Respect the @cancellable flag for RestServlets and BaseFederationServlets (#12699)

Both `RestServlet`s and `BaseFederationServlet`s register their handlers
with `HttpServer.register_paths` / `JsonResource.register_paths`. Update
`JsonResource` to respect the `@cancellable` flag on handlers registered
in this way.

Although `ReplicationEndpoint` also registers itself using
`register_paths`, it does not pass the handler method that would have the
`@cancellable` flag directly, and so needs separate handling.

Signed-off-by: Sean Quah <seanq@element.io>

tests/federation/transport/server/test__base.py

@@ -0,0 +1,112 @@
+# Copyright 2022 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from http import HTTPStatus
+from typing import Dict, List, Tuple
+
+from synapse.api.errors import Codes
+from synapse.federation.transport.server import BaseFederationServlet
+from synapse.federation.transport.server._base import Authenticator
+from synapse.http.server import JsonResource, cancellable
+from synapse.server import HomeServer
+from synapse.types import JsonDict
+from synapse.util.ratelimitutils import FederationRateLimiter
+
+from tests import unittest
+from tests.http.server._base import EndpointCancellationTestHelperMixin
+
+
+class CancellableFederationServlet(BaseFederationServlet):
+    PATH = "/sleep"
+
+    def __init__(
+        self,
+        hs: HomeServer,
+        authenticator: Authenticator,
+        ratelimiter: FederationRateLimiter,
+        server_name: str,
+    ):
+        super().__init__(hs, authenticator, ratelimiter, server_name)
+        self.clock = hs.get_clock()
+
+    @cancellable
+    async def on_GET(
+        self, origin: str, content: None, query: Dict[bytes, List[bytes]]
+    ) -> Tuple[int, JsonDict]:
+        await self.clock.sleep(1.0)
+        return HTTPStatus.OK, {"result": True}
+
+    async def on_POST(
+        self, origin: str, content: JsonDict, query: Dict[bytes, List[bytes]]
+    ) -> Tuple[int, JsonDict]:
+        await self.clock.sleep(1.0)
+        return HTTPStatus.OK, {"result": True}
+
+
+class BaseFederationServletCancellationTests(
+    unittest.FederatingHomeserverTestCase, EndpointCancellationTestHelperMixin
+):
+    """Tests for `BaseFederationServlet` cancellation."""
+
+    path = f"{CancellableFederationServlet.PREFIX}{CancellableFederationServlet.PATH}"
+
+    def create_test_resource(self):
+        """Overrides `HomeserverTestCase.create_test_resource`."""
+        resource = JsonResource(self.hs)
+
+        CancellableFederationServlet(
+            hs=self.hs,
+            authenticator=Authenticator(self.hs),
+            ratelimiter=self.hs.get_federation_ratelimiter(),
+            server_name=self.hs.hostname,
+        ).register(resource)
+
+        return resource
+
+    def test_cancellable_disconnect(self) -> None:
+        """Test that handlers with the `@cancellable` flag can be cancelled."""
+        channel = self.make_signed_federation_request(
+            "GET", self.path, await_result=False
+        )
+
+        # Advance past all the rate limiting logic. If we disconnect too early, the
+        # request won't be processed.
+        self.pump()
+
+        self._test_disconnect(
+            self.reactor,
+            channel,
+            expect_cancellation=True,
+            expected_body={"error": "Request cancelled", "errcode": Codes.UNKNOWN},
+        )
+
+    def test_uncancellable_disconnect(self) -> None:
+        """Test that handlers without the `@cancellable` flag cannot be cancelled."""
+        channel = self.make_signed_federation_request(
+            "POST",
+            self.path,
+            content={},
+            await_result=False,
+        )
+
+        # Advance past all the rate limiting logic. If we disconnect too early, the
+        # request won't be processed.
+        self.pump()
+
+        self._test_disconnect(
+            self.reactor,
+            channel,
+            expect_cancellation=False,
+            expected_body={"result": True},
+        )