[Gc] Cygwin and HANDLE_FORK/GC_handle_fork

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Greetings,

I recently encountered, by way of a bug report in Sage [1], a problem
with using GC in a forked child process on Cygwin.  The symptom is an
unhandled SIGABRT issued from within GC_suspend during GC_stop_world
in a child process forked from a process using GC.

I've narrowed this down to a simple test program demonstrating the
problem, attached.  The output of a sample run (demonstrating the
issue) looks like:

parent pid: 17500
child pid: 2616
SuspendThread failed

In this case, GC only knows about one thread, the one whose structure
is in the first_thread variable.  The problem is that post-fork, the
member first_thread->handle no longer points to a valid HANDLE to any
thread.  So when GC_stop_world is hit and it tries to suspend all
threads it knows about, SuspendThread fails and this abort is raised.
This is with GC 7.2 configured with

 $ ./configure --enable-threads=posix

This is easily gotten around with the pthread_atexit handlers that are
installed if CAN_HANDLE_FORK is set and the global GC_handle_fork = 1.
This appears to clean up the no longer valid thread struct, so there
are no further errors.

The problem, then, as I see it, is that GC can't work reliably on
Cygwin (in a forking context) *without* HANDLE_FORK--that is,
specifying --enable-handle-fork at configure time.  So my proposal is
to enable this feature by default on Cygwin.  It works, and I see no
reason *not* to have it.

Thanks,
Erik

[1] https://trac.sagemath.org/ticket/22694

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