manager: Try looking up session based on PID first

Unfortunately, GDM may be running multiple greeters, and each greeter is
currently using the same user. So while in a lot of setups each user
should only have one graphical session and also only one DBus session
bus, this is not true for the gdm greeter.

Lacking another solution (e.g. separate users), we need to be able to
correctly lookup the session information for all greeter instances. We
can do so by using sd_pid_get_session and using this information is safe
if it does return something.

See: #526

2 files changed, 30 insertions, 7 deletions

diff --git a/common/gdm-common.c b/common/gdm-common.c
index 41bdb389..b8de7555 100644
--- a/common/gdm-common.c
+++ b/common/gdm-common.c
@@ -497,7 +497,7 @@ goto_login_session (GDBusConnection  *connection,
          * since the data allocated is from libsystemd-logind, which
          * does not use GLib's g_malloc (). */
 
-        if (!gdm_find_display_session_for_uid (getuid (), &our_session, &local_error)) {
+        if (!gdm_find_display_session (0, getuid (), &our_session, &local_error)) {
                 g_propagate_prefixed_error (error, local_error, _("Could not identify the current session: "));
 
                 return FALSE;
@@ -898,16 +898,38 @@ _systemd_session_is_active (const char *session_id)
 }
 
 gboolean
-gdm_find_display_session_for_uid (const uid_t uid,
-                                  char      **out_session_id,
-                                  GError    **error)
+gdm_find_display_session (GPid        pid,
+                          const uid_t uid,
+                          char      **out_session_id,
+                          GError    **error)
 {
         char *local_session_id = NULL;
         g_auto(GStrv) sessions = NULL;
         int n_sessions;
+        int res;
 
         g_return_val_if_fail (out_session_id != NULL, FALSE);
 
+        /* First try to look up the session using the pid. We need this
+         * at least for the greeter, because it currently runs multiple
+         * sessions under the same user.
+         * See also commit 2b52d8933c8ab38e7ee83318da2363d00d8c5581
+         * which added an explicit dbus-run-session for all but seat0.
+         */
+        res = sd_pid_get_session (pid, &local_session_id);
+        if (res >= 0) {
+                g_debug ("GdmCommon: Found session %s for PID %d, using", local_session_id, pid);
+
+                *out_session_id = g_strdup (local_session_id);
+                free (local_session_id);
+
+                return TRUE;
+        } else {
+                if (res != -ENODATA)
+                        g_warning ("GdmCommon: Failed to retrieve session information for pid %d: %s",
+                                   pid, strerror (-res));
+        }
+
         g_debug ("Finding a graphical session for user %d", uid);
 
         n_sessions = sd_uid_get_sessions (uid,
diff --git a/common/gdm-common.h b/common/gdm-common.h
index 5c3fe137..001b70c1 100644
--- a/common/gdm-common.h
+++ b/common/gdm-common.h
@@ -51,9 +51,10 @@ int            gdm_wait_on_and_disown_pid (int pid,
 int            gdm_signal_pid            (int pid,
                                           int signal);
 
-gboolean       gdm_find_display_session_for_uid (const uid_t uid,
-                                                 char      **out_session_id,
-                                                 GError    **error);
+gboolean       gdm_find_display_session (GPid        pid,
+                                         const uid_t uid,
+                                         char      **out_session_id,
+                                         GError    **error);
 
 gboolean       gdm_get_pwent_for_name    (const char     *name,
                                           struct passwd **pwentp);