From f94bddf72cefe1abd30b54474d774ff08dfd181e Mon Sep 17 00:00:00 2001
From: Harald Alvestrand <hta@webrtc.org>
Date: Sun, 12 Jan 2025 13:44:44 +0000
Subject: [PATCH] Split PushNewMediaChannelAndDeleteChannel

This admits to the fact that a transceiver's channel can't change, it's just
either created or deleted.

Bug: webrtc:42224170
Change-Id: I9a44bf0c0bace74eda6cdf1a1d6967eb8c697594
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/372380
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#43713}
---
 pc/rtp_transceiver.cc | 41 ++++++++++++++++++++++++++---------------
 pc/rtp_transceiver.h  |  8 +++++---
 2 files changed, 31 insertions(+), 18 deletions(-)

diff --git a/pc/rtp_transceiver.cc b/pc/rtp_transceiver.cc
index 6f4b5ae20c..788b49145e 100644
--- a/pc/rtp_transceiver.cc
+++ b/pc/rtp_transceiver.cc
@@ -334,7 +334,7 @@ void RtpTransceiver::SetChannel(
               SafeTask(std::move(flag), [this]() { OnFirstPacketSent(); }));
         });
   });
-  PushNewMediaChannelAndDeleteChannel(nullptr);
+  PushNewMediaChannel();
 
   RTC_DCHECK_BLOCK_COUNT_NO_MORE_THAN(2);
 }
@@ -358,35 +358,46 @@ void RtpTransceiver::ClearChannel() {
   });
 
   RTC_DCHECK_BLOCK_COUNT_NO_MORE_THAN(1);
-  PushNewMediaChannelAndDeleteChannel(std::move(channel_));
+  DeleteChannel();
 
   RTC_DCHECK_BLOCK_COUNT_NO_MORE_THAN(2);
 }
 
-void RtpTransceiver::PushNewMediaChannelAndDeleteChannel(
-    std::unique_ptr<cricket::ChannelInterface> channel_to_delete) {
-  // The clumsy combination of pushing down media channel and deleting
-  // the channel is due to the desire to do both things in one Invoke().
-  if (!channel_to_delete && senders_.empty() && receivers_.empty()) {
+void RtpTransceiver::PushNewMediaChannel() {
+  RTC_DCHECK(channel_);
+  if (senders_.empty() && receivers_.empty()) {
     return;
   }
   context()->worker_thread()->BlockingCall([&]() {
-    // Push down the new media_channel, if any, otherwise clear it.
-    auto* media_send_channel =
-        channel_ ? channel_->media_send_channel() : nullptr;
+    // Push down the new media_channel.
+    auto* media_send_channel = channel_->media_send_channel();
     for (const auto& sender : senders_) {
       sender->internal()->SetMediaChannel(media_send_channel);
     }
 
-    auto* media_receive_channel =
-        channel_ ? channel_->media_receive_channel() : nullptr;
+    auto* media_receive_channel = channel_->media_receive_channel();
     for (const auto& receiver : receivers_) {
       receiver->internal()->SetMediaChannel(media_receive_channel);
     }
+  });
+}
 
-    // Destroy the channel, if we had one, now _after_ updating the receivers
-    // who might have had references to the previous channel.
-    channel_to_delete = nullptr;
+void RtpTransceiver::DeleteChannel() {
+  RTC_DCHECK(channel_);
+  // Ensure that channel_ is not reachable via transceiver, but is deleted
+  // only after clearing the references in senders_ and receivers_.
+  context()->worker_thread()->BlockingCall([&]() {
+    auto channel_to_delete = std::move(channel_);
+    // Clear the media channel reference from senders and receivers.
+    for (const auto& sender : senders_) {
+      sender->internal()->SetMediaChannel(nullptr);
+    }
+    for (const auto& receiver : receivers_) {
+      receiver->internal()->SetMediaChannel(nullptr);
+    }
+    // The channel is destroyed here, on the worker thread as it needs to
+    // be.
+    channel_to_delete.reset();
   });
 }
 
diff --git a/pc/rtp_transceiver.h b/pc/rtp_transceiver.h
index d9c38c1fce..e544cece34 100644
--- a/pc/rtp_transceiver.h
+++ b/pc/rtp_transceiver.h
@@ -304,10 +304,12 @@ class RtpTransceiver : public RtpTransceiverInterface {
   void OnFirstPacketReceived();
   void OnFirstPacketSent();
   void StopSendingAndReceiving();
-  // Delete a channel, and ensure that references to its media channel
+  // Tell the senders and receivers about possibly-new media channels
+  // in a newly created `channel_`.
+  void PushNewMediaChannel();
+  // Delete `channel_`, and ensure that references to its media channels
   // are updated before deleting it.
-  void PushNewMediaChannelAndDeleteChannel(
-      std::unique_ptr<cricket::ChannelInterface> channel_to_delete);
+  void DeleteChannel();
 
   // Enforce that this object is created, used and destroyed on one thread.
   TaskQueueBase* const thread_;