[glib] gmain: Document constraints on waitpid

[Colin Walters <walters src gnome org>]

commit 00f4c12bf97432a50e49749895d2c8174e204907
Author: Paul Eggert <eggert cs ucla edu>
Date:   Mon Oct 29 10:19:20 2012 -0400

    gmain: Document constraints on waitpid
    
    Applications that use glib should not invoke waitpid with a first
    argument that is nonpositive, because when such a waitpid is run in
    one thread and glib waits for a subprocess in another, there is a race
    condition, and the former waitpid can reap a process that was intended
    for the latter.  Mention this in the documentation for
    g_child_watch_source_new, and in the diagnostic generated by
    g_spawn_sync when its waitpid fails with errno equal to ECHILD.
    
    Signed-off-by: Colin Walters <walters verbum org>
    
    http://bugzilla.gnome.org/show_bug.cgi?id=687075

 glib/gmain.c  |    8 ++++----
 glib/gspawn.c |    2 +-
 2 files changed, 5 insertions(+), 5 deletions(-)
---
diff --git a/glib/gmain.c b/glib/gmain.c
index c0c4581..133b0f1 100644
--- a/glib/gmain.c
+++ b/glib/gmain.c
@@ -4639,10 +4639,10 @@ g_unix_signal_handler (int signum)
  * source is still active. Typically, you will want to call
  * g_spawn_close_pid() in the callback function for the source.
  *
- * Note further that using g_child_watch_source_new() is not 
- * compatible with calling <literal>waitpid(-1)</literal> in 
- * the application. Calling waitpid() for individual pids will
- * still work fine. 
+ * Note further that using g_child_watch_source_new() is not
+ * compatible with calling <literal>waitpid</literal> with a
+ * nonpositive first argument in the application. Calling waitpid()
+ * for individual pids will still work fine.
  * 
  * Return value: the newly-created child watch source
  *
diff --git a/glib/gspawn.c b/glib/gspawn.c
index 14d1b4c..5e4922d 100644
--- a/glib/gspawn.c
+++ b/glib/gspawn.c
@@ -415,7 +415,7 @@ g_spawn_sync (const gchar          *working_directory,
         {
           if (exit_status)
             {
-              g_warning ("In call to g_spawn_sync(), exit status of a child process was requested but SIGCHLD action was set to SIG_IGN and ECHILD was received by waitpid(), so exit status can't be returned. This is a bug in the program calling g_spawn_sync(); either don't request the exit status, or don't set the SIGCHLD action.");
+              g_warning ("In call to g_spawn_sync(), 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_spawn_sync either directly or indirectly.");
             }
           else
             {