[Bug gdb/17247] gdb freezes on multi threaded app (test-case attached)

["palves at redhat dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=17247

Pedro Alves <palves at redhat dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |palves at redhat dot com

--- Comment #33 from Pedro Alves <palves at redhat dot com> ---
Comment on attachment 7762
  --> https://sourceware.org/bugzilla/attachment.cgi?id=7762
17247-experiment-1.patch

> +#ifdef HAVE_SIGPROCMASK
> +  /* Before we initialize Guile, block SIGCHLD.
> +     This is done so that all threads created during Guile initialization
> +     have SIGCHLD blocked.  PR 17247.  */
> +  sigprocmask (SIG_SETMASK, NULL, &sigset_for_guile);
> +  sigaddset (&sigset_for_guile, SIGCHLD);
> +  sigprocmask (SIG_SETMASK, &sigset_for_guile, NULL);

Using SIG_BLOCK lets this be a single syscall.

> +#endif
> +
>    /* scm_with_guile is the most portable way to initialize Guile.
>       Plus we need to initialize the Guile support while in Guile mode
>       (e.g., called from within a call to scm_with_guile).  */
>    scm_with_guile (call_initialize_gdb_module, NULL);
>  
> +#ifdef HAVE_SIGPROCMASK
> +  /* Undo the blocking of SIGCHLD.  */
> +  sigdelset (&sigset_for_guile, SIGCHLD);
> +  sigprocmask (SIG_SETMASK, &sigset_for_guile, NULL);

This assumes SIGCHLD wasn't blocked before.  Best avoid that, like
in the below pseudo-patch.

+ sigset_t sigchld_set, prev_set;
...
+#ifdef HAVE_SIGPROCMASK
+  /* Before we initialize Guile, make sure SIGCHLD is blocked.
+     This is done so that all threads created during Guile initialization
+     have SIGCHLD blocked.  PR 17247.  */
+  sigemptyset (sigchld_set);
+  sigaddset (&sigchld_set, SIGCHLD);
+  sigprocmask (SIG_BLOCK, &sigchld_set, &prev_set);
+#endif
...
+#ifdef HAVE_SIGPROCMASK
+  /* Restore the previous mask.  */
+  sigprocmask (SIG_SETMASK, &prev_set, NULL);
+#endif

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