webrtc: In all blocking pad probes except for sink pads also handle serialized events

Otherwise it can happen that e.g. the stream-start event is tried to be
sent as part of pushing the first buffer. Downstream might not be in
PAUSED/PLAYING yet, so the event is rejected with GST_FLOW_FLUSHING and
because it's an event would not cause the blocking pad probe to trigger
first. This would then return GST_FLOW_FLUSHING for the buffer and shut
down all of upstream.

To solve this we return GST_PAD_PROBE_DROP for all events. In case of
sticky events they would be resent again later once we unblocked after
blocking on the buffer and everything works fine.

Don't handle events specifically in sink pad blocking pad probes as here
downstream is not linked yet and we are actually waiting for the
following CAPS event before unblocking can happen.

Fixes https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/issues/1172

ext/webrtc/gstwebrtcbin.c

@@ -1857,8 +1857,19 @@ _on_sctp_notify_dtls_state (GstWebRTCDTLSTransport * transport,
 }
 
 static GstPadProbeReturn
-pad_block (GstPad * pad, GstPadProbeInfo * info, gpointer unused)
+sctp_pad_block (GstPad * pad, GstPadProbeInfo * info, gpointer unused)
 {
+  /* Drop all events: we don't care about them and don't want to block on
+   * them. Sticky events would be forwarded again later once we unblock
+   * and we don't want to forward them here already because that might
+   * cause a spurious GST_FLOW_FLUSHING */
+  if (GST_IS_EVENT (info->data))
+    return GST_PAD_PROBE_DROP;
+
+  /* But block on any actual data-flow so we don't accidentally send that
+   * to a pad that is not ready yet, causing GST_FLOW_FLUSHING and everything
+   * to silently stop.
+   */
   GST_LOG_OBJECT (pad, "blocking pad with data %" GST_PTR_FORMAT, info->data);
 
   return GST_PAD_PROBE_OK;
@@ -1912,9 +1923,8 @@ _get_or_create_data_channel_transports (GstWebRTCBin * webrtc, guint session_id)
           "data_src");
       sctp_transport->sctpdec_block_id =
           gst_pad_add_probe (receive_srcpad,
-          GST_PAD_PROBE_TYPE_BLOCK |
+          GST_PAD_PROBE_TYPE_BLOCK | GST_PAD_PROBE_TYPE_DATA_DOWNSTREAM,
-          GST_PAD_PROBE_TYPE_BUFFER | GST_PAD_PROBE_TYPE_BUFFER_LIST,
+          (GstPadProbeCallback) sctp_pad_block, NULL, NULL);
-          (GstPadProbeCallback) pad_block, NULL, NULL);
       gst_object_unref (receive_srcpad);
     }
 
@@ -5457,6 +5467,14 @@ gst_webrtc_bin_change_state (GstElement * element, GstStateChange transition)
   return ret;
 }
 
+static GstPadProbeReturn
+sink_pad_block (GstPad * pad, GstPadProbeInfo * info, gpointer unused)
+{
+  GST_LOG_OBJECT (pad, "blocking pad with data %" GST_PTR_FORMAT, info->data);
+
+  return GST_PAD_PROBE_OK;
+}
+
 static GstPad *
 gst_webrtc_bin_request_new_pad (GstElement * element, GstPadTemplate * templ,
     const gchar * name, const GstCaps * caps)
@@ -5494,7 +5512,7 @@ gst_webrtc_bin_request_new_pad (GstElement * element, GstPadTemplate * templ,
 
     pad->block_id = gst_pad_add_probe (GST_PAD (pad), GST_PAD_PROBE_TYPE_BLOCK |
         GST_PAD_PROBE_TYPE_BUFFER | GST_PAD_PROBE_TYPE_BUFFER_LIST,
-        (GstPadProbeCallback) pad_block, NULL, NULL);
+        (GstPadProbeCallback) sink_pad_block, NULL, NULL);
     webrtc->priv->pending_sink_transceivers =
         g_list_append (webrtc->priv->pending_sink_transceivers,
         gst_object_ref (pad));

