Re: SIGCHLD ignored

[Pedro Alves <pedro at codesourcery dot com>]

[moved to gdb-patches@]

Original report here:
 SIGCHLD ignored
 http://sourceware.org/ml/gdb/2008-06/msg00099.html

A Wednesday 11 June 2008 23:45:22, Pedro Alves wrote:
> A Wednesday 11 June 2008 18:21:05, Vladimir Prus wrote:
> > Pedro, I think this SIGCHLD magic is your doing -- do you have any ideas
> > how to fix it?
>
> Dang, I totally missed this could be a problem.
>
> Right, so, normal_mask has SIGCHLD blocked in _initialize_linux_nat,
> and since forking a child inherits the signal mask, the child gets
> SIGCHLD blocked by default.  This would have gone unnoticed
> if Qt unblocked SIGCHLD in it's setup (one may argue it should).
>
> It's not safe to just remove that SIGCHLD blocking, we're not
> taking care of blocking it in places we used to in sync mode
> (we used to block it the first time we hit linux_nat_wait, which
> is reached after forking an inferior).
>
> For async mode, we also need to do something about SIGCHLD
> blocking.  We have:
>
> linux_nat_create_inferior
>   -> linux_nat_async_mask
>      -> linux_nat_async
>         -> linux_nat_async_events
>            -> block SIGCHLD.
>
>   -> linux_ops->to_create_inferior (forks a child)
>
> Attached is a patch to fix this.
>
> Basically, I turned linux_nat_async_events_enabled into a
> 3-state instead of a bool, with the new state being
> "SIGCHLD unblocked with action set to whatever we get
> on startup".  Most of the SIGCHLD state management stays with
> the same logic, except that linux_nat_create_inferior gets a
> switch to the new state before forking a child, and
> linux_nat_wait, gets an unconditional switch to the blocked
> state.  The rest of the patch is mostly updating to the
> new interface.
>
> Tested on x86-64-unknown-linux-gnu sync and async modes
> without regressions.

Same code patch, but updated with a new testcase.  I imagine
we're going to touch these issues again with full
multi-process support.

-- 
Pedro Alves

gdb/
2008-06-12  Pedro Alves  <pedro@codesourcery.com>

	* linux-nat.c (enum sigchld_state): New.
	(linux_nat_async_events_state): Renamed from
	linux_nat_async_events_enabled.
	(linux_nat_event_pipe_push, my_waitpid): Adjust.
	(sigchld_default_action): New.
	(lin_lwp_attach_lwp): Adjust.  Call linux_nat_async_events
	unconditionally.
	(linux_nat_create_inferior): Set events state to sigchld_default
	state.
	(linux_nat_resume): Adjust.
	(linux_nat_wait): Call linux_nat_async_events unconditionally.
	(sigchld_handler): Adjust.
	(linux_nat_async_mask): Don't set SIGCHLD actions here.
	(get_pending_events): Adjust.
	(linux_nat_async_events): Rewrite to handle enum sigchld_state
	instead of a boolean.
	(linux_nat_async): Adjust.
	(_initialize_linux_nat): Capture default SIGCHLD action into
	sigchld_default_action.

gdb/testsuite/
2008-06-12  Pedro Alves  <pedro@codesourcery.com>

	* gdb.base/sigchld.c, gdb.base/sigchld.exp: New test.
	
---
 gdb/linux-nat.c                    |  154 +++++++++++++++++++++++--------------
 gdb/testsuite/gdb.base/sigchld.c   |   37 ++++++++
 gdb/testsuite/gdb.base/sigchld.exp |   45 ++++++++++
 3 files changed, 179 insertions(+), 57 deletions(-)

Index: src/gdb/linux-nat.c
===================================================================
--- src.orig/gdb/linux-nat.c	2008-06-11 23:37:14.000000000 +0100
+++ src/gdb/linux-nat.c	2008-06-12 00:12:14.000000000 +0100
@@ -196,11 +196,24 @@ static int linux_nat_event_pipe[2] = { -
 /* Number of queued events in the pipe.  */
 static volatile int linux_nat_num_queued_events;
 
-/* If async mode is on, true if we're listening for events; false if
-   target events are blocked.  */
-static int linux_nat_async_events_enabled;
+/* The possible SIGCHLD handling states.  */
 
