[PATCH][PR guile/17247] Block SIGCHLD while initializing Guile

[Doug Evans <xdje42 at gmail dot com>]

Hi.

This patch fixes PR 17247.

Basically, current Guile (git) starts an internal thread
(the "finalizer" thread), and libgc as of 7.4 now starts several
marker threads by default (before 7.4.0 one needed to configure
libgc with --enable-parallel-mark).

When other threads are running, and they haven't blocked SIGCHLD,
then the kernel may send SIGCHLD to these threads, leaving gdb
hung in the sigsuspend calls in linux-nat.c.

P.S. I have a tentative patch for PR 17314, which also fixes 17247,
but for a bit of robustness I'm applying both.

2014-08-31  Doug Evans  <xdje42@gmail.com>

	PR 17247
	* guile.c: #include <signal.h>.
	(_initialize_guile): Block SIGCHLD while initializing Guile.

	Replaces the following, which is reverted.

	2014-07-26  Doug Evans  <xdje42@gmail.com>

	PR 17185
	* configure.ac: Add check for header gc/gc.h.
	Add check for function setenv.
	* configure: Regenerate.
	* config.in: Regenerate.
	* guile/guile.c (_initialize_guile): Add workaround for libgc 7.4.0.

diff --git a/gdb/configure.ac b/gdb/configure.ac
index 26c8ecf..ab7b1c2 100644
--- a/gdb/configure.ac
+++ b/gdb/configure.ac
@@ -1233,11 +1233,6 @@ AC_SUBST(GUILE_CPPFLAGS)
 AC_SUBST(GUILE_LIBS)
 AM_CONDITIONAL(HAVE_GUILE, test "${have_libguile}" != no)
 
-# PR 17185, see if we can get the libgc version to see if we need
-# to apply the workaround.
-AC_CHECK_HEADERS(gc/gc.h)
-AC_CHECK_FUNCS([setenv])
-
 # --------------------- #
 # Check for libmcheck.  #
 # --------------------- #
diff --git a/gdb/guile/guile.c b/gdb/guile/guile.c
index 575bb6c..40208d2 100644
--- a/gdb/guile/guile.c
+++ b/gdb/guile/guile.c
@@ -34,10 +34,8 @@
 #ifdef HAVE_GUILE
 #include "guile.h"
 #include "guile-internal.h"
-#ifdef HAVE_GC_GC_H
-#include <gc/gc.h> /* PR 17185 */
-#endif
 #endif
+#include <signal.h>
 
 /* The Guile version we're using.
    We *could* use the macros in libguile/version.h but that would preclude
@@ -833,7 +831,9 @@ extern initialize_file_ftype _initialize_guile;
 void
 _initialize_guile (void)
 {
-  char *msg;
+#ifdef HAVE_SIGPROCMASK
+  sigset_t sigchld_mask, prev_mask;
+#endif
 
   install_gdb_commands ();
 
@@ -842,16 +842,13 @@ _initialize_guile (void)
      side to define module "gdb" which imports "_gdb".  There is evidently no
      similar convention in Guile so we skip this.  */
 
-  /* PR 17185 There are problems with using libgc 7.4.0.
-     Copy over the workaround Guile uses (Guile is working around a different
-     problem, but the workaround is the same).  */
-#if (GC_VERSION_MAJOR == 7 && GC_VERSION_MINOR == 4 && GC_VERSION_MICRO == 0)
-  /* The bug is only known to appear with pthreads.  We assume any system
-     using pthreads also uses setenv (and not putenv).  That is why we don't
-     have a similar call to putenv here.  */
-#if defined (HAVE_SETENV)
-  setenv ("GC_MARKERS", "1", 1);
-#endif
+#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.  */
+  sigemptyset (&sigchld_mask);
+  sigaddset (&sigchld_mask, SIGCHLD);
+  sigprocmask (SIG_BLOCK, &sigchld_mask, &prev_mask);
 #endif
 
   /* scm_with_guile is the most portable way to initialize Guile.
@@ -859,6 +856,10 @@ _initialize_guile (void)
      (e.g., called from within a call to scm_with_guile).  */
   scm_with_guile (call_initialize_gdb_module, NULL);
 
+#ifdef HAVE_SIGPROCMASK
+  sigprocmask (SIG_SETMASK, &prev_mask, NULL);
+#endif
+
   /* Set Guile's backtrace to match the "set guile print-stack" default.
      [N.B. The two settings are still separate.]
      But only do this after we've initialized Guile, it's nice to see a