[gdm] gdm-{x,wayland}-session: don't start dbus-daemon if unneeded

[Ray Strode <halfline src gnome org>]

commit 37c30a497908c969898e4a15bea88aea91cb2db5
Author: David Herrmann <dh herrmann gmail com>
Date:   Wed Jun 3 20:07:18 2015 +0200

    gdm-{x,wayland}-session: don't start dbus-daemon if unneeded
    
    Desktop sessions require a message bus to be present for IPC between
    the various desktop services and applications.
    
    GDM handles that by manually, unconditionally spawning an instance of
    dbus-daemon at login time before starting the user session (from the
    gdm-x-session and gdm-wayland-session launcher programs).
    
    Newer versions of the kernel will support this message bus functionality
    without needing to start dbus-daemon (via kdbus). The message bus is
    initialized as part of the login process by a kernel interface exercised
    from pam_systemd.
    
    Since gdm-x-session and gdm-wayland-session spawn a dbus-daemon
    unconditionally as part of session startup, users using kdbus end up with
    two message buses per session. The extra bus gets in the way, confusing
    programs and wasting resources.
    
    This commit changes gdm-x-session and gdm-wayland-session to check for
    DBUS_SESSION_BUS_ADDRESS in the environment, and if set, skip spawning
    a dbus-daemon based session bus.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=750358

 daemon/gdm-wayland-session.c |    8 ++++++++
 daemon/gdm-x-session.c       |    8 ++++++++
 2 files changed, 16 insertions(+), 0 deletions(-)
---
diff --git a/daemon/gdm-wayland-session.c b/daemon/gdm-wayland-session.c
index b2f3808..b6eccc2 100644
--- a/daemon/gdm-wayland-session.c
+++ b/daemon/gdm-wayland-session.c
@@ -95,6 +95,7 @@ spawn_bus (State        *state,
         GInputStream        *input_stream = NULL;
         GDataInputStream    *data_stream = NULL;
         GError              *error = NULL;
+        const char          *bus_env = NULL;
         char                *bus_address_fd_string = NULL;
         char                *bus_address = NULL;
         gsize                bus_address_size;
@@ -105,6 +106,13 @@ spawn_bus (State        *state,
 
         g_debug ("Running session message bus");
 
+        bus_env = g_getenv ("DBUS_SESSION_BUS_ADDRESS");
+        if (bus_env != NULL) {
+                g_debug ("session message bus already running, not starting another one");
+                state->bus_address = g_strdup (bus_env);
+                return TRUE;
+        }
+
         ret = g_unix_open_pipe (pipe_fds, FD_CLOEXEC, &error);
 
         if (!ret) {
diff --git a/daemon/gdm-x-session.c b/daemon/gdm-x-session.c
index 3cc7d40..bcb884c 100644
--- a/daemon/gdm-x-session.c
+++ b/daemon/gdm-x-session.c
@@ -367,6 +367,7 @@ spawn_bus (State        *state,
         GInputStream        *input_stream = NULL;
         GDataInputStream    *data_stream = NULL;
         GError              *error = NULL;
+        const char          *bus_env = NULL;
         char                *bus_address_fd_string;
         char                *bus_address = NULL;
         gsize                bus_address_size;
@@ -377,6 +378,13 @@ spawn_bus (State        *state,
 
         g_debug ("Running session message bus");
 
+        bus_env = g_getenv ("DBUS_SESSION_BUS_ADDRESS");
+        if (bus_env != NULL) {
+                g_debug ("session message bus already running, not starting another one");
+                state->bus_address = g_strdup (bus_env);
+                return TRUE;
+        }
+
         ret = g_unix_open_pipe (pipe_fds, FD_CLOEXEC, &error);
 
         if (!ret) {