[glib] Do not attempt to autolaunch a session dbus-daemon with no DISPLAY

[Matthias Clasen <matthiasc src gnome org>]

commit 0f2e4fd01c5cb07039aaf29f4f3219582c0f145c
Author: Simon McVittie <smcv debian org>
Date:   Mon Feb 3 01:14:45 2014 +0000

    Do not attempt to autolaunch a session dbus-daemon with no DISPLAY
    
    The two known use-cases for autolaunching are:
    
    * X-forwarding: "ssh -Y myhost myapp" resulting in a
      session bus on myhost but an X server on the original host
    
    * Legacy desktop environments on OSs without D-Bus integration:
      e.g. running a single GNOME or KDE app under fvwm or something,
      without a session dbus-daemon being started by either systemd,
      gnome-session, or OS integration scripts analogous to Debian's
      /etc/X11/Xsession.d/75dbus_dbus-launch
    
    In either case, an X11 DISPLAY is also needed.
    
    "dbus-launch --autolaunch" doesn't do anything useful when unable
    to connect to an X11 display; this has been the case since the feature
    was added in 2006, and is useful to avoid "split brain" situations in
    which two processes that ought to be part of the same session end up
    on separate session buses. Since dbus commit 407c111 in 2011,
    libdbus hasn't even attempted to run "dbus-launch --autolaunch"
    unless getenv("DISPLAY") returns non-null in the parent: this avoids
    doing a relatively complicated fork-and-exec that is clearly not
    going to lead to success. This commit gives GDBus the same policy.
    
    This change was originally made to work around a race condition in
    subprocess spawning (Debian bug #737380, GNOME bug #711090) but
    it seems valid in its own right.
    
    In my opinion as D-Bus maintainer, "dbus-launch --autolaunch" should
    be considered to be an X11 feature, and any future D-Bus enhancements
    (e.g. kdbus) or successors for X11 (e.g. Wayland, Mir) should obtain
    a session bus address by other means - either a session manager
    such as "systemd --user", gnome-session or Upstart, or a wrapper
    for the user session like dbus-run-session(1).
    
    Related to dbus bug <https://bugs.freedesktop.org/show_bug.cgi?id=19997>.
    
    Bug: https://bugzilla.gnome.org/show_bug.cgi?id=723506
    Bug-Debian: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=737380
    Signed-off-by: Simon McVittie <simon mcvittie collabora co uk>

 gio/gdbusaddress.c |    7 +++++++
 1 files changed, 7 insertions(+), 0 deletions(-)
---
diff --git a/gio/gdbusaddress.c b/gio/gdbusaddress.c
index a004011..d058666 100644
--- a/gio/gdbusaddress.c
+++ b/gio/gdbusaddress.c
@@ -1072,6 +1072,13 @@ get_session_address_dbus_launch (GError **error)
       goto out;
     }
 
+  if (g_getenv ("DISPLAY") == NULL)
+    {
+      g_set_error (error, G_IO_ERROR, G_IO_ERROR_FAILED,
+                   _("Cannot autolaunch D-Bus without X11 $DISPLAY"));
+      goto out;
+    }
+
   /* We're using private libdbus facilities here. When everything
    * (X11, Mac OS X, Windows) is spec'ed out correctly (not even the
    * X11 property is correctly documented right now) we should