Fix rtcp_sender to support sdes with 31 chunk

webrtc/modules/rtp_rtcp/source/rtcp_sender.cc

Index: webrtc/modules/rtp_rtcp/source/rtcp_sender.cc
diff --git a/webrtc/modules/rtp_rtcp/source/rtcp_sender.cc b/webrtc/modules/rtp_rtcp/source/rtcp_sender.cc
index 5f2eb759c96937cccb48d44f421e4f8a22a4c534..186f4e980016c18399dd216ce93518c605604c8c 100644
--- a/webrtc/modules/rtp_rtcp/source/rtcp_sender.cc
+++ b/webrtc/modules/rtp_rtcp/source/rtcp_sender.cc
@@ -327,7 +327,8 @@ int32_t RTCPSender::AddMixedCNAME(uint32_t SSRC, const char* c_name) {
   RTC_DCHECK(c_name);
   RTC_DCHECK_LT(strlen(c_name), RTCP_CNAME_SIZE);
   rtc::CritScope lock(&critical_section_rtcp_sender_);
-  if (csrc_cnames_.size() >= kRtpCsrcSize)
+  // One spot is reserved for ssrc_/cname_.
+  if (csrc_cnames_.size() >= rtcp::Sdes::kMaxNumberOfChunks - 1)

[nisse-webrtc, 2017/03/16 13:42:44]

I'd need a bit more context to understand what's going on. 

I've had a quick look at RFC 3550. So kRtpCsrsSize == 15 is the maximum number
of csrs in an RTP header, i.e., a limit of the number of streams that can be
mixed together in a single RTP packet.

SDES is the source description RTCP message (RFC3550), and there can be at most
31 chunks, of which 30 can be CSRSs (and you can have even more, put into
additional SDES packets). Why would one have a larger number here? We have 50
sources to describe, even though at any point in time the mixer includes only a
few of them?

And what are we using the cname attribute for?

[danilchap, 2017/03/16 13:58:02]

imagine 30 participant meeting with server-side audio mixing.
Sdes list all 30 participiant, and each rtp packet include csrcs of the
currently speaking people (thus it need place for only few csrcs). Using csrcs
client can show who was actually speaking.
cname is a way to identify the participant. same cname might be mapped to
several srcs (e.g. 1 audio, 3 video ssrcs).

     return -1;
 
   csrc_cnames_[SSRC] = c_name;
@@ -463,8 +464,8 @@ std::unique_ptr<rtcp::RtcpPacket> RTCPSender::BuildSDES(
   rtcp::Sdes* sdes = new rtcp::Sdes();
   sdes->AddCName(ssrc_, cname_);
 
-  for (const auto it : csrc_cnames_)
-    sdes->AddCName(it.first, it.second);
+  for (const auto& it : csrc_cnames_)
+    RTC_CHECK(sdes->AddCName(it.first, it.second));
 
   return std::unique_ptr<rtcp::RtcpPacket>(sdes);
 }