Replace lock with checker in TransportFeedbackDemuxer

Bug: webrtc:13517, webrtc:11993
Change-Id: If44c7a7428454c0bf5a6fee6d12d816e3dc8e27c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/248163
Reviewed-by: Niels Moller <nisse@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35772}

modules/congestion_controller/rtp/BUILD.gn

@@ -61,6 +61,7 @@ rtc_library("transport_feedback") {
     "../../../rtc_base:rtc_base_approved",
     "../../../rtc_base/network:sent_packet",
     "../../../rtc_base/synchronization:mutex",
+    "../../../rtc_base/system:no_unique_address",
     "../../../system_wrappers",
     "../../../system_wrappers:field_trial",
     "../../rtp_rtcp:rtp_rtcp_format",

modules/congestion_controller/rtp/transport_feedback_demuxer.cc

@@ -15,10 +15,17 @@ namespace webrtc {
 namespace {
 static const size_t kMaxPacketsInHistory = 5000;
 }
+
+TransportFeedbackDemuxer::TransportFeedbackDemuxer() {
+  // In case the construction thread is different from where the registration
+  // and callbacks occur, detach from the construction thread.
+  observer_checker_.Detach();
+}
+
 void TransportFeedbackDemuxer::RegisterStreamFeedbackObserver(
     std::vector<uint32_t> ssrcs,
     StreamFeedbackObserver* observer) {
-  MutexLock lock(&observers_lock_);
+  RTC_DCHECK_RUN_ON(&observer_checker_);
   RTC_DCHECK(observer);
   RTC_DCHECK(absl::c_find_if(observers_, [=](const auto& pair) {
                return pair.second == observer;
@@ -28,7 +35,7 @@ void TransportFeedbackDemuxer::RegisterStreamFeedbackObserver(
 
 void TransportFeedbackDemuxer::DeRegisterStreamFeedbackObserver(
     StreamFeedbackObserver* observer) {
-  MutexLock lock(&observers_lock_);
+  RTC_DCHECK_RUN_ON(&observer_checker_);
   RTC_DCHECK(observer);
   const auto it = absl::c_find_if(
       observers_, [=](const auto& pair) { return pair.second == observer; });
@@ -65,14 +72,14 @@ void TransportFeedbackDemuxer::OnTransportFeedback(
       if (it != history_.end()) {
         auto packet_info = it->second;
         packet_info.received = packet.received();
-        stream_feedbacks.push_back(packet_info);
+        stream_feedbacks.push_back(std::move(packet_info));
         if (packet.received())
           history_.erase(it);
       }
     }
   }
 
-  MutexLock lock(&observers_lock_);
+  RTC_DCHECK_RUN_ON(&observer_checker_);
   for (auto& observer : observers_) {
     std::vector<StreamFeedbackObserver::StreamPacketInfo> selected_feedback;
     for (const auto& packet_info : stream_feedbacks) {

modules/congestion_controller/rtp/transport_feedback_demuxer.h

@@ -14,14 +14,27 @@
 #include <utility>
 #include <vector>
 
+#include "api/sequence_checker.h"
 #include "modules/include/module_common_types_public.h"
 #include "modules/rtp_rtcp/include/rtp_rtcp_defines.h"
 #include "rtc_base/synchronization/mutex.h"
+#include "rtc_base/system/no_unique_address.h"
 
 namespace webrtc {
 
-class TransportFeedbackDemuxer : public StreamFeedbackProvider {
+// Implementation of StreamFeedbackProvider that provides a way for
+// implementations of StreamFeedbackObserver to register for feedback callbacks
+// for a given set of SSRCs.
+// Registration methods need to be called from the same execution context
+// (thread or task queue) and callbacks to
+// StreamFeedbackObserver::OnPacketFeedbackVector will be made in that same
+// context.
+// TODO(tommi): This appears to be the only implementation of this interface.
+// Do we need the interface?
+class TransportFeedbackDemuxer final : public StreamFeedbackProvider {
  public:
+  TransportFeedbackDemuxer();
+
   // Implements StreamFeedbackProvider interface
   void RegisterStreamFeedbackObserver(
       std::vector<uint32_t> ssrcs,
@@ -40,9 +53,9 @@ class TransportFeedbackDemuxer : public StreamFeedbackProvider {
   // Maps a set of ssrcs to corresponding observer. Vectors are used rather than
   // set/map to ensure that the processing order is consistent independently of
   // the randomized ssrcs.
-  Mutex observers_lock_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker observer_checker_;
   std::vector<std::pair<std::vector<uint32_t>, StreamFeedbackObserver*>>
-      observers_ RTC_GUARDED_BY(&observers_lock_);
+      observers_ RTC_GUARDED_BY(&observer_checker_);
 };
 }  // namespace webrtc