Add @cancellable decorator, for use on request handlers

changelog.d/12586.misc

@@ -0,0 +1 @@
+Add `@cancellable` decorator, for use on endpoint methods that can be cancelled when clients disconnect.

synapse/http/server.py

@@ -33,6 +33,7 @@
  Optional,
  Pattern,
  Tuple,
+ TypeVar,
  Union,
 )
 
@@ -92,6 +93,66 @@
 HTTP_STATUS_REQUEST_CANCELLED = 499
 
 
+F = TypeVar("F", bound=Callable[..., Any])
+
+
+_cancellable_method_names = frozenset(
+ {
+ # `RestServlet`, `BaseFederationServlet` and `BaseFederationServerServlet`
+ # methods
+ "on_GET",
+ "on_PUT",
+ "on_POST",
+ "on_DELETE",
+ # `_AsyncResource`, `DirectServeHtmlResource` and `DirectServeJsonResource`
+ # methods
+ "_async_render_GET",
+ "_async_render_PUT",
+ "_async_render_POST",
+ "_async_render_DELETE",
+ "_async_render_OPTIONS",
+ # `ReplicationEndpoint` methods
+ "_handle_request",
+ }
+)
+
+
+def cancellable(method: F) -> F:
+ """Marks a servlet method as cancellable.
+
+ Methods with this decorator will be cancelled if the client disconnects before we
+ finish processing the request.
+
+ 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 endpoint, 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.
+
+ Usage:
+ class SomeServlet(RestServlet):
+ @cancellable
+ async def on_GET(self, request: SynapseRequest) -> ...:
+ ...
+ """
+ if method.__name__ not in _cancellable_method_names:
+ raise ValueError(
+ "@cancellable decorator can only be applied to servlet methods."
+ )
+
+ method.cancellable = True # type: ignore[attr-defined]
+ return method
+
+
+def is_method_cancellable(method: Callable[..., Any]) -> bool:
+ """Checks whether a servlet method has the `@cancellable` flag."""
+ return getattr(method, "cancellable", False)
+
+
 def return_json_error(f: failure.Failure, request: SynapseRequest) -> None:
  """Sends a JSON error response to clients."""