[glib] gmain: Add private API to create Unix child watch that uses waitid()

[Colin Walters <walters src gnome org>]

commit 93bf37ce1507380f74d4cb4cab6640fc7d2eb7d1
Author: Colin Walters <walters verbum org>
Date:   Mon May 21 17:09:06 2012 -0400

    gmain: Add private API to create Unix child watch that uses waitid()
    
    This avoids collecting the zombie child, which means that the PID
    can't be reused.  This prevents possible race conditions that might
    occur were one to send e.g. SIGTERM to a child.
    
    This race condition has always existed due to the way we called
    waitpid() for the app, but the window was widened when we moved the
    waitpid() calls into a separate thread.
    
    If waitid() isn't available, we return NULL, and consumers of this
    private API (namely, GSubprocess) will need to handle that.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=672102

 configure.ac        |    2 +-
 glib/glib-private.c |    3 +-
 glib/glib-private.h |    3 +
 glib/gmain.c        |  178 ++++++++++++++++++++++++++++++++++++++------------
 4 files changed, 141 insertions(+), 45 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 6a820ca..2daebd1 100644
--- a/configure.ac
+++ b/configure.ac
@@ -989,7 +989,7 @@ AC_MSG_RESULT(unsigned $glib_size_type)
 
 # Check for some functions
 AC_CHECK_FUNCS(lstat strerror strsignal memmove vsnprintf stpcpy strcasecmp strncasecmp poll getcwd vasprintf setenv unsetenv getc_unlocked readlink symlink fdwalk memmem)
-AC_CHECK_FUNCS(chown lchmod lchown fchmod fchown link utimes getgrgid getpwuid getresuid)
+AC_CHECK_FUNCS(chown lchmod lchown fchmod fchown link utimes getgrgid getpwuid getresuid waitid)
 AC_CHECK_FUNCS(getmntent_r setmntent endmntent hasmntopt getfsstat getvfsstat)
 # Check for high-resolution sleep functions
 AC_CHECK_FUNCS(splice)
diff --git a/glib/glib-private.c b/glib/glib-private.c
index e7838a6..c2346c5 100644
--- a/glib/glib-private.c
+++ b/glib/glib-private.c
@@ -41,7 +41,8 @@ glib__private__ (void)
     g_get_worker_context,
 
     g_check_setuid,
-    g_main_context_new_with_next_id
+    g_main_context_new_with_next_id,
+    g_child_watch_source_new_nowait
   };
 
   return &table;
diff --git a/glib/glib-private.h b/glib/glib-private.h
index 23bfb36..1ea840d 100644
--- a/glib/glib-private.h
+++ b/glib/glib-private.h
@@ -29,6 +29,8 @@ G_GNUC_INTERNAL
 gboolean                g_check_setuid                  (void);
 G_GNUC_INTERNAL
 GMainContext *          g_main_context_new_with_next_id (guint next_id);
+G_GNUC_INTERNAL
+GSource *               g_child_watch_source_new_nowait (GPid     pid);
 
 #define GLIB_PRIVATE_CALL(symbol) (glib__private__()->symbol)
 
@@ -46,6 +48,7 @@ typedef struct {
 
   gboolean              (* g_check_setuid)              (void);
   GMainContext *        (* g_main_context_new_with_next_id) (guint next_id);
+  GSource *             (* g_child_watch_source_new_nowait) (GPid pid);
   /* Add other private functions here, initialize them in glib-private.c */
 } GLibPrivateVTable;
 
diff --git a/glib/gmain.c b/glib/gmain.c
index c1776a8..6b43126 100644
--- a/glib/gmain.c
+++ b/glib/gmain.c
@@ -293,7 +293,9 @@ struct _GChildWatchSource
 #ifdef G_OS_WIN32
   GPollFD     poll;
 #else /* G_OS_WIN32 */
-  gboolean    child_exited;
+  guint       child_exited : 1;
+  guint       wnowait : 1;
+  guint       reserved : 30;
 #endif /* G_OS_WIN32 */
 };
 
