webrtcsrc: ensure source pad has msid when added

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1800>

net/webrtc/src/webrtcsrc/imp.rs

@@ -351,6 +351,7 @@ impl Session {
             n_audio_pads: AtomicU16::new(0),
             flow_combiner: Mutex::new(gst_base::UniqueFlowCombiner::new()),
             request_counter: 0,
+            pending_srcpads: HashMap::new(),
         })
     }
 
@@ -370,6 +371,18 @@ impl Session {
         })
     }
 
+    // Maps the `webrtcbin` pad to our exposed source pad using the pad stream ID.
+    fn take_pending_src_pad(
+        &mut self,
+        webrtcbin_src: &gst::Pad,
+    ) -> Option<(WebRTCSrcPad, gst::Caps)> {
+        self.get_stream_id(
+            Some(webrtcbin_src.property::<gst_webrtc::WebRTCRTPTransceiver>("transceiver")),
+            None,
+        )
+        .and_then(|stream_id| self.pending_srcpads.remove(&stream_id))
+    }
+
     // Maps the `webrtcbin` pad to our exposed source pad using the pad stream ID.
     fn get_src_pad_from_webrtcbin_pad(
         &self,
@@ -469,13 +482,13 @@ impl Session {
     //      - otherwise, encoded filter's srcpad, if requested
     //      - otherwise, webrtcbin's src pad.
     fn handle_webrtc_src_pad(
-        &self,
+        &mut self,
         bin: &gst::Bin,
         webrtcbin_pad: &gst::Pad,
         element: &super::BaseWebRTCSrc,
     ) -> gst::GhostPad {
-        let srcpad = self.get_src_pad_from_webrtcbin_pad(webrtcbin_pad, element);
-        if let Some(ref srcpad) = srcpad {
+        let srcpad_and_caps = self.take_pending_src_pad(webrtcbin_pad);
+        if let Some((ref srcpad, ref caps)) = srcpad_and_caps {
             let stream_id = srcpad.imp().stream_id();
             let mut builder = gst::event::StreamStart::builder(&stream_id);
             if let Some(stream_start) = webrtcbin_pad.sticky_event::<gst::event::StreamStart>(0) {
@@ -493,6 +506,34 @@ impl Session {
             webrtcbin_pad.store_sticky_event(&builder.build()).ok();
 
             srcpad.imp().set_webrtc_pad(webrtcbin_pad.downgrade());
+
+            element
+                .add_pad(srcpad)
+                .expect("Adding ghost pad should never fail");
+            let media_type = caps
+                .structure(0)
+                .expect("Passing empty caps is invalid")
+                .get::<&str>("media")
+                .expect("Only caps with a `media` field are expected when creating the pad");
+
+            let raw_caps = if media_type == "video" {
+                VIDEO_CAPS.to_owned()
+            } else if media_type == "audio" {
+                AUDIO_CAPS.to_owned()
+            } else {
+                unreachable!()
+            };
+
+            let caps_with_raw = [caps.clone(), raw_caps.clone()]
+                .into_iter()
+                .collect::<gst::Caps>();
+
+            let downstream_caps = srcpad.peer_query_caps(Some(&caps_with_raw));
+            if let Some(first_struct) = downstream_caps.structure(0) {
+                if first_struct.has_name(raw_caps.structure(0).unwrap().name()) {
+                    srcpad.imp().set_needs_decoding(true)
+                }
+            }
         }
 
         let ghostpad = gst::GhostPad::builder(gst::PadDirection::Src)
@@ -601,7 +642,7 @@ impl Session {
             );
         }
 
-        if let Some(srcpad) = srcpad {
+        if let Some((srcpad, _)) = srcpad_and_caps {
             let signaller = element.imp().signaller();
 
