Fix server streaming handler not cancelled on client disconnect

When a client disconnects during a server streaming RPC, the async
generator continued yielding indefinitely because receive() was never
consulted after the initial request was consumed.

Fix this by spawning a background task in the `EndpointServerStream`
case that monitors receive() for http.disconnect. When detected, an
Event is set; the streaming loop checks it between yields and raises
ConnectError(CANCELED) if set. The response stream is also explicitly
`aclose()`'d in the finally block so generator finally-clauses run
promptly rather than being deferred to GC.

Fixes #174.

Signed-off-by: Stefan VanBuren <svanburen@buf.build>

Author: stefanvanburen

src/connectrpc/_server_async.py

@@ -4,7 +4,7 @@
 import functools
 import inspect
 from abc import ABC, abstractmethod
-from asyncio import CancelledError, sleep
+from asyncio import CancelledError, Event, create_task, sleep
 from dataclasses import replace
 from http import HTTPStatus
 from typing import TYPE_CHECKING, Generic, TypeVar, cast
@@ -385,6 +385,9 @@ async def _handle_stream(
                 self._read_max_bytes,
             )
 
+            disconnect_detected: Event | None = None
+            monitor_task = None
+
             match endpoint:
                 case EndpointUnary():
                     request = await _consume_single_request(request_stream)
@@ -396,22 +399,48 @@ async def _handle_stream(
                 case EndpointServerStream():
                     request = await _consume_single_request(request_stream)
                     response_stream = endpoint.function(request, ctx)
+
+                    # The request has been fully consumed; monitor receive() for a
+                    # client disconnect so we can stop streaming promptly.
+                    disconnect_detected = Event()
+
+                    async def _watch_for_disconnect() -> None:
+                        while True:
+                            msg = await receive()
+                            if msg["type"] == "http.disconnect":
+                                disconnect_detected.set()
+                                return
+
+                    monitor_task = create_task(_watch_for_disconnect())
                 case EndpointBidiStream():
                     response_stream = endpoint.function(request_stream, ctx)
 
-            async for message in response_stream:
-                # Don't send headers until the first message to allow logic a chance to add
-                # response headers.
-                if not sent_headers:
-                    await _send_stream_response_headers(
-                        send, protocol, codec, resp_compression.name(), ctx
+            try:
+                async for message in response_stream:
+                    if disconnect_detected is not None and disconnect_detected.is_set():
+                        raise ConnectError(Code.CANCELED, "Client disconnected")
+                    # Don't send headers until the first message to allow logic a chance to add
+                    # response headers.
+                    if not sent_headers:
+                        await _send_stream_response_headers(
+                            send, protocol, codec, resp_compression.name(), ctx
+                        )
+                        sent_headers = True
+
+                    body = writer.write(message)
+                    await send(
+                        {"type": "http.response.body", "body": body, "more_body": True}
                     )
-                    sent_headers = True
-
-                body = writer.write(message)
-                await send(
-                    {"type": "http.response.body", "body": body, "more_body": True}
-                )
+            finally:
+                # Explicitly close the stream so that any generator finally-blocks
+                # run promptly (Python defers async-generator cleanup to GC otherwise).
+                await response_stream.aclose()
+                if monitor_task is not None:
+                    monitor_task.cancel()
+                    try:
+                        await monitor_task
+                    except (CancelledError, Exception):
+                        pass
         except CancelledError as e:
             raise ConnectError(Code.CANCELED, "Request was cancelled") from e
         except Exception as e:

test/test_roundtrip.py

@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+import asyncio
+import struct
 from typing import TYPE_CHECKING
 
 import pytest
@@ -280,3 +282,64 @@ async def request_stream():
         else:
             assert len(requests) == 2
             assert len(responses) == 1
+
+
+@pytest.mark.asyncio
+async def test_server_stream_client_disconnect() -> None:
+    """Server streaming generator should be closed when the client disconnects.
+
+    Regression test for https://github.com/connectrpc/connect-python/issues/174.
+    """
+    generator_closed = asyncio.Event()
+
+    class InfiniteHaberdasher(Haberdasher):
+        async def make_similar_hats(self, request, ctx):
+            try:
+                while True:
+                    yield Hat(size=request.inches, color="green")
+                    await asyncio.sleep(0)  # yield control to event loop
+            finally:
+                generator_closed.set()
+
+    app = HaberdasherASGIApplication(InfiniteHaberdasher())
+
+    # Encode a Connect protocol (application/connect+proto) request for Size(inches=10).
+    request_bytes = Size(inches=10).SerializeToString()
+    request_body = struct.pack(">BI", 0, len(request_bytes)) + request_bytes
+
+    disconnect_trigger = asyncio.Event()
+    response_count = 0
+    call_count = 0
+
+    async def receive():
+        nonlocal call_count
+        call_count += 1
+        if call_count == 1:
+            return {"type": "http.request", "body": request_body, "more_body": False}
+        # Block until the test is ready to simulate a disconnect.
+        await disconnect_trigger.wait()
+        return {"type": "http.disconnect"}
+
+    async def send(message):
+        nonlocal response_count
+        if message.get("type") == "http.response.body" and message.get("more_body", False):
+            response_count += 1
+            if response_count >= 3:
+                disconnect_trigger.set()
+
+    scope = {
+        "type": "http",
+        "method": "POST",
+        "path": "/connectrpc.example.Haberdasher/MakeSimilarHats",
+        "query_string": b"",
+        "root_path": "",
+        "headers": [
+            (b"content-type", b"application/connect+proto"),
+        ],
+    }
+
+    # Without the fix the app hangs forever (generator never stopped), causing a
+    # TimeoutError here.  With the fix it terminates promptly after the disconnect.
+    await asyncio.wait_for(app(scope, receive, send), timeout=5.0)
+
+    assert generator_closed.is_set(), "generator should be closed after client disconnect"