@@ -4460,6 +4462,82 @@ wake_source (GSource *source)
   G_UNLOCK(main_context_list);
 }
 
+/* This is a hack; we want to use the newer waitid() call, but the use
+ * of the waitpid() status code API is baked into the GChildWatchSource
+ * callback signature, so we need to synthesize it from the siginfo_t
+ * we get from waitid().
+ */ 
+#ifdef HAVE_WAITID
+#define __G_MAKE_EXITSTATUS(ecode, signum) ((ecode) << 8 | (signum))
+static gint
+waitpid_status_from_siginfo (siginfo_t *siginfo)
+{
+  G_STATIC_ASSERT(((WIFEXITED (__G_MAKE_EXITSTATUS (1, 0))) &&		\
+		   !(WIFSIGNALED (__G_MAKE_EXITSTATUS (1, 0))) &&	\
+		   (WEXITSTATUS (__G_MAKE_EXITSTATUS (1, 0)) == 1)));
+  G_STATIC_ASSERT((!WIFEXITED (__G_MAKE_EXITSTATUS (0, 1))) &&		\
+		  (WIFSIGNALED (__G_MAKE_EXITSTATUS (0, 1))) &&		\
+		  (WTERMSIG (__G_MAKE_EXITSTATUS (0, 1)) == 1));
+  if (siginfo->si_code == CLD_EXITED)
+    return __G_MAKE_EXITSTATUS(siginfo->si_status, 0);
+ else if (siginfo->si_code == CLD_KILLED || siginfo->si_code == CLD_DUMPED)
+   return __G_MAKE_EXITSTATUS(0, siginfo->si_status);
+ else
+   return __G_MAKE_EXITSTATUS(0xFF, 0);
+}
+#undef __G_MAKE_EXITSTATUS
+#endif /* HAVE_WAITID */
+
+/* Returns TRUE if we need to wake the source */
+static gboolean
+unix_child_watch_source_waitpid (GChildWatchSource    *source)
+{
+  pid_t pid;
+
+  if (source->child_exited)
+    return FALSE;
+
+#ifdef HAVE_WAITID
+  if (source->wnowait)
+    {
+      siginfo_t siginfo;
+      int r;
+
+      siginfo.si_pid = 0;
+      do
+        r = waitid (P_PID, (id_t)source->pid, &siginfo, WEXITED | WNOHANG | WNOWAIT);
+      while (r == -1 && errno == EINTR);
+
+      if (r == 0 && siginfo.si_pid == source->pid)
+	{
+	  source->child_exited = TRUE;
+	  source->child_status = waitpid_status_from_siginfo (&siginfo);
+	  return TRUE;
+	}
+    } else
+#endif
+    {
+      do
+        pid = waitpid (source->pid, &source->child_status, WNOHANG);
+      while (pid == -1 && errno == EINTR);
+
+      if (pid > 0)
+        {
+          source->child_exited = TRUE;
+          return TRUE;
+        }
+      else if (pid == -1 && errno == ECHILD)
+        {
+          g_warning ("GChildWatchSource: Exit status of a child process was requested but ECHILD was received by waitpid(). Most likely the process is ignoring SIGCHLD, or some other thread is invoking waitpid() with a nonpositive first argument; either behavior can break applications that use g_child_watch_add()/g_spawn_sync() either directly or indirectly.");
+          source->child_exited = TRUE;
+          source->child_status = 0;
+          return TRUE;
+        }
+    }
+
+  return FALSE;
+}
+
 static void
 dispatch_unix_signals (void)
 {
@@ -4486,28 +4564,9 @@ dispatch_unix_signals (void)
       for (node = unix_child_watches; node; node = node->next)
         {
           GChildWatchSource *source = node->data;
-
-          if (!source->child_exited)
-            {
-              pid_t pid;
-              do
-                {
-                  pid = waitpid (source->pid, &source->child_status, WNOHANG);
-                  if (pid > 0)
-                    {
-                      source->child_exited = TRUE;
-                      wake_source ((GSource *) source);
-                    }
-                  else if (pid == -1 && errno == ECHILD)
-                    {
-                      g_warning ("GChildWatchSource: Exit status of a child process was requested but ECHILD was received by waitpid(). Most likely the process is ignoring SIGCHLD, or some other thread is invoking waitpid() with a nonpositive first argument; either behavior can break applications that use g_child_watch_add()/g_spawn_sync() either directly or indirectly.");
-                      source->child_exited = TRUE;
-                      source->child_status = 0;
-                      wake_source ((GSource *) source);
-                    }
-                }
-              while (pid == -1 && errno == EINTR);
-            }
+	  
+	  if (unix_child_watch_source_waitpid (source))
+	    wake_source ((GSource *) source);
         }
     }
 
