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diff --git a/synapse/util/cancellation.py b/synapse/util/cancellation.py
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+# 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 typing import Any, Callable, TypeVar
+
+F = TypeVar("F", bound=Callable[..., Any])
+
+
+def cancellable(function: F) -> F:
+    """Marks a function as cancellable.
+
+    Servlet methods with this decorator will be cancelled if the client disconnects before we
+    finish processing the request.
+
+    Although this annotation is particularly useful for servlet methods, it's also
+    useful for intermediate functions, where it documents the fact that the function has
+    been audited for cancellation safety and needs to preserve that.
+    This then simplifies auditing new functions that call those same intermediate
+    functions.
+
+    During cancellation, `Deferred.cancel()` will be invoked on the `Deferred` wrapping
+    the method. The `cancel()` call will propagate down to the `Deferred` that is
+    currently being waited on. That `Deferred` will raise a `CancelledError`, which will
+    propagate up, as per normal exception handling.
+
+    Before applying this decorator to a new function, you MUST recursively check
+    that all `await`s in the function are on `async` functions or `Deferred`s that
+    handle cancellation cleanly, otherwise a variety of bugs may occur, ranging from
+    premature logging context closure, to stuck requests, to database corruption.
+
+    See the documentation page on Cancellation for more information.
+
+    Usage:
+        class SomeServlet(RestServlet):
+            @cancellable
+            async def on_GET(self, request: SynapseRequest) -> ...:
+                ...
+    """
+
+    function.cancellable = True  # type: ignore[attr-defined]
+    return function
+
+
+def is_function_cancellable(function: Callable[..., Any]) -> bool:
+    """Checks whether a servlet method has the `@cancellable` flag."""
+    return getattr(function, "cancellable", False)