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Fail pending send_raw_request waiters when the read stream yields an exception by chaucerj · Pull Request #3402 · modelcontextprotocol/python-sdk · GitHub

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28 changes: 21 additions & 7 deletions src/mcp/shared/jsonrpc_dispatcher.py
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters. Learn more about bidirectional Unicode characters
Original file line number Diff line number Diff line change
Expand Up @@ -120,8 +120,8 @@ def cancelled_request_id_from_params(params: Mapping[str, Any] | None) -> Reques
class _Pending:
"""An outbound request awaiting its response."""

send: MemoryObjectSendStream[dict[str, Any] | ErrorData]
receive: MemoryObjectReceiveStream[dict[str, Any] | ErrorData]
send: MemoryObjectSendStream[dict[str, Any] | ErrorData | Exception]
receive: MemoryObjectReceiveStream[dict[str, Any] | ErrorData | Exception]
on_progress: ProgressFnT | None = None


Expand Down Expand Up @@ -329,6 +329,8 @@ async def send_raw_request(
MCPError: Peer error response; `REQUEST_TIMEOUT` if
`opts["timeout"]` elapsed; `CONNECTION_CLOSED` if the
transport closed or the dispatcher shut down.
Exception: The read stream yielded an exception (transport
fault) while awaiting; re-raised as-is.
RuntimeError: Called before `run()`.
"""
# Post-close sends get the same CONNECTION_CLOSED contract as in-flight waiters.
Expand Down Expand Up @@ -363,7 +365,7 @@ async def send_raw_request(

# buffer=1: a close signal can arrive before the waiter parks in receive();
# a WouldBlock later just means the waiter already has its one outcome.
send, receive = anyio.create_memory_object_stream[dict[str, Any] | ErrorData](1)
send, receive = anyio.create_memory_object_stream[dict[str, Any] | ErrorData | Exception](1)
pending = _Pending(send=send, receive=receive, on_progress=on_progress)
self._pending[pending_key] = pending

Expand Down Expand Up @@ -442,6 +444,10 @@ async def send_raw_request(

if isinstance(outcome, ErrorData):
raise MCPError(code=outcome.code, message=outcome.message, data=outcome.data)
if isinstance(outcome, Exception):
# Read stream faulted mid-await: re-raise the transport's exception
# as-is so callers see the original type (e.g. httpx.ReadTimeout).
raise outcome
return outcome

async def notify(
Expand Down Expand Up @@ -536,6 +542,9 @@ async def _dispatch(
are awaited; any other `await` would head-of-line block the read loop.
"""
if isinstance(item, Exception):
# No response can arrive over a faulted transport: fail the pending
# waiters now instead of parking them until their timeout elapses.
self._fail_pending(item)
if self.on_stream_exception is None:
logger.debug("transport yielded exception: %r", item)
return
Expand Down Expand Up @@ -686,14 +695,19 @@ def _spawn(
self._tg.start_soon(fn, *args)

def _fan_out_closed(self) -> None:
"""Wake every pending `send_raw_request` waiter with `CONNECTION_CLOSED`.
"""Wake every pending `send_raw_request` waiter with `CONNECTION_CLOSED`."""
self._fail_pending(ErrorData(code=CONNECTION_CLOSED, message="Connection closed"))

Synchronous: callers may be inside a cancelled scope. Idempotent.
def _fail_pending(self, outcome: ErrorData | Exception) -> None:
"""Wake every pending `send_raw_request` waiter with `outcome`.

`CONNECTION_CLOSED` on EOF, the transport's exception on a faulted
read stream. Synchronous: callers may be inside a cancelled scope.
Idempotent.
"""
closed = ErrorData(code=CONNECTION_CLOSED, message="Connection closed")
for pending in self._pending.values():
try:
pending.send.send_nowait(closed)
pending.send.send_nowait(outcome)
except (anyio.WouldBlock, anyio.BrokenResourceError, anyio.ClosedResourceError):
pass
self._pending.clear()
Expand Down
51 changes: 49 additions & 2 deletions tests/shared/test_jsonrpc_dispatcher.py
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters. Learn more about bidirectional Unicode characters
Original file line number Diff line number Diff line change
Expand Up @@ -316,6 +316,53 @@ async def caller() -> None:
s.close()


