rtp/recv: support pushing buffer lists from the jitterbuffer

Multiple concurrent buffers produced by the jitterbuffer will be
combined into a single buffer list which will be sent downstream.

Events or queries that interrupt the buffer flow will cause a split in
the output buffer list.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1618>

net/rtp/src/rtpbin2/rtprecv.rs

@@ -55,6 +55,7 @@ impl Default for Settings {
 struct JitterBufferStream {
     store: Arc<Mutex<JitterBufferStore>>,
     sleep: Pin<Box<tokio::time::Sleep>>,
+    pending_item: Option<JitterBufferItem>,
 }
 
 impl JitterBufferStream {
@@ -62,6 +63,7 @@ impl JitterBufferStream {
         Self {
             store,
             sleep: Box::pin(tokio::time::sleep(Duration::from_secs(1))),
+            pending_item: None,
         }
     }
 }
@@ -70,13 +72,20 @@ impl futures::stream::Stream for JitterBufferStream {
     type Item = JitterBufferItem;
 
     fn poll_next(
-        self: Pin<&mut Self>,
+        mut self: Pin<&mut Self>,
         cx: &mut std::task::Context<'_>,
     ) -> std::task::Poll<Option<Self::Item>> {
         let now = Instant::now();
         let mut lowest_wait = None;
 
+        if let Some(item) = self.pending_item.take() {
+            return Poll::Ready(Some(item));
+        }
+
         let mut jitterbuffer_store = self.store.lock().unwrap();
+        let mut pending_item = None;
+        let mut next_pending_item = None;
+        loop {
             let ret = jitterbuffer_store.jitterbuffer.poll(now);
             gst::trace!(CAT, "jitterbuffer poll ret: {ret:?}");
             match ret {
@@ -88,7 +97,6 @@ impl futures::stream::Stream for JitterBufferStream {
                         .store
                         .remove(&id)
                         .unwrap_or_else(|| panic!("Buffer with id {id} not in store!"));
-                cx.waker().wake_by_ref();
                 }
                 jitterbuffer::PollResult::Forward { id, discont } => {
                     let mut item = jitterbuffer_store
@@ -102,20 +110,64 @@ impl futures::stream::Stream for JitterBufferStream {
                             packet_mut.set_flags(gst::BufferFlags::DISCONT);
                         }
                     }
+
+                    match item {
+                        // we don't currently push packet lists into the jitterbuffer
+                        JitterBufferItem::PacketList(_list) => unreachable!(),
+                        // forward events and queries as-is
+                        JitterBufferItem::Event(_) | JitterBufferItem::Query(_, _) => {
+                            if pending_item.is_some() {
+                                // but only after sending the previous pending item
+                                next_pending_item = Some(item);
+                                break
+                            }
+                        }
+                        JitterBufferItem::Packet(ref packet) => {
+                            match pending_item {
+                                Some(JitterBufferItem::Event(_) | JitterBufferItem::Query(_, _)) => unreachable!(),
+                                Some(JitterBufferItem::Packet(pending_buffer)) => {
+                                    let mut list = gst::BufferList::new();
+                                    let list_mut = list.make_mut();
+                                    list_mut.add(pending_buffer);
+                                    list_mut.add(packet.clone());
+                                    pending_item = Some(JitterBufferItem::PacketList(list));
+                                }
+                                Some(JitterBufferItem::PacketList(mut pending_list)) => {
+                                    let list_mut = pending_list.make_mut();
+                                    list_mut.add(packet.clone());
+                                    pending_item = Some(JitterBufferItem::PacketList(pending_list));
+                                }
+                                None => {
+                                    pending_item = Some(item);
+                                }
+                            }
+                            continue;
+                        }
+                    }
                     return Poll::Ready(Some(item));
                 }
                 jitterbuffer::PollResult::Timeout(timeout) => {
                     if lowest_wait.map_or(true, |lowest_wait| timeout < lowest_wait) {
                         lowest_wait = Some(timeout);
                     }
+                    break;
                 }
                 // Will be woken up when necessary
-            jitterbuffer::PollResult::Empty => (),
+                jitterbuffer::PollResult::Empty => break,
+            }
         }
 
         jitterbuffer_store.waker = Some(cx.waker().clone());
         drop(jitterbuffer_store);
 
+        if next_pending_item.is_some() {
+            self.pending_item = next_pending_item;
+        }
+
+        if pending_item.is_some() {
+            return Poll::Ready(pending_item);
+        }
+
         if let Some(timeout) = lowest_wait {
             let this = self.get_mut();
             this.sleep.as_mut().reset(timeout.into());
@@ -131,6 +183,7 @@ impl futures::stream::Stream for JitterBufferStream {
 #[derive(Debug)]
 enum JitterBufferItem {
     Packet(gst::Buffer),
+    PacketList(gst::BufferList),
     Event(gst::Event),
     Query(
         std::ptr::NonNull<gst::QueryRef>,
@@ -297,6 +350,13 @@ impl RecvSession {
                 let mut stream = JitterBufferStream::new(store);
                 while let Some(item) = stream.next().await {
                     match item {
+                        JitterBufferItem::PacketList(list) => {
+                            let flow = pad.push_list(list);
+                            gst::trace!(CAT, obj: pad, "Pushed buffer list, flow ret {:?}", flow);
+                            let mut recv_flow_combiner = recv_flow_combiner.lock().unwrap();
+                            let _combined_flow = recv_flow_combiner.update_pad_flow(&pad, flow);
+                            // TODO: store flow, return only on session pads?
+                        }
                         JitterBufferItem::Packet(buffer) => {
                             let flow = pad.push(buffer);
                             gst::trace!(CAT, obj: pad, "Pushed buffer, flow ret {:?}", flow);