rtspclientsink: Add "update-sdp" signal that allows updating the SDP before sending it to the server

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-rtsp-server/-/merge_requests/178>

docs/gst_plugins_cache.json

@@ -414,6 +414,15 @@
                         ],
                         "return-type": "GstCaps",
                         "when": "last"
+                    },
+                    "update-sdp": {
+                        "args": [
+                            {
+                                "name": "arg0",
+                                "type": "GstSDPMessage"
+                            }
+                        ],
+                        "return-type": "void"
                     }
                 }
             }

gst/rtsp-sink/gstrtspclientsink.c

@@ -246,6 +246,7 @@ enum
   SIGNAL_NEW_PAYLOADER,
   SIGNAL_REQUEST_RTCP_KEY,
   SIGNAL_ACCEPT_CERTIFICATE,
+  SIGNAL_UPDATE_SDP,
   LAST_SIGNAL
 };
 
@@ -805,6 +806,22 @@ gst_rtsp_client_sink_class_init (GstRTSPClientSinkClass * klass)
       G_TYPE_BOOLEAN, 3, G_TYPE_TLS_CONNECTION, G_TYPE_TLS_CERTIFICATE,
       G_TYPE_TLS_CERTIFICATE_FLAGS);
 
+  /**
+   * GstRTSPClientSink::update-sdp:
+   * @rtsp_client_sink: a #GstRTSPClientSink
+   * @sdp: a #GstSDPMessage
+   *
+   * Emitted right before the ANNOUNCE request is sent to the server with the
+   * generated SDP. The SDP can be updated from signal handlers but the order
+   * and number of medias must not be changed.
+   *
+   * Since: 1.20
+   */
+  gst_rtsp_client_sink_signals[SIGNAL_UPDATE_SDP] =
+      g_signal_new_class_handler ("update-sdp", G_TYPE_FROM_CLASS (klass),
+      0, 0, NULL, NULL, NULL,
+      G_TYPE_NONE, 1, GST_TYPE_SDP_MESSAGE | G_SIGNAL_TYPE_STATIC_SCOPE);
+
   gstelement_class->provide_clock = gst_rtsp_client_sink_provide_clock;
   gstelement_class->change_state = gst_rtsp_client_sink_change_state;
   gstelement_class->request_new_pad =
@@ -4453,6 +4470,8 @@ gst_rtsp_client_sink_record (GstRTSPClientSink * sink, gboolean async)
   if (res < 0)
     goto create_request_failed;
 
+  g_signal_emit (sink, gst_rtsp_client_sink_signals[SIGNAL_UPDATE_SDP], 0, sdp);
+
   gst_rtsp_message_add_header (&request, GST_RTSP_HDR_CONTENT_TYPE,
       "application/sdp");