ext/webrtc/transportreceivebin.c

@@ -101,7 +101,17 @@ _receive_state_to_string (ReceiveState state)
 static GstPadProbeReturn
 pad_block (GstPad * pad, GstPadProbeInfo * info, TransportReceiveBin * receive)
 {
+  /* Drop all events: we don't care about them and don't want to block on
+   * them. Sticky events would be forwarded again later once we unblock
+   * and we don't want to forward them here already because that might
+   * cause a spurious GST_FLOW_FLUSHING */
+  if (GST_IS_EVENT (info->data))
+    return GST_PAD_PROBE_DROP;
 
+  /* But block on any actual data-flow so we don't accidentally send that
+   * to a pad that is not ready yet, causing GST_FLOW_FLUSHING and everything
+   * to silently stop.
+   */
   GST_LOG_OBJECT (pad, "blocking pad with data %" GST_PTR_FORMAT, info->data);
 
   return GST_PAD_PROBE_OK;
@@ -222,7 +232,7 @@ transport_receive_bin_change_state (GstElement * element,
       receive->rtp_block->block_id =
           gst_pad_add_probe (pad,
           GST_PAD_PROBE_TYPE_BLOCK |
-          GST_PAD_PROBE_TYPE_BUFFER | GST_PAD_PROBE_TYPE_BUFFER_LIST,
+          GST_PAD_PROBE_TYPE_DATA_DOWNSTREAM,
           (GstPadProbeCallback) pad_block, receive, NULL);
       gst_object_unref (pad);
 
@@ -238,7 +248,7 @@ transport_receive_bin_change_state (GstElement * element,
       receive->rtcp_block->block_id =
           gst_pad_add_probe (pad,
           GST_PAD_PROBE_TYPE_BLOCK |
-          GST_PAD_PROBE_TYPE_BUFFER | GST_PAD_PROBE_TYPE_BUFFER_LIST,
+          GST_PAD_PROBE_TYPE_DATA_DOWNSTREAM,
           (GstPadProbeCallback) pad_block, receive, NULL);
       gst_object_unref (pad);
 

ext/webrtc/transportsendbin.c

@@ -146,6 +146,17 @@ transport_send_bin_get_property (GObject * object, guint prop_id,
 static GstPadProbeReturn
 pad_block (GstPad * pad, GstPadProbeInfo * info, gpointer unused)
 {
+  /* Drop all events: we don't care about them and don't want to block on
+   * them. Sticky events would be forwarded again later once we unblock
+   * and we don't want to forward them here already because that might
+   * cause a spurious GST_FLOW_FLUSHING */
+  if (GST_IS_EVENT (info->data))
+    return GST_PAD_PROBE_DROP;
+
+  /* But block on any actual data-flow so we don't accidentally send that
+   * to a pad that is not ready yet, causing GST_FLOW_FLUSHING and everything
+   * to silently stop.
+   */
   GST_LOG_OBJECT (pad, "blocking pad with data %" GST_PTR_FORMAT, info->data);
 
   return GST_PAD_PROBE_OK;
@@ -165,9 +176,8 @@ block_peer_pad (GstElement * elem, const gchar * pad_name)
   peer = gst_pad_get_peer (pad);
   block = _create_pad_block (elem, peer, 0, NULL, NULL);
   block->block_id = gst_pad_add_probe (peer,
-      GST_PAD_PROBE_TYPE_BLOCK | GST_PAD_PROBE_TYPE_BUFFER |
+      GST_PAD_PROBE_TYPE_BLOCK | GST_PAD_PROBE_TYPE_DATA_DOWNSTREAM,
-      GST_PAD_PROBE_TYPE_BUFFER_LIST, (GstPadProbeCallback) pad_block, NULL,
+      (GstPadProbeCallback) pad_block, NULL, NULL);
-      NULL);
   gst_object_unref (pad);
   gst_object_unref (peer);
   return block;