logind-session-dbus: allow to set display name via dbus

Currently, the only way to set display name of a graphical session is to
pass it to CreateSession(). But modern display managers like gdm start
the display server as part of the user session, which means that the
display name isn't known yet when the session is being created. Hence,
let's make it possible to set it later.

man/org.freedesktop.login1.xml

@@ -1075,6 +1075,7 @@ node /org/freedesktop/login1/session/1 {
       TakeControl(in  b force);
       ReleaseControl();
       SetType(in  s type);
+      SetDisplay(in  s display);
       TakeDevice(in  u major,
                  in  u minor,
                  out h fd,
@@ -1172,6 +1173,8 @@ node /org/freedesktop/login1/session/1 {
 
     <variablelist class="dbus-method" generated="True" extra-ref="SetType()"/>
 
+    <variablelist class="dbus-method" generated="True" extra-ref="SetDisplay()"/>
+
     <variablelist class="dbus-method" generated="True" extra-ref="TakeDevice()"/>
 
     <variablelist class="dbus-method" generated="True" extra-ref="ReleaseDevice()"/>
@@ -1268,6 +1271,11 @@ node /org/freedesktop/login1/session/1 {
       connection. This should help prevent a session from entering an inconsistent state, for example if the
       controller crashes. The only argument <varname>type</varname> is the new session type.</para>
 
+      <para><function>SetDisplay()</function> allows the display name of the graphical session to be changed. This is
+      useful if the display server is started as part of the session. It can only be called by session's current
+      controller. If <function>TakeControl()</function> has not been called, this method will fail. The only argument
+      <varname>display</varname> is the new display name.</para>
+
       <para><function>TakeDevice()</function> allows a session controller to get a file descriptor for a
       specific device. Pass in the major and minor numbers of the character device and
       <filename>systemd-logind</filename> will return a file descriptor for the device. Only a limited set of

src/login/logind-session-dbus.c

@@ -413,6 +413,30 @@ static int method_set_type(sd_bus_message *message, void *userdata, sd_bus_error
         return sd_bus_reply_method_return(message, NULL);
 }
 
+static int method_set_display(sd_bus_message *message, void *userdata, sd_bus_error *error) {
+        Session *s = ASSERT_PTR(userdata);
+        const char *display;
+        int r;
+
+        assert(message);
+
+        r = sd_bus_message_read(message, "s", &display);
+        if (r < 0)
+                return r;
+
+        if (!session_is_controller(s, sd_bus_message_get_sender(message)))
+                return sd_bus_error_set(error, BUS_ERROR_NOT_IN_CONTROL, "You must be in control of this session to set display");
+
+        if (!SESSION_TYPE_IS_GRAPHICAL(s->type))
+                return sd_bus_error_set(error, SD_BUS_ERROR_NOT_SUPPORTED, "Setting display is only supported for graphical sessions");
+
+        r = session_set_display(s, display);
+        if (r < 0)
+                return r;
+
+        return sd_bus_reply_method_return(message, NULL);
+}
+
 static int method_take_device(sd_bus_message *message, void *userdata, sd_bus_error *error) {
         Session *s = userdata;
         uint32_t major, minor;
@@ -901,6 +925,11 @@ static const sd_bus_vtable session_vtable[] = {
                                 SD_BUS_NO_RESULT,
                                 method_set_type,
                                 SD_BUS_VTABLE_UNPRIVILEGED),
+        SD_BUS_METHOD_WITH_ARGS("SetDisplay",
+                                SD_BUS_ARGS("s", display),
+                                SD_BUS_NO_RESULT,
+                                method_set_display,
+                                SD_BUS_VTABLE_UNPRIVILEGED),
         SD_BUS_METHOD_WITH_ARGS("TakeDevice",
                                 SD_BUS_ARGS("u", major, "u", minor),
                                 SD_BUS_RESULT("h", fd, "b", inactive),

src/login/logind-session.c

@@ -1044,6 +1044,26 @@ void session_set_type(Session *s, SessionType t) {
         session_send_changed(s, "Type", NULL);
 }
 
+int session_set_display(Session *s, const char *display) {
+        int r;
+
+        assert(s);
+        assert(display);
+
+        if (streq(s->display, display))
+                return 0;
+
+        r = free_and_strdup(&s->display, display);
+        if (r < 0)
+                return r;
+
+        session_save(s);
+
+        session_send_changed(s, "Display", NULL);
+
+        return 1;
+}
+
 static int session_dispatch_fifo(sd_event_source *es, int fd, uint32_t revents, void *userdata) {
         Session *s = userdata;
 

src/login/logind-session.h

@@ -137,6 +137,7 @@ int session_set_idle_hint(Session *s, bool b);
 int session_get_locked_hint(Session *s);
 void session_set_locked_hint(Session *s, bool b);
 void session_set_type(Session *s, SessionType t);
+int session_set_display(Session *s, const char *display);
 int session_create_fifo(Session *s);
 int session_start(Session *s, sd_bus_message *properties, sd_bus_error *error);
 int session_stop(Session *s, bool force);

src/login/org.freedesktop.login1.conf

@@ -346,6 +346,10 @@
                        send_interface="org.freedesktop.login1.User"
                        send_member="Kill"/>
 
+                <allow send_destination="org.freedesktop.login1"
+                       send_interface="org.freedesktop.login1.Session"
+                       send_member="SetDisplay"/>
+
                 <allow receive_sender="org.freedesktop.login1"/>
         </policy>