systemd: add api for detecting if this is the last session for a user

https://bugzilla.gnome.org/show_bug.cgi?id=764029
Adapted from https://gitlab.gnome.org/GNOME/gnome-session/commit/f708bbbf

mate-session/gsm-systemd.c

@@ -421,6 +421,71 @@ emit_stop_complete (GsmSystemd *manager,
     }
 }
 
+gboolean
+gsm_systemd_is_last_session_for_user (GsmSystemd *manager)
+{
+        char **sessions = NULL;
+        char *session = NULL;
+        gboolean is_last_session;
+        int ret, i;
+
+        ret = sd_pid_get_session (getpid (), &session);
+
+        if (ret != 0) {
+                return FALSE;
+        }
+
+        ret = sd_uid_get_sessions (getuid (), FALSE, &sessions);
+
+        if (ret <= 0) {
+                free (session);
+                return FALSE;
+        }
+
+        is_last_session = TRUE;
+        for (i = 0; sessions[i]; i++) {
+                char *state = NULL;
+                char *type = NULL;
+
+                if (g_strcmp0 (sessions[i], session) == 0)
+                        continue;
+
+                ret = sd_session_get_state (sessions[i], &state);
+
+                if (ret != 0)
+                        continue;
+
+                if (g_strcmp0 (state, "closing") == 0) {
+                        free (state);
+                        continue;
+                }
+                free (state);
+
+                ret = sd_session_get_type (sessions[i], &type);
+
+                if (ret != 0)
+                        continue;
+
+                if (g_strcmp0 (type, "x11") != 0 &&
+                    g_strcmp0 (type, "wayland") != 0) {
+                        free (type);
+                        continue;
+                }
+                free (type);
+
+                is_last_session = FALSE;
+
+                break;
+        }
+
+        for (i = 0; sessions[i]; i++)
+                free (sessions[i]);
+        free (sessions);
+        free (session);
+
+        return is_last_session;
+}
+
 void
 gsm_systemd_attempt_restart (GsmSystemd *manager)
 {

mate-session/gsm-systemd.h

@@ -76,6 +76,8 @@ gboolean         gsm_systemd_can_hibernate     (GsmSystemd *manager);
 
 gboolean         gsm_systemd_can_suspend     (GsmSystemd *manager);
 
+gboolean         gsm_systemd_is_last_session_for_user (GsmSystemd *manager);
+
 void             gsm_systemd_attempt_stop    (GsmSystemd *manager);
 
 void             gsm_systemd_attempt_restart (GsmSystemd *manager);