             // Signalers like WhipServer do not need a peer producer id as they run as a server
@@ -727,7 +768,7 @@ impl Session {
     }
 
     fn handle_offer(
-        &self,
+        &mut self,
         offer: &gst_webrtc::WebRTCSessionDescription,
         element: &super::BaseWebRTCSrc,
     ) -> (gst::Promise, gst::Bin) {
@@ -1109,8 +1150,8 @@ impl BaseWebRTCSrc {
                             assert_eq!(desc.type_(), gst_webrtc::WebRTCSDPType::Offer);
                             let this = instance.imp();
                             gst::info!(CAT, imp = this, "got sdp offer");
-                            let state = this.state.lock().unwrap();
-                            let Some(session) = state.sessions.get(session_id) else {
+                            let mut state = this.state.lock().unwrap();
+                            let Some(session) = state.sessions.get_mut(session_id) else {
                                 gst::error!(CAT, imp = this, "session {session_id:?} not found");
                                 return;
                             };
@@ -1165,7 +1206,7 @@ impl BaseWebRTCSrc {
         &self,
         caps: &gst::Caps,
         stream_id: &str,
-        session: &Session,
+        session: &mut Session,
     ) -> bool {
         gst::log!(CAT, "Creating pad for {caps:?}, stream: {stream_id}");
 
@@ -1176,7 +1217,7 @@ impl BaseWebRTCSrc {
             .get::<&str>("media")
             .expect("Only caps with a `media` field are expected when creating the pad");
 
-        let (template, name, raw_caps) = if media_type == "video" {
+        let (template, name) = if media_type == "video" {
             (
                 obj.pad_template("video_%s_%u").unwrap(),
                 format!(
@@ -1184,7 +1225,6 @@ impl BaseWebRTCSrc {
                     session.id,
                     session.n_video_pads.fetch_add(1, Ordering::SeqCst)
                 ),
-                VIDEO_CAPS.to_owned(),
             )
         } else if media_type == "audio" {
             (
@@ -1194,7 +1234,6 @@ impl BaseWebRTCSrc {
                     session.id,
                     session.n_audio_pads.fetch_add(1, Ordering::SeqCst)
                 ),
-                AUDIO_CAPS.to_owned(),
             )
         } else {
             gst::info!(
@@ -1206,24 +1245,15 @@ impl BaseWebRTCSrc {
             return false;
         };
 
-        let caps_with_raw = [caps.clone(), raw_caps.clone()]
-            .into_iter()
-            .collect::<gst::Caps>();
         let ghost = gst::GhostPad::builder_from_template(&template)
             .name(name)
             .build()
             .downcast::<WebRTCSrcPad>()
             .unwrap();
         ghost.imp().set_stream_id(stream_id);
-        obj.add_pad(&ghost)
-            .expect("Adding ghost pad should never fail");
-
-        let downstream_caps = ghost.peer_query_caps(Some(&caps_with_raw));
-        if let Some(first_struct) = downstream_caps.structure(0) {
-            if first_struct.has_name(raw_caps.structure(0).unwrap().name()) {
-                ghost.imp().set_needs_decoding(true)
-            }
-        }
+        session
+            .pending_srcpads
+            .insert(stream_id.to_string(), (ghost.clone(), caps.clone()));
 
         true
     }
@@ -1603,6 +1633,7 @@ struct Session {
     n_audio_pads: AtomicU16,
     flow_combiner: Mutex<gst_base::UniqueFlowCombiner>,
     request_counter: u64,
+    pending_srcpads: HashMap<String, (WebRTCSrcPad, gst::Caps)>,
 }
 struct State {
     sessions: HashMap<String, Session>,

net/webrtc/src/webrtcsrc/pad.rs

@@ -57,18 +57,14 @@ impl ObjectImpl for WebRTCSrcPad {
     }
     fn property(&self, _id: usize, pspec: &glib::ParamSpec) -> glib::Value {
         match pspec.name() {
-            "msid" => {
-                let msid = self
+            "msid" => self
                 .webrtcbin_pad
                 .lock()
                 .unwrap()
                 .as_ref()
                 .and_then(|p| p.upgrade())
-                    .map(|p| p.property::<String>("msid"))
-                    .unwrap_or_else(|| String::from(""));
-
-                msid.to_value()
-            }
+                .and_then(|p| p.property::<Option<String>>("msid"))
+                .to_value(),
             name => panic!("no readable property {name:?}"),
         }
     }