-static int linux_nat_async_events (int enable);
+enum sigchld_state
+{
+  /* SIGCHLD disabled, with action set to sigchld_handler, for the
+     sigsuspend in linux_nat_wait.  */
+  sigchld_sync,
+  /* SIGCHLD enabled, with action set to async_sigchld_handler.  */
+  sigchld_async,
+  /* Set SIGCHLD to default action.  Used while creating an
+     inferior.  */
+  sigchld_default
+};
+
+/* The current SIGCHLD handling state.  */
+static enum sigchld_state linux_nat_async_events_state;
+
+static enum sigchld_state linux_nat_async_events (enum sigchld_state enable);
 static void pipe_to_local_event_queue (void);
 static void local_event_queue_to_pipe (void);
 static void linux_nat_event_pipe_push (int pid, int status, int options);
@@ -234,8 +247,8 @@ queued_waitpid (int pid, int *status, in
   if (debug_linux_nat_async)
     fprintf_unfiltered (gdb_stdlog,
 			"\
-QWPID: linux_nat_async_events_enabled(%d), linux_nat_num_queued_events(%d)\n",
-			linux_nat_async_events_enabled,
+QWPID: linux_nat_async_events_state(%d), linux_nat_num_queued_events(%d)\n",
+			linux_nat_async_events_state,
 			linux_nat_num_queued_events);
 
   if (flags & __WALL)
@@ -381,7 +394,7 @@ my_waitpid (int pid, int *status, int fl
   int ret;
 
   /* There should be no concurrent calls to waitpid.  */
-  gdb_assert (!linux_nat_async_events_enabled);
+  gdb_assert (linux_nat_async_events_state != sigchld_async);
 
   ret = queued_waitpid (pid, status, flags);
   if (ret != -1)
@@ -813,6 +826,9 @@ struct sigaction sync_sigchld_action;
 
 /* SIGCHLD action for asynchronous mode.  */
 static struct sigaction async_sigchld_action;
+
+/* SIGCHLD default action, to pass to new inferiors.  */
+static struct sigaction sigchld_default_action;
 
 
 /* Prototypes for local functions.  */
@@ -1136,12 +1152,11 @@ int
 lin_lwp_attach_lwp (ptid_t ptid)
 {
   struct lwp_info *lp;
-  int async_events_were_enabled = 0;
+  enum sigchld_state async_events_original_state;
 
   gdb_assert (is_lwp (ptid));
 
-  if (target_can_async_p ())
-    async_events_were_enabled = linux_nat_async_events (0);
+  async_events_original_state = linux_nat_async_events (sigchld_sync);
 
   lp = find_lwp_pid (ptid);
 
@@ -1206,9 +1221,7 @@ lin_lwp_attach_lwp (ptid_t ptid)
       lp->stopped = 1;
     }
 
-  if (async_events_were_enabled)
-    linux_nat_async_events (1);
-
+  linux_nat_async_events (async_events_original_state);
   return 0;
 }
 
@@ -1222,6 +1235,8 @@ linux_nat_create_inferior (char *exec_fi
      we have to mask the async mode.  */
 
   if (target_can_async_p ())
+    /* Mask async mode.  Creating a child requires a loop calling
+       wait_for_inferior currently.  */
     saved_async = linux_nat_async_mask (0);
   else
     {
@@ -1232,6 +1247,12 @@ linux_nat_create_inferior (char *exec_fi
       sigdelset (&suspend_mask, SIGCHLD);
     }
 
+  /* Set SIGCHLD to the default action, until after execing the child,
+     since the inferior inherits the superior's signal mask.  It will
+     be blocked again in linux_nat_wait, which is only reached after
+     the inferior execing.  */
+  linux_nat_async_events (sigchld_default);
+
   linux_ops->to_create_inferior (exec_file, allargs, env, from_tty);
 
   if (saved_async)
@@ -1463,7 +1484,7 @@ linux_nat_resume (ptid_t ptid, int step,
 
   if (target_can_async_p ())
     /* Block events while we're here.  */
-    linux_nat_async_events (0);
+    linux_nat_async_events (sigchld_sync);
 
   /* A specific PTID means `step only this process id'.  */
   resume_all = (PIDGET (ptid) == -1);
@@ -2525,9 +2546,8 @@ linux_nat_wait (ptid_t ptid, struct targ
 
   sigemptyset (&flush_mask);
 
-  if (target_can_async_p ())
-    /* Block events while we're here.  */
-    target_async (NULL, 0);
+  /* Block events while we're here.  */
+  linux_nat_async_events (sigchld_sync);
 
 retry:
 
