bug#36609: 27.0.50; Possible race-condition in threading implementation

[Pip Cet <pipcet@gmail.com>]

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On Thu, Jul 11, 2019 at 8:52 PM Andreas Politz
<politza@hochschule-trier.de> wrote:
> I think there is a race-condition in the implementation of threads.  I
> tried to find a minimal test-case, without success.  Thus, I've attached
> a lengthy source-file.  Loading that file should trigger this bug and
> may freeze your session.

It does here, so I can provide further debugging information if
needed. On first glance, it appears that xgselect returns abnormally
with g_main_context acquired in one thread, and then other threads
fail to acquire it and loop endlessly.

Can you confirm that this very dirty patch avoids (it's not a fix by
any means!) the problem?

[-- Attachment #2: Avoid-glib-issue.patch --]
[-- Type: text/x-patch, Size: 1349 bytes --]

diff --git a/src/xgselect.c b/src/xgselect.c
index 9982a1f0e9..9ffeb6c66c 100644
--- a/src/xgselect.c
+++ b/src/xgselect.c
@@ -58,8 +58,16 @@ xg_select (int fds_lim, fd_set *rfds, fd_set *wfds, fd_set *efds,
   int i, nfds, tmo_in_millisec, must_free = 0;
   bool need_to_dispatch;
 
+  ptrdiff_t count = SPECPDL_INDEX ();
   context = g_main_context_default ();
-  context_acquired = g_main_context_acquire (context);
+  context_acquired = true;
+
+  auto void release_g_main_context (void)
+  {
+  }
+
+  if (context_acquired)
+    record_unwind_protect_void (release_g_main_context);
   /* FIXME: If we couldn't acquire the context, we just silently proceed
      because this function handles more than just glib file descriptors.
      Note that, as implemented, this failure is completely silent: there is
@@ -164,9 +172,6 @@ xg_select (int fds_lim, fd_set *rfds, fd_set *wfds, fd_set *efds,
       errno = pselect_errno;
     }
 
-  if (context_acquired)
-    g_main_context_release (context);
-
   /* To not have to recalculate timeout, return like this.  */
   if ((our_fds > 0 || (nfds == 0 && tmop == &tmo)) && (retval == 0))
     {
@@ -174,6 +179,6 @@ xg_select (int fds_lim, fd_set *rfds, fd_set *wfds, fd_set *efds,
       errno = EINTR;
     }
 
-  return retval;
+  return unbind_to (count, Qnil), retval;
 }
 #endif /* HAVE_GLIB */