sendrecv: Implement remote-offerer option for JS example

Now you can check the "Remote offerer" checkbox in the JS example to
force the peer to send the SDP offer. This involved implementing
support for receiving the OFFER_REQUEST message in the C example.

As a side-effect of this, the C example will no longer send
OFFER_REQUEST automatically when the --our-id option is passed. It
will only do so when the --remote-offerer option is explicitly passed.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-examples/-/merge_requests/31>

webrtc/sendrecv/gst/webrtc-sendrecv.c

@@ -285,11 +285,12 @@ on_offer_created (GstPromise * promise, gpointer user_data)
 static void
 on_negotiation_needed (GstElement * element, gpointer user_data)
 {
+  gboolean create_offer = GPOINTER_TO_INT (user_data);
   app_state = PEER_CALL_NEGOTIATING;
 
-  if (remote_is_offerer || our_id) {
+  if (remote_is_offerer) {
     soup_websocket_connection_send_text (ws_conn, "OFFER_REQUEST");
-  } else {
+  } else if (create_offer) {
     GstPromise *promise =
         gst_promise_new_with_change_func (on_offer_created, NULL, NULL);
     g_signal_emit_by_name (webrtc1, "create-offer", NULL, promise);
@@ -372,7 +373,7 @@ on_ice_gathering_state_notify (GstElement * webrtcbin, GParamSpec * pspec,
 }
 
 static gboolean
-start_pipeline (void)
+start_pipeline (gboolean create_offer)
 {
   GstStateChangeReturn ret;
   GError *error = NULL;
@@ -397,7 +398,7 @@ start_pipeline (void)
   /* This is the gstwebrtc entry point where we create the offer and so on. It
    * will be called when the pipeline goes to PLAYING. */
   g_signal_connect (webrtc1, "on-negotiation-needed",
-      G_CALLBACK (on_negotiation_needed), NULL);
+      G_CALLBACK (on_negotiation_needed), GINT_TO_POINTER (create_offer));
   /* We need to transmit this ICE candidate to the browser via the websockets
    * signalling server. Incoming ice candidates from the browser need to be
    * added by us too, see on_server_message() */
@@ -601,7 +602,17 @@ on_server_message (SoupWebsocketConnection * conn, SoupWebsocketDataType type,
 
     app_state = PEER_CONNECTED;
     /* Start negotiation (exchange SDP and ICE candidates) */
-    if (!start_pipeline ())
+    if (!start_pipeline (TRUE))
+      cleanup_and_quit_loop ("ERROR: failed to start pipeline",
+          PEER_CALL_ERROR);
+  } else if (g_strcmp0 (text, "OFFER_REQUEST") == 0) {
+    if (app_state != SERVER_REGISTERED) {
+      gst_printerr ("Received OFFER_REQUEST at a strange time, ignoring\n");
+      goto out;
+    }
+    gst_print ("Received OFFER_REQUEST, sending offer\n");
+    /* Peer wants us to start negotiation (exchange SDP and ICE candidates) */
+    if (!start_pipeline (TRUE))
       cleanup_and_quit_loop ("ERROR: failed to start pipeline",
           PEER_CALL_ERROR);
   } else if (g_str_has_prefix (text, "ERROR")) {
@@ -645,7 +656,7 @@ on_server_message (SoupWebsocketConnection * conn, SoupWebsocketDataType type,
     /* If peer connection wasn't made yet and we are expecting peer will
      * connect to us, launch pipeline at this moment */
     if (!webrtc1 && our_id) {
-      if (!start_pipeline ()) {
+      if (!start_pipeline (FALSE)) {
         cleanup_and_quit_loop ("ERROR: failed to start pipeline",
             PEER_CALL_ERROR);
       }

webrtc/sendrecv/js/index.html

@@ -31,6 +31,9 @@
       <label for="peer-connect">Enter peer id</label>
       <input id="peer-connect" type="text" name="text">
       <input id="peer-connect-button" onclick="onConnectClicked();" type="button" value="Connect">
+      <!-- Request the peer to send the offer by sending the OFFER_REQUEST message.
+        Same as the -​-remote-offerer flag in the sendrecv C example -->
+      <input id="remote-offerer" type="checkbox" autocomplete="off"><span>Remote offerer</span>
     </div>
 
     <div>Our id is <b id="peer-id">unknown</b></div>

webrtc/sendrecv/js/webrtc.js

@@ -29,6 +29,10 @@ function setConnectButtonState(value) {
     document.getElementById("peer-connect-button").value = value;
 }
 
+function wantRemoteOfferer() {
+   return document.getElementById("remote-offerer").checked;
+}
+
 function onConnectClicked() {
     if (document.getElementById("peer-connect-button").value == "Disconnect") {
         resetState();
@@ -137,6 +141,11 @@ function onServerMessage(event) {
             return;
         case "SESSION_OK":
             setStatus("Starting negotiation");
+            if (wantRemoteOfferer()) {
+                ws_conn.send("OFFER_REQUEST");
+                setStatus("Sent OFFER_REQUEST, waiting for offer");
+                return;
+            }
             if (!peer_connection)
                 createCall(null).then (generateOffer);
             return;