From 79503740d0cb18ce94a767b3e7d7a835aa7ca75d Mon Sep 17 00:00:00 2001
From: "Ilya (Marshal)" <ilya@marshal.dev>
Date: Wed, 30 Sep 2026 22:33:55 +0200
Subject: [PATCH] Settle set_parameters when libwebrtc drops the callback

---
 .../cpp/src/interfaces/rtc_rtp_sender.cpp     | 50 ++++++++++++++++---
 tests/test_rtp_sender_receiver.py             | 11 ++++
 2 files changed, 54 insertions(+), 7 deletions(-)

diff --git a/python-webrtc/cpp/src/interfaces/rtc_rtp_sender.cpp b/python-webrtc/cpp/src/interfaces/rtc_rtp_sender.cpp
index 709a8f0..d9bc00b 100644
--- a/python-webrtc/cpp/src/interfaces/rtc_rtp_sender.cpp
+++ b/python-webrtc/cpp/src/interfaces/rtc_rtp_sender.cpp
@@ -8,6 +8,10 @@
 #include "rtc_rtp_sender.h"
 #include "../utils/python_callback.h"
 
+#include <functional>
+#include <memory>
+#include <utility>
+
 #include <pybind11/functional.h>
 #include <pybind11/stl.h>
 
@@ -17,6 +21,43 @@
 
 namespace python_webrtc {
 
+  namespace {
+
+    // libwebrtc drops the callback of SetParametersAsync without calling it when the sender has no media channel
+    // (like after a rollback of its offer) or loses it meanwhile: rejects like it does once the channel is gone
+    class SetParametersCompletion final {
+    public:
+      SetParametersCompletion(std::function<void()> onSuccess, std::function<void(RTCCallbackException)> onFailure)
+          : _onSuccess(std::move(onSuccess)), _onFailure(std::move(onFailure)) {}
+      SetParametersCompletion(const SetParametersCompletion &) = delete;
+      SetParametersCompletion(SetParametersCompletion &&) = delete;
+      SetParametersCompletion &operator=(const SetParametersCompletion &) = delete;
+      SetParametersCompletion &operator=(SetParametersCompletion &&) = delete;
+
+      ~SetParametersCompletion() {
+        if (_onFailure) {
+          _onFailure(RTCCallbackException(webrtc::RTCErrorType::INVALID_STATE,
+                                          "The sender was detached before the parameters were set"));
+        }
+      }
+
+      void operator()(webrtc::RTCError error) {
+        auto onSuccess = std::exchange(_onSuccess, nullptr);
+        auto onFailure = std::exchange(_onFailure, nullptr);
+        if (error.ok()) {
+          onSuccess();
+        } else {
+          onFailure(RTCCallbackException(std::move(error)));
+        }
+      }
+
+    private:
+      std::function<void()> _onSuccess;
+      std::function<void(RTCCallbackException)> _onFailure;
+    };
+
+  } // namespace
+
   RTCRtpSender::RTCRtpSender(std::shared_ptr<PeerConnectionFactory> factory,
                              webrtc::scoped_refptr<webrtc::RtpSenderInterface> sender)
       : _factory(std::move(factory)), _sender(std::move(sender)) {}
@@ -184,13 +225,8 @@ namespace python_webrtc {
     current.transaction_id = fresh.transaction_id;
     // read-only: getParameters() shows the negotiated codecs this side can send, libwebrtc takes its own list
     current.codecs = fresh.codecs;
-    _sender->SetParametersAsync(current, [onSuccess, onFailure](webrtc::RTCError error) {
-      if (error.ok()) {
-        onSuccess();
-      } else {
-        onFailure(RTCCallbackException(std::move(error)));
-      }
-    });
+    _sender->SetParametersAsync(current, [completion = std::make_unique<SetParametersCompletion>(onSuccess, onFailure)](
+                                             webrtc::RTCError error) { (*completion)(std::move(error)); });
   }
 
   bool RTCRtpSender::ReplaceTrack(std::optional<std::reference_wrapper<MediaStreamTrack>> track) {
diff --git a/tests/test_rtp_sender_receiver.py b/tests/test_rtp_sender_receiver.py
index 6fd1b1f..c7d9a78 100644
--- a/tests/test_rtp_sender_receiver.py
+++ b/tests/test_rtp_sender_receiver.py
@@ -7,6 +7,7 @@
 
 """Senders and receivers: capabilities, parameters, codecs, tracks, DTMF and synchronization sources."""
 
+import asyncio
 import time
 
 import pytest
@@ -187,6 +188,16 @@ async def test_set_parameters_key_frames(caller, callee):
     await sender.set_parameters(sender.get_parameters(), key_frames=[True])
 
 
+@pytest.mark.asyncio
+async def test_set_parameters_after_rollback(pc):
+    """A sender rolled back out of its offer has no media channel: setting parameters rejects, not hangs"""
+    sender = pc.add_transceiver(webrtc.MediaType.video).sender
+    await pc.set_local_description()
+    await pc.set_local_description({'type': 'rollback'})
+    with pytest.raises(webrtc.InvalidStateError):
+        await asyncio.wait_for(sender.set_parameters(sender.get_parameters()), 5)
+
+
 @pytest.mark.asyncio
 async def test_simulcast_receiver_parameters(caller, callee):
     """The receiver of simulcast has the negotiated codecs and header extensions"""