chore: drop write-only _InFlight.cancelled_by_peer; add assertion to notify-drop test

cancelled_by_peer was set by _dispatch_notification but never read; the
peer-vs-outer-cancel distinction in _handle_request relies on
scope.cancel_called alone (and works because nothing else cancels the
per-request scope).

test_runner_on_notify_drops_before_init_and_unknown_methods now
registers a handler and asserts only the post-init notification reaches
it (was assertionless before).

Author: maxisbey

src/mcp/shared/jsonrpc_dispatcher.py

@@ -102,7 +102,6 @@ class _InFlight(Generic[TransportT]):
 
     scope: anyio.CancelScope
     dctx: _JSONRPCDispatchContext[TransportT]
-    cancelled_by_peer: bool = False
 
 
 @dataclass
@@ -489,7 +488,6 @@ def _dispatch_notification(
         if msg.method == "notifications/cancelled":
             match msg.params:
                 case {"requestId": str() | int() as rid} if (in_flight := self._in_flight.get(rid)) is not None:
-                    in_flight.cancelled_by_peer = True
                     in_flight.dctx.cancel_requested.set()
                     if self._peer_cancel_mode == "interrupt":
                         in_flight.scope.cancel()

tests/server/test_runner.py

@@ -255,11 +255,19 @@ async def on_level(ctx: Ctx, params: SetLevelRequestParams) -> None:
 
 @pytest.mark.anyio
 async def test_runner_on_notify_drops_before_init_and_unknown_methods(server: SrvT):
+    seen: list[Any] = []
+
+    async def on_roots(ctx: Ctx, params: NotificationParams | None) -> None:
+        seen.append(params)
+
+    server.add_notification_handler("notifications/roots/list_changed", NotificationParams, on_roots)
     async with connected_runner(server, initialized=False) as (client, _):
         await client.notify("notifications/roots/list_changed", None)  # before init: dropped
         await client.notify("notifications/initialized", None)
         await client.notify("notifications/unknown", None)  # no handler: dropped
-    # No exception raised; both drops are silent.
+        await client.notify("notifications/roots/list_changed", None)  # post-init: delivered
+        await anyio.wait_all_tasks_blocked()
+    assert seen == [None]  # only the post-init one reached the handler
 
 
 @pytest.mark.anyio