Respect the `@cancellable` flag for `ReplicationEndpoint`s (#12700)

While `ReplicationEndpoint`s register themselves via `JsonResource`,
they pass a method that calls the handler, instead of the handler itself,
to `register_paths`. As a result, `JsonResource` will not correctly pick
up the `@cancellable` flag and we have to apply it ourselves.

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

4 files changed, 139 insertions, 2 deletions

diff --git a/changelog.d/12700.misc b/changelog.d/12700.misc
new file mode 100644
index 0000000000..d93eb5dada
--- /dev/null
+++ b/changelog.d/12700.misc
@@ -0,0 +1 @@
+Respect the `@cancellable` flag for `ReplicationEndpoint`s.
diff --git a/synapse/replication/http/_base.py b/synapse/replication/http/_base.py
index 2bd244ed79..a4ae4040c3 100644
--- a/synapse/replication/http/_base.py
+++ b/synapse/replication/http/_base.py
@@ -26,7 +26,8 @@ from twisted.web.server import Request
 
 from synapse.api.errors import HttpResponseException, SynapseError
 from synapse.http import RequestTimedOutError
-from synapse.http.server import HttpServer
+from synapse.http.server import HttpServer, is_method_cancellable
+from synapse.http.site import SynapseRequest
 from synapse.logging import opentracing
 from synapse.logging.opentracing import trace
 from synapse.types import JsonDict
@@ -310,6 +311,12 @@ class ReplicationEndpoint(metaclass=abc.ABCMeta):
         url_args = list(self.PATH_ARGS)
         method = self.METHOD
 
+        if self.CACHE and is_method_cancellable(self._handle_request):
+            raise Exception(
+                f"{self.__class__.__name__} has been marked as cancellable, but CACHE "
+                "is set. The cancellable flag would have no effect."
+            )
+
         if self.CACHE:
             url_args.append("txn_id")
 
@@ -324,7 +331,7 @@ class ReplicationEndpoint(metaclass=abc.ABCMeta):
         )
 
     async def _check_auth_and_handle(
-        self, request: Request, **kwargs: Any
+        self, request: SynapseRequest, **kwargs: Any
     ) -> Tuple[int, JsonDict]:
         """Called on new incoming requests when caching is enabled. Checks
         if there is a cached response for the request and returns that,
@@ -340,8 +347,18 @@ class ReplicationEndpoint(metaclass=abc.ABCMeta):
         if self.CACHE:
             txn_id = kwargs.pop("txn_id")
 
+            # We ignore the `@cancellable` flag, since cancellation wouldn't interupt
+            # `_handle_request` and `ResponseCache` does not handle cancellation
+            # correctly yet. In particular, there may be issues to do with logging
+            # context lifetimes.
+
             return await self.response_cache.wrap(
                 txn_id, self._handle_request, request, **kwargs
             )
 
+        # The `@cancellable` decorator may be applied to `_handle_request`. But we
+        # told `HttpServer.register_paths` that our handler is `_check_auth_and_handle`,
+        # so we have to set up the cancellable flag ourselves.
+        request.is_render_cancellable = is_method_cancellable(self._handle_request)
+
         return await self._handle_request(request, **kwargs)
diff --git a/tests/replication/http/__init__.py b/tests/replication/http/__init__.py
new file mode 100644
index 0000000000..3a5f22c022
--- /dev/null
+++ b/tests/replication/http/__init__.py
@@ -0,0 +1,13 @@
+# 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.
diff --git a/tests/replication/http/test__base.py b/tests/replication/http/test__base.py
new file mode 100644
index 0000000000..a5ab093a27
--- /dev/null
+++ b/tests/replication/http/test__base.py
@@ -0,0 +1,106 @@
+# 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 Tuple
+
+from twisted.web.server import Request
+
+from synapse.api.errors import Codes
+from synapse.http.server import JsonResource, cancellable
+from synapse.replication.http import REPLICATION_PREFIX
+from synapse.replication.http._base import ReplicationEndpoint
+from synapse.server import HomeServer
+from synapse.types import JsonDict
+
+from tests import unittest
+from tests.http.server._base import EndpointCancellationTestHelperMixin
+
+
+class CancellableReplicationEndpoint(ReplicationEndpoint):
+    NAME = "cancellable_sleep"
+    PATH_ARGS = ()
+    CACHE = False
+
+    def __init__(self, hs: HomeServer):
+        super().__init__(hs)
+        self.clock = hs.get_clock()
+
+    @staticmethod
+    async def _serialize_payload() -> JsonDict:
+        return {}
+
+    @cancellable
+    async def _handle_request(  # type: ignore[override]
+        self, request: Request
+    ) -> Tuple[int, JsonDict]:
+        await self.clock.sleep(1.0)
+        return HTTPStatus.OK, {"result": True}
+
+
+class UncancellableReplicationEndpoint(ReplicationEndpoint):
+    NAME = "uncancellable_sleep"
+    PATH_ARGS = ()
+    CACHE = False
+
+    def __init__(self, hs: HomeServer):
+        super().__init__(hs)
+        self.clock = hs.get_clock()
+
+    @staticmethod
+    async def _serialize_payload() -> JsonDict:
+        return {}
+
+    async def _handle_request(  # type: ignore[override]
+        self, request: Request
+    ) -> Tuple[int, JsonDict]:
+        await self.clock.sleep(1.0)
+        return HTTPStatus.OK, {"result": True}
+
+
+class ReplicationEndpointCancellationTestCase(
+    unittest.HomeserverTestCase, EndpointCancellationTestHelperMixin
+):
+    """Tests for `ReplicationEndpoint` cancellation."""
+
+    def create_test_resource(self):
+        """Overrides `HomeserverTestCase.create_test_resource`."""
+        resource = JsonResource(self.hs)
+
+        CancellableReplicationEndpoint(self.hs).register(resource)
+        UncancellableReplicationEndpoint(self.hs).register(resource)
+
+        return resource
+
+    def test_cancellable_disconnect(self) -> None:
+        """Test that handlers with the `@cancellable` flag can be cancelled."""
+        path = f"{REPLICATION_PREFIX}/{CancellableReplicationEndpoint.NAME}/"
+        channel = self.make_request("POST", path, await_result=False)
+        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."""
+        path = f"{REPLICATION_PREFIX}/{UncancellableReplicationEndpoint.NAME}/"
+        channel = self.make_request("POST", path, await_result=False)
+        self._test_disconnect(
+            self.reactor,
+            channel,
+            expect_cancellation=False,
+            expected_body={"result": True},
+        )