This addresses an extremely difficult to debug deadlock when running
under emulation on ARM64.
A relatively easy way to trigger this bug is to call `fork()`, then within the
child process immediately call another `fork()` and then `exit()` the
intermediate process.
It would seem that there is a "code emulation" lock on the wait thread at
this stage, and if the thread is terminated too early, that lock still exists
albeit without a thread, and nothing moves forward.
It seems that a `SuspendThread()` combined with a `GetThreadContext()`
(to force the thread to _actually_ be suspended, for more details see
https://devblogs.microsoft.com/oldnewthing/20150205-00/?p=44743)
makes sure the thread is "booted" from emulation before it is suspended.
Hopefully this means it won't be holding any locks or otherwise leave
emulation in a bad state when the thread is terminated.
Also, attempt to use `CancelSynchonousIo()` (as seen in `flock.cc`) to avoid
the need for `TerminateThread()` altogether. This doesn't always work,
however, so was not a complete fix for the deadlock issue.
Addresses: https://cygwin.com/pipermail/cygwin-developers/2024-May/012694.html
Signed-off-by: Jeremy Drake <cygwin@jdrake.com>
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