@pytest.mark.anyio
@pytest.mark.parametrize("anyio_backend", ["asyncio", "trio"], indirect=True)
async def test_send_raw_request_raises_transport_exception_yielded_mid_await():
"""A blocked send_raw_request is woken with the transport's own exception, not parked
until its timeout elapses; the dispatcher keeps serving once the stream recovers (#1401)."""
c2s_send, c2s_recv = anyio.create_memory_object_stream[SessionMessage | Exception](32)
s2c_send, s2c_recv = anyio.create_memory_object_stream[SessionMessage | Exception](32)
client: JSONRPCDispatcher[TransportContext] = JSONRPCDispatcher(s2c_recv, c2s_send)
server: JSONRPCDispatcher[TransportContext] = JSONRPCDispatcher(c2s_recv, s2c_send)
release_first = anyio.Event()

async def server_on_request(ctx: DCtx, method: str, params: Mapping[str, Any] | None) -> dict[str, Any]:
# Park the first request so the caller is mid-await when the fault lands.
await release_first.wait()
return {"echoed": method, "params": {}}

async def on_notify(ctx: DCtx, method: str, params: Mapping[str, Any] | None) -> None:
raise NotImplementedError

fault_consumed = anyio.Event()

async def caller() -> None:
with pytest.raises(RuntimeError, match="transport fault"):
await client.send_raw_request("ping", None)
fault_consumed.set()

try:
async with anyio.create_task_group() as tg:
await tg.start(client.run, *echo_handlers(Recorder()))
await tg.start(server.run, server_on_request, on_notify)

tg.start_soon(caller)
await anyio.sleep(0)
# Fault the client's read side mid-await. The buffered send yields no
# checkpoint, so wait for the waiter to consume the fault first.
await s2c_send.send(RuntimeError("transport fault"))
await fault_consumed.wait()
release_first.set() # the parked first response arrives late and is dropped
# The stream stays open, so a later round-trip must still work.
assert await client.send_raw_request("ping", None) == {"echoed": "ping", "params": {}}
s2c_send.close() # EOF both read streams so run() loops exit and the tg joins
c2s_send.close()
finally:
for s in (c2s_send, c2s_recv, s2c_send, s2c_recv):
s.close()


@pytest.mark.anyio
async def test_run_returns_cleanly_when_read_stream_receive_end_is_closed():
"""Iterating a closed receive end is EOF, not a crash (stateless SHTTP closes it during teardown)."""
Expand Down Expand Up @@ -1826,7 +1873,7 @@ def test_resolve_pending_drops_outcome_when_waiter_stream_already_closed():
c2s_send, c2s_recv = anyio.create_memory_object_stream[SessionMessage | Exception](1)
s2c_send, s2c_recv = anyio.create_memory_object_stream[SessionMessage | Exception](1)
d: JSONRPCDispatcher[TransportContext] = JSONRPCDispatcher(s2c_recv, c2s_send)
send, recv = anyio.create_memory_object_stream[dict[str, Any] | ErrorData](1)
send, recv = anyio.create_memory_object_stream[dict[str, Any] | ErrorData | Exception](1)
d._pending[1] = _Pending(send=send, receive=recv) # pyright: ignore[reportPrivateUsage]
recv.close() # waiter gone - send_nowait will raise BrokenResourceError
d._resolve_pending(1, {"late": True}) # pyright: ignore[reportPrivateUsage]
Expand All @@ -1839,7 +1886,7 @@ def test_fan_out_closed_drops_signal_when_waiter_already_has_outcome():
c2s_send, c2s_recv = anyio.create_memory_object_stream[SessionMessage | Exception](1)
s2c_send, s2c_recv = anyio.create_memory_object_stream[SessionMessage | Exception](1)
d: JSONRPCDispatcher[TransportContext] = JSONRPCDispatcher(s2c_recv, c2s_send)
send, recv = anyio.create_memory_object_stream[dict[str, Any] | ErrorData](1)
send, recv = anyio.create_memory_object_stream[dict[str, Any] | ErrorData | Exception](1)
d._pending[1] = _Pending(send=send, receive=recv) # pyright: ignore[reportPrivateUsage]
send.send_nowait({"real": "result"})
d._fan_out_closed() # pyright: ignore[reportPrivateUsage]
Expand Down
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