Fix channel worker stalling when stopping ffmpeg with unread stdout

src/restreamer/ffmpeg_runner.py

@@ -74,6 +74,10 @@ class FfmpegStreamProcess:
         if self._process is None or self._process.returncode is not None:
             return
 
+        # wait() only returns once every pipe hits EOF, so unread stdout left over
+        # from the consumer breaking out of iter_chunks() must be drained, or it hangs forever.
+        stdout_drain_task = asyncio.create_task(self._drain_stdout())
+
         self._process.terminate()
         try:
             await asyncio.wait_for(self._process.wait(), timeout=FFMPEG_TERMINATE_TIMEOUT_SECONDS)
@@ -82,9 +86,16 @@ class FfmpegStreamProcess:
             self._process.kill()
             await self._process.wait()
 
+        await stdout_drain_task
         if self._stderr_task is not None:
             await self._stderr_task
 
+    async def _drain_stdout(self) -> None:
+        if self._process is None or self._process.stdout is None:
+            return
+        while await self._process.stdout.read(FFMPEG_READ_CHUNK_SIZE):
+            pass
+
     async def _drain_stderr(self) -> None:
         if self._process is None or self._process.stderr is None:
             return