[PATCH] Cygwin: Remove waitloop argument from try_to_debug()

Jon Turney jon.turney@dronecode.org.uk
Sun Aug 30 15:00:20 GMT 2020

On 30/08/2020 13:47, Corinna Vinschen wrote:
> On Aug 29 15:43, Jon Turney wrote:
>> Currently, when using CYGWIN='error_start=dumper', the core dump written
>> in response to an exception is non-deterministic, as the faulting
>> process isn't stopped while the dumper is started (it even seems
>> possible in theory that the faulting process could have exited before
>> the dumper process attaches).
>> Remove the waitloop argument, only used in this case, so the faulting
>> process busy-waits until the dump starts.
>> Code archaeology to determine why the code is this way didn't really turn
>> up any answers, but this seems a low-risk change, as this only changes
>> the behaviour when:
>>   - a debugger isn't already attached
>>   - an error_start is specified in CYGWIN env var
>>   - an exception has occurred which will be translated to a signal
>> Future work: This probably can be further simplified to make it
>> completely synchronous by waiting for the dumper process to exit. This
>> would avoid the race condition of the dumper attaching and detaching
>> before we get around to checking for that (which we try to work around
>> by juggling thread priorities), and the failure state where the dumper
>> doesn't attach and we spin indefinitely.

So, on reflection, this idea is wrong, and it currently is the way it 
has to be.

If we use CYGWIN='error_start=gdb', we should be able to continue the 
thread which encountered an exception, which we can't do if it's blocked 
waiting for the error_start process to exit.

So I'll tweak the patch commentary before pushing.

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