@@ -4699,6 +4758,58 @@ g_unix_signal_handler (int signum)
 
 #endif /* !G_OS_WIN32 */
 
+static GSource *
+g_child_watch_source_new_internal (GPid     pid,
+                                   gboolean wnowait)
+{
+  GSource *source;
+  GChildWatchSource *child_watch_source;
+
+#if defined(G_OS_UNIX) && !defined(HAVE_WAITID)
+  if (wnowait)
+    return NULL;
+#else
+  source = g_source_new (&g_child_watch_funcs, sizeof (GChildWatchSource));
+  child_watch_source = (GChildWatchSource *)source;
+
+  child_watch_source->pid = pid;
+#if defined(G_OS_UNIX) && defined(HAVE_WAITID)
+  if (wnowait)
+    child_watch_source->wnowait = TRUE;
+#endif
+
+#ifdef G_OS_WIN32
+  child_watch_source->poll.fd = (gintptr) pid;
+  child_watch_source->poll.events = G_IO_IN;
+
+  g_source_add_poll (source, &child_watch_source->poll);
+#else /* G_OS_WIN32 */
+  G_LOCK (unix_signal_lock);
+  ensure_unix_signal_handler_installed_unlocked (SIGCHLD);
+  unix_child_watches = g_slist_prepend (unix_child_watches, child_watch_source);
+  unix_child_watch_source_waitpid (child_watch_source);
+  G_UNLOCK (unix_signal_lock);
+#endif /* G_OS_WIN32 */
+
+  return source;
+#endif
+}
+
+/* internal
+ * Create a child watch, using @wnowait to enable a mode
+ * where the child won't be reaped.  This is used by
+ * GSubprocess to ensure that the pid is valid for
+ * the lifetime of the GSubprocess object.  Otherwise
+ * there'd be a possible race condition where we reap
+ * the child with waitpid() in the worker thread,
+ * while the main thread is calling kill() on it.
+ */
+GSource *
+g_child_watch_source_new_nowait (GPid     pid)
+{
+  return g_child_watch_source_new_internal (pid, TRUE);
+}
+
 /**
  * g_child_watch_source_new:
  * @pid: process to watch. On POSIX the pid of a child process. On
@@ -4731,26 +4842,7 @@ g_unix_signal_handler (int signum)
 GSource *
 g_child_watch_source_new (GPid pid)
 {
-  GSource *source = g_source_new (&g_child_watch_funcs, sizeof (GChildWatchSource));
-  GChildWatchSource *child_watch_source = (GChildWatchSource *)source;
-
-  child_watch_source->pid = pid;
-
-#ifdef G_OS_WIN32
-  child_watch_source->poll.fd = (gintptr) pid;
-  child_watch_source->poll.events = G_IO_IN;
-
-  g_source_add_poll (source, &child_watch_source->poll);
-#else /* G_OS_WIN32 */
-  G_LOCK (unix_signal_lock);
-  ensure_unix_signal_handler_installed_unlocked (SIGCHLD);
-  unix_child_watches = g_slist_prepend (unix_child_watches, child_watch_source);
-  if (waitpid (pid, &child_watch_source->child_status, WNOHANG) > 0)
-    child_watch_source->child_exited = TRUE;
-  G_UNLOCK (unix_signal_lock);
-#endif /* G_OS_WIN32 */
-
-  return source;
+  return g_child_watch_source_new_internal (pid, FALSE);
 }
 
 /**