Need to block SIGCHLD in libgc and guile internal threads

[Doug Evans <xdje42@gmail.com>]

Hi.

I think(!) I understand why gdb is hanging when used with libgc 7.4.x.
This is gdb bug 17247.
https://sourceware.org/bugzilla/show_bug.cgi?id=17247#c30

First, libgc 7.4.x was the first release to default PARALLEL_MARK to
on, so I'm guessing the same "bug" exists in 7.2, it's just not
visible unless one builds libgc with --enable-parallel-mark.

gdb/linux-nat.c calls sigsuspend when the inferior is running and gdb
needs to wait for it to stop.
gdb is waiting on a SIGCHLD at this point.

However, if the SIGCHLD goes to a different thread, say the guile
finalizer thread or a libgc marker thread then the sigsuspend that gdb
calls doesn't wake up and gdb is hung.

So question: Any suggestions for how to approach this?

Here's the hack that I applied to Guile to see if this removes the gdb hang.
I'm not suggesting checking this in.  It's just data to help advance
the discussion.
I think there's a general issue here that these threads should block
every signal
they're not expecting, or at least provide a hook to let the app
specify which signals
to block.  gdb's need to use SIGCHLD is just one example of a general problem.

diff --git a/libguile/finalizers.c b/libguile/finalizers.c
index 82f292c..95a022c 100644
--- a/libguile/finalizers.c
+++ b/libguile/finalizers.c
@@ -239,6 +239,12 @@ finalization_thread_proc (void *unused)
 static void*
 run_finalization_thread (void *arg)
 {
+  {
+    sigset_t blocked_mask;
+    sigemptyset (&blocked_mask);
+    sigaddset (&blocked_mask, SIGCHLD);
+    sigprocmask (SIG_BLOCK, &blocked_mask, NULL);
+  }
   return scm_with_guile (finalization_thread_proc, arg);
 }