@@ -2986,7 +3006,7 @@ static void
 sigchld_handler (int signo)
 {
   if (linux_nat_async_enabled
-      && linux_nat_async_events_enabled
+      && linux_nat_async_events_state != sigchld_sync
       && signo == SIGCHLD)
     /* It is *always* a bug to hit this.  */
     internal_error (__FILE__, __LINE__,
@@ -3845,15 +3865,9 @@ linux_nat_async_mask (int mask)
 	{
 	  linux_nat_async (NULL, 0);
 	  linux_nat_async_mask_value = mask;
-	  /* We're in sync mode.  Make sure SIGCHLD isn't handled by
-	     async_sigchld_handler when we come out of sigsuspend in
-	     linux_nat_wait.  */
-	  sigaction (SIGCHLD, &sync_sigchld_action, NULL);
 	}
       else
 	{
-	  /* Restore the async handler.  */
-	  sigaction (SIGCHLD, &async_sigchld_action, NULL);
 	  linux_nat_async_mask_value = mask;
 	  linux_nat_async (inferior_event_handler, 0);
 	}
@@ -3911,7 +3925,8 @@ get_pending_events (void)
 {
   int status, options, pid;
 
-  if (!linux_nat_async_enabled || !linux_nat_async_events_enabled)
+  if (!linux_nat_async_enabled
+      || linux_nat_async_events_state != sigchld_async)
     internal_error (__FILE__, __LINE__,
 		    "get_pending_events called with async masked");
 
@@ -3965,44 +3980,71 @@ async_sigchld_handler (int signo)
   get_pending_events ();
 }
 
-/* Enable or disable async SIGCHLD handling.  */
+/* Set SIGCHLD handling state to STATE.  Returns previous state.  */
 
-static int
-linux_nat_async_events (int enable)
+static enum sigchld_state
+linux_nat_async_events (enum sigchld_state state)
 {
-  int current_state = linux_nat_async_events_enabled;
+  enum sigchld_state current_state = linux_nat_async_events_state;
 
   if (debug_linux_nat_async)
     fprintf_unfiltered (gdb_stdlog,
-			"LNAE: enable(%d): linux_nat_async_events_enabled(%d), "
+			"LNAE: state(%d): linux_nat_async_events_state(%d), "
 			"linux_nat_num_queued_events(%d)\n",
-			enable, linux_nat_async_events_enabled,
+			state, linux_nat_async_events_state,
 			linux_nat_num_queued_events);
 
-  if (current_state != enable)
+  if (current_state != state)
     {
       sigset_t mask;
       sigemptyset (&mask);
       sigaddset (&mask, SIGCHLD);
-      if (enable)
-	{
-	  /* Unblock target events.  */
-	  linux_nat_async_events_enabled = 1;
 
-	  local_event_queue_to_pipe ();
-	  /* While in masked async, we may have not collected all the
-	     pending events.  Get them out now.  */
-	  get_pending_events ();
-	  sigprocmask (SIG_UNBLOCK, &mask, NULL);
-	}
-      else
+      linux_nat_async_events_state = state;
+
+      switch (state)
 	{
-	  /* Block target events.  */
-	  sigprocmask (SIG_BLOCK, &mask, NULL);
-	  linux_nat_async_events_enabled = 0;
-	  /* Get events out of queue, and make them available to
-	     queued_waitpid / my_waitpid.  */
-	  pipe_to_local_event_queue ();
+	case sigchld_sync:
+	  {
+	    /* Block target events.  */
+	    sigprocmask (SIG_BLOCK, &mask, NULL);
+	    sigaction (SIGCHLD, &sync_sigchld_action, NULL);
+	    /* Get events out of queue, and make them available to
+	       queued_waitpid / my_waitpid.  */
+	    pipe_to_local_event_queue ();
+	  }
+	  break;
+	case sigchld_async:
+	  {
+	    /* Unblock target events for async mode.  */
+
+	    sigprocmask (SIG_BLOCK, &mask, NULL);
+
+	    /* Put events we already waited on, in the pipe first, so
+	       events are FIFO.  */
+	    local_event_queue_to_pipe ();
+	    /* While in masked async, we may have not collected all
+	       the pending events.  Get them out now.  */
+	    get_pending_events ();
+
+	    /* Let'em come.   */
+	    sigaction (SIGCHLD, &async_sigchld_action, NULL);
+	    sigprocmask (SIG_UNBLOCK, &mask, NULL);
+	  }
+	  break;
+	case sigchld_default:
+	  {
+	    /* SIGCHLD default mode.  */
+	    sigaction (SIGCHLD, &sigchld_default_action, NULL);
+
+	    /* Get events out of queue, and make them available to
+	       queued_waitpid / my_waitpid.  */
+	    pipe_to_local_event_queue ();
+
+	    /* Unblock SIGCHLD.  */
+	    sigprocmask (SIG_UNBLOCK, &mask, NULL);
+	  }
+	  break;
 	}
     }
 
