordinary exit-after-panic happens.
}

[ptdiscus]

@ianlancetaylor
I believe calling a "crash" function in lastcontinuehandler would cause the exception context record in the dump to point to the crash function. Returning _EXCEPTION_CONTINUE_SEARCH should cause the exception context record to point to the original source of the exception.

[alexbrainman]

Two suggestions:

Sounds good to me.

We can submit a PR if this is agreeable.

That is agreeble from me, thank you.
We use this https://golang.org/doc/contribute.html procedure instead of Github PRs.

I didn't know about GOTRACEBACK

I did know about GOTRACEBACK, but I did not know about "crashes in an operating system-specific manner instead of exiting" if GOTRACEBACK=crash. So the "crashing" part is not implemented on Windows.

I believe calling a "crash" function in lastcontinuehandler would cause the exception context record in the dump to point to the crash function.

Correct.

Returning _EXCEPTION_CONTINUE_SEARCH should cause the exception context record to point to the original source of the exception.

Correct. And that is what @bhiggins is proposing to add.

I think we are all on the same page.

Alex

[usirsiwal]

We have a go+C program on Windows. In absence of coredump, it is hard for us to root cause errors in the C code. Will really appreciate resolution of this issue.

[alexbrainman]

Will really appreciate resolution of this issue.

@usirsiwal try changing runtime yourself and see if it helps you. And if it works for you, submit it here to help others.

If you have problems, just ask for help here.

Alex

[usirsiwal]

@alexbrainman I will try the runtime change and see if it works for us.

[usirsiwal]

Hello,
We came up with the following solution. We had to conditionally remove disableWER to get the crashdump. We are not sure if that is the right thing to do. Please let us know if there is a better way of doing it.

Thanks,
Umesh

diff --git "a/C:\\go1.10.windows-amd64\\go\\src\\runtime\\signal_windows.go.orig" "b/C:\\go1.10.windows-amd64\\go\\src\\runtime\\signal_windows.go"
index 518aac3c4..939379ce4 100644
--- "a/C:\\go1.10.windows-amd64\\go\\src\\runtime\\signal_windows.go.orig"
+++ "b/C:\\go1.10.windows-amd64\\go\\src\\runtime\\signal_windows.go"
@@ -148,7 +148,7 @@ func lastcontinuehandler(info *exceptionrecord, r *context, gp *g) int32 {
        }

        if docrash {
-               crash()
+               return _EXCEPTION_CONTINUE_SEARCH
        }

        exit(2)
@@ -228,6 +228,10 @@ func crash() {
        // Something like that should happen here.
        // It's okay to leave this empty for now: if crash returns
        // the ordinary exit-after-panic happens.
+       _, _, docrash := gotraceback()
+       if docrash {
+               raiseException(0xdead)
+       }
 }

 // gsignalStack is unused on Windows.

 
 
 diff --git "a/C:\\go1.10.windows-amd64\\go\\src\\runtime\\os_windows.go.orig" "b/C:\\go1.10.windows-amd64\\go\\src\\runtime\\os_windows.go"
index 7aeadd9ef..18acc4dcf 100644
--- "a/C:\\go1.10.windows-amd64\\go\\src\\runtime\\os_windows.go.orig"
+++ "b/C:\\go1.10.windows-amd64\\go\\src\\runtime\\os_windows.go"
@@ -80,6 +80,7 @@ var (
        _LoadLibraryA,
        _QueryPerformanceCounter,
        _QueryPerformanceFrequency,
+       _RaiseException,
        _ResumeThread,
        _SetConsoleCtrlHandler,
        _SetErrorMode,
@@ -302,7 +303,10 @@ func osinit() {

        useLoadLibraryEx = (_LoadLibraryExW != nil && _AddDllDirectory != nil)

-       disableWER()
+       _, _, docrash := gotraceback()
+       if docrash {
+               disableWER()
+       }

        externalthreadhandlerp = funcPC(externalthreadhandler)

@@ -445,6 +449,15 @@ func exit(code int32) {
        stdcall1(_ExitProcess, uintptr(code))
 }

+//go:nosplit
+func raiseException(code int32) {
+       stdcall4(_RaiseException,
+               uintptr(code),
+               0,
+               0,
+               0)
+}
+
 //go:nosplit
 func write(fd uintptr, buf unsafe.Pointer, n int32) int32 {
        const (