bug#1058: 23.0.60; emacs --daemon should not return until socket is ready

[Romain Francoise]

Here's version one of a patch for this, please let me know what you
think.

diff --git a/lisp/startup.el b/lisp/startup.el
index 5bd73f9..22c92c5 100644
--- a/lisp/startup.el
+++ b/lisp/startup.el
@@ -1224,7 +1224,8 @@ opening the first frame (e.g. open a connection to an X 
server).")
   ;; processing all command line arguments to allow e.g. `server-name'
   ;; to be changed before the server starts.
   (when (daemonp)
-    (server-start))
+    (server-start)
+    (daemon-initialized))
 
   ;; Run emacs-session-restore (session management) if started by
   ;; the session manager and we have a session manager connection.
diff --git a/src/emacs.c b/src/emacs.c
index f94d2d3..d4d16b7 100644
--- a/src/emacs.c
+++ b/src/emacs.c
@@ -235,6 +235,13 @@ int noninteractive1;
 /* Nonzero means Emacs was started as a daemon.  */
 int is_daemon = 0;
 
+/* Set nonzero after the daemon has finished initializing.  */
+int daemon_initialized = 0;
+
+/* Pipe used to send exit notification to the daemon parent at
+   startup.  */
+int daemon_pipe[2];
+
 /* Save argv and argc.  */
 char **initial_argv;
 int initial_argc;
@@ -1074,16 +1081,56 @@ main (int argc, char **argv)
   if (argmatch (argv, argc, "-daemon", "--daemon", 5, NULL, &skip_args))
     {
 #ifndef DOS_NT
-      pid_t f = fork ();
+      pid_t f;
       int nfd;
+
+      /* Start as a daemon: fork a new child process which will run the
+        rest of the initialization code, then exit.
+
+        We want to avoid exiting before the server socket is ready, so
+        use a pipe for synchronization.  The parent waits for the child
+        to close its end of the pipe (using `daemon-initialized')
+        before exiting.  */
+      if (pipe (daemon_pipe) == -1)
+       {
+         fprintf (stderr, "Cannot pipe!\n");
+         exit (1);
+       }
+
+      f = fork ();
       if (f > 0)
-       exit (0);
+       {
+         int retval;
+         char buf[1];
+
+         /* Close unused writing end of the pipe.  */
+         close (daemon_pipe[1]);
+
+         /* Just wait for the child to close its end of the pipe.  */
+         do
+           {
+             retval = read (daemon_pipe[0], &buf, 1);
+           }
+         while (retval == -1 && errno == EINTR);
+
+         if (retval < 0)
+           {
+             fprintf (stderr, "Error reading status from child\n");
+             exit (1);
+           }
+
+         close (daemon_pipe[0]);
+         exit (0);
+       }
       if (f < 0)
        {
          fprintf (stderr, "Cannot fork!\n");
          exit (1);
        }
 
+      /* Close unused reading end of the pipe.  */
+      close (daemon_pipe[0]);
+
       nfd = open ("/dev/null", O_RDWR);
       dup2 (nfd, 0);
       dup2 (nfd, 1);
@@ -2389,6 +2436,24 @@ DEFUN ("daemonp", Fdaemonp, Sdaemonp, 0, 0, 0,
   return is_daemon ? Qt : Qnil;
 }
 
+DEFUN ("daemon-initialized", Fdaemon_initialized, Sdaemon_initialized, 0, 0, 0,
+       doc: /* Mark the Emacs daemon as being initialized.  */)
+  ()
+{
+  if (is_daemon)
+    {
+      if (! daemon_initialized)
+       {
+         /* Closing the pipe will notify the parent that it can exit. */
+         close (daemon_pipe[1]);
+         daemon_initialized = 1;
+       }
+    }
+  else
+    error ("Emacs was not started as a daemon");
+  return daemon_initialized ? Qt : Qnil;
+}
+
 void
 syms_of_emacs ()
 {
@@ -2408,6 +2473,7 @@ syms_of_emacs ()
   defsubr (&Sinvocation_name);
   defsubr (&Sinvocation_directory);
   defsubr (&Sdaemonp);
+  defsubr (&Sdaemon_initialized);
 
   DEFVAR_LISP ("command-line-args", &Vcommand_line_args,
               doc: /* Args passed by shell to Emacs, as a list of strings.