Re: Need to block SIGCHLD in libgc and guile internal threads

[Doug Evans]

On Tue, Aug 26, 2014 at 1:14 AM, Doug Evans <address@hidden> wrote:
> 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?

Since threads inherit the sigmask of their creator one thing I could
do, I think, is block SIGCHLD before initializing Guile and then
unblock it afterwards.  The four threads I see being created during
Guile initialization (3 marker threads plus Guile's finalizer thread)
are created during that time.

Question: Does Guile create any threads lazily?

[I'm referring to internal Guile-created threads.
Users who create their own threads are on their own,
at least for now. IWBN though if there were a mechanism
to have all threads that Guile created have SIGCHLD
initially blocked.  I wouildn't make it SIGCHLD-specific
of course - just have a way to augment the initial
sigmask of newly created threads.]