webrtc: add all SSRC attributes getting CAPS for a PT

The transport stream only returned the CAPS for the first matching PT entry
from the `ptmap`. Other SSRC with the same PT where not included. For a stream
which bundled multiple audio streams for instance, only the first SSRC was
knowed to the SSRC demux and downstream elements.

This commit adds all the `ssrc-` attributes from the matching PT entries.

The RTP jitter buffer can now find the CNAME corresponding its SSRC even if it
was not the first to be registered for a particular PT.

The RTP PT demux removes `ssrc-*` attributes cooresponding to other SSRCs
before pushing SSRC specific CAPS to downstream elements.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/6119>

subprojects/gst-plugins-bad/ext/webrtc/gstwebrtcbin.c

@@ -5122,6 +5122,8 @@ _set_internal_rtpbin_element_props_from_stream (GstWebRTCBin * webrtc,
 
     GST_LOG_OBJECT (stream, "setting rtx mapping: %s -> %u", apt, rtx_pt[i]);
     gst_structure_set (pt_map, apt, G_TYPE_UINT, rtx_pt[i], NULL);
+
+    gst_caps_unref (rtx_caps);
   }
 
   GST_DEBUG_OBJECT (stream, "setting payload map on %" GST_PTR_FORMAT " : %"
@@ -7446,8 +7448,7 @@ on_rtpbin_request_pt_map (GstElement * rtpbin, guint session_id, guint pt,
   if (!stream)
     goto unknown_session;
 
-  if ((ret = transport_stream_get_caps_for_pt (stream, pt)))
-    gst_caps_ref (ret);
+  ret = transport_stream_get_caps_for_pt (stream, pt);
 
   GST_DEBUG_OBJECT (webrtc, "Found caps %" GST_PTR_FORMAT " for pt %d in "
       "session %d", ret, pt, session_id);

subprojects/gst-plugins-bad/ext/webrtc/transportstream.c

@@ -47,15 +47,37 @@ enum
 GstCaps *
 transport_stream_get_caps_for_pt (TransportStream * stream, guint pt)
 {
-  guint i, len;
+  GstCaps *ret = NULL;
+  GstStructure *ret_s, *item_s;
+  guint i, len, si, slen;
+  const gchar *field_name;
 
   len = stream->ptmap->len;
   for (i = 0; i < len; i++) {
     PtMapItem *item = &g_array_index (stream->ptmap, PtMapItem, i);
-    if (item->pt == pt)
-      return item->caps;
+    if (item->pt == pt) {
+      if (item->caps) {
+        if (ret == NULL) {
+          ret = gst_caps_copy (item->caps);
+          ret_s = gst_caps_get_structure (ret, 0);
+        } else {
+          /* Append the "ssrc-*" fields for current PT entry to ret */
+          item_s = gst_caps_get_structure (item->caps, 0);
+          slen = gst_structure_n_fields (item_s);
+          for (si = 0; si < slen; ++si) {
+            field_name = gst_structure_nth_field_name (item_s, si);
+            if (!g_str_has_prefix (field_name, "ssrc-"))
+              continue;
+
+            gst_structure_set (ret_s, field_name, G_TYPE_STRING,
+                gst_structure_get_string (item_s, field_name), NULL);
+          }
+        }
+      }
+    }
   }
-  return NULL;
+
+  return ret;
 }
 
 int

subprojects/gst-plugins-good/gst/rtpmanager/gstrtpptdemux.c

@@ -312,6 +312,29 @@ gst_rtp_pt_demux_finalize (GObject * object)
   G_OBJECT_CLASS (parent_class)->finalize (object);
 }
 
+/* Removes "ssrc-*" attributes matching other SSRCs. */
+static gboolean
+_filter_ssrc (GQuark field_id, GValue * value, gpointer ssrc)
+{
+  const gchar *field_name = g_quark_to_string (field_id);
+
+  if (!g_str_has_prefix (field_name, "ssrc-"))
+    return TRUE;
+
+  gchar *endptr;
+  guint32 field_ssrc = g_ascii_strtoll (field_name + 5, &endptr, 10);
+
+  if (!endptr || *endptr != '-')
+    return TRUE;
+
+  /* Found a valid "ssrc-*" */
+  if (field_ssrc != *(guint32 *) ssrc)
+    /* Not the expected SSRC => remove this field */
+    return FALSE;
+
+  return TRUE;
+}
+
 static GstCaps *
 gst_rtp_pt_demux_get_caps (GstRtpPtDemux * rtpdemux, guint pt)
 {
@@ -349,7 +372,12 @@ gst_rtp_pt_demux_get_caps (GstRtpPtDemux * rtpdemux, guint pt)
   }
 
   if (caps != NULL) {
+    GstStructure *s;
+
     caps = gst_caps_make_writable (caps);
+    s = gst_caps_get_structure (caps, 0);
+    gst_structure_filter_and_map_in_place (s, _filter_ssrc, &ssrc);
+
     gst_caps_set_simple (caps, "payload", G_TYPE_INT, pt, NULL);
     if (have_ssrc)
       gst_caps_set_simple (caps, "ssrc", G_TYPE_UINT, ssrc, NULL);