@@ -4094,14 +4136,14 @@ linux_nat_async (void (*callback) (enum 
       add_file_handler (linux_nat_event_pipe[0],
 			linux_nat_async_file_handler, NULL);
 
-      linux_nat_async_events (1);
+      linux_nat_async_events (sigchld_async);
     }
   else
     {
       async_client_callback = callback;
       async_client_context = context;
 
-      linux_nat_async_events (0);
+      linux_nat_async_events (sigchld_sync);
       delete_file_handler (linux_nat_event_pipe[0]);
     }
   return;
@@ -4117,21 +4159,15 @@ linux_nat_set_async_mode (int on)
       if (on)
 	{
 	  gdb_assert (waitpid_queue == NULL);
-	  sigaction (SIGCHLD, &async_sigchld_action, NULL);
-
 	  if (pipe (linux_nat_event_pipe) == -1)
 	    internal_error (__FILE__, __LINE__,
 			    "creating event pipe failed.");
-
 	  fcntl (linux_nat_event_pipe[0], F_SETFL, O_NONBLOCK);
 	  fcntl (linux_nat_event_pipe[1], F_SETFL, O_NONBLOCK);
 	}
       else
 	{
-	  sigaction (SIGCHLD, &sync_sigchld_action, NULL);
-
 	  drain_queued_events (-1);
-
 	  linux_nat_num_queued_events = 0;
 	  close (linux_nat_event_pipe[0]);
 	  close (linux_nat_event_pipe[1]);
@@ -4248,6 +4284,10 @@ Tells gdb whether to control the GNU/Lin
 			   &maintenance_set_cmdlist,
 			   &maintenance_show_cmdlist);
 
+  /* Get the default SIGCHLD action.  Used while forking an inferior
+     (see linux_nat_create_inferior/linux_nat_async_events).  */
+  sigaction (SIGCHLD, NULL, &sigchld_default_action);
+
   /* Block SIGCHLD by default.  Doing this early prevents it getting
      unblocked if an exception is thrown due to an error while the
      inferior is starting (sigsetjmp/siglongjmp).  */
Index: src/gdb/testsuite/gdb.base/sigchld.c
===================================================================
--- /dev/null	1970-01-01 00:00:00.000000000 +0000
+++ src/gdb/testsuite/gdb.base/sigchld.c	2008-06-12 00:05:36.000000000 +0100
@@ -0,0 +1,37 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2008 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+/* Check that GDB isn't messing the SIGCHLD mask while creating an
+   inferior.  */
+
+#include <signal.h>
+#include <stdlib.h>
+
+int
+main ()
+{
+  sigset_t mask;
+
+  sigemptyset (&mask);
+  sigprocmask (SIG_BLOCK, NULL, &mask);
+
+  if (!sigismember (&mask, SIGCHLD))
+    return 0; /* good, not blocked */
+  else
+    return 1; /* bad, blocked */
+}
Index: src/gdb/testsuite/gdb.base/sigchld.exp
===================================================================
--- /dev/null	1970-01-01 00:00:00.000000000 +0000
+++ src/gdb/testsuite/gdb.base/sigchld.exp	2008-06-12 00:10:54.000000000 +0100
@@ -0,0 +1,45 @@
+# Copyright (C) 2008 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Check that GDB isn't messing the SIGCHLD mask while creating an
+# inferior.
+
+if [target_info exists gdb,nosignals] {
+    verbose "Skipping sigchld.exp because of nosignals."
+    continue
+}
+
+set testfile "sigchld"
+set srcfile ${testfile}.c
+set binfile ${objdir}/${subdir}/${testfile}
+
+if {[gdb_compile "${srcdir}/${subdir}/${srcfile}" "${binfile}" executable {debug}] != "" } {
+    return -1
+}
+
+gdb_exit
+gdb_start
+gdb_reinitialize_dir $srcdir/$subdir
+gdb_load ${binfile}
+
+runto_main
+
+gdb_test "b [gdb_get_line_number "good, not blocked"]" \
+         ".*Breakpoint .*sigchld.*" "set breakpoint at success exit"
+
+gdb_test "b [gdb_get_line_number "bad, blocked"]" \
+         ".*Breakpoint .*sigchld.*" "set breakpoint at failure exit"
+
+gdb_test "continue" ".*good, not blocked.*" "SIGCHLD blocked in inferior"