Fix SDP stream ID mismatch issue when a track's stream changes.

webrtc/pc/mediasession.cc

 /*
  *  Copyright 2004 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 430 matching lines @@
 
   const bool include_flexfec_stream =
       ContainsFlexfecCodec(content_description->codecs());
 
   MediaSessionOptions::Streams::const_iterator stream_it;
   for (stream_it = streams.begin();
        stream_it != streams.end(); ++stream_it) {
     if (stream_it->type != media_type)
       continue;  // Wrong media type.
 
-    const StreamParams* param =
-        GetStreamByIds(*current_streams, "", stream_it->id);
+    StreamParams* param = GetStreamByIds(*current_streams, "", stream_it->id);

[pthatcher1, 2017/04/13 21:31:21]

Instead of making this non-const and adding all this functions to return a
non-const, why not just make a new non-const StreamParams from the const one and
then add that to the content_description down below?  

[Taylor Brandstetter, 2017/04/14 15:52:08]

Just because this way is more convenient/efficient, and I don't see any reason
to not return non-const pointers.

     // groupid is empty for StreamParams generated using
     // MediaSessionDescriptionFactory.
     if (!param) {
       // This is a new stream.
       std::vector<uint32_t> ssrcs;
       GenerateSsrcs(*current_streams, stream_it->num_sim_layers, &ssrcs);
       StreamParams stream_param;
       stream_param.id = stream_it->id;
       // Add the generated ssrc.
       for (size_t i = 0; i < ssrcs.size(); ++i) {
@@ skipping 30 matching lines @@
         }
       }
       stream_param.cname = options.rtcp_cname;
       stream_param.sync_label = stream_it->sync_label;
       content_description->AddStream(stream_param);
 
       // Store the new StreamParams in current_streams.
       // This is necessary so that we can use the CNAME for other media types.
       current_streams->push_back(stream_param);
     } else {
+      // Use existing generated SSRCs/groups, but update the sync_label and
+      // cname if necessary.

[pthatcher1, 2017/04/13 21:31:22]

Can you explain why this is necessary in the comment?

Also, why does the cname come from options instead of stream_it->cname?

[Taylor Brandstetter, 2017/04/14 15:52:08]

Done. And there is no stream_it->cname.

+      param->cname = options.rtcp_cname;
+      param->sync_label = stream_it->sync_label;
       content_description->AddStream(*param);
     }
   }
   return true;
 }
 
 // Updates the transport infos of the |sdesc| according to the given
 // |bundle_group|. The transport infos of the content names within the
 // |bundle_group| should be updated to use the ufrag, pwd and DTLS role of the
 // first content within the |bundle_group|.
@@ skipping 1670 matching lines @@
       GetFirstMediaContentDescription(sdesc, MEDIA_TYPE_VIDEO));
 }
 
 DataContentDescription* GetFirstDataContentDescription(
     SessionDescription* sdesc) {
   return static_cast<DataContentDescription*>(
       GetFirstMediaContentDescription(sdesc, MEDIA_TYPE_DATA));
 }
 
 }  // namespace cricket