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Add @cancellable
decorator, for use on request handlers (#12586)
Signed-off-by: Sean Quah <seanq@element.io>
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changelog.d/12586.misc
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changelog.d/12586.misc
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Add `@cancellable` decorator, for use on endpoint methods that can be cancelled when clients disconnect.
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@ -33,6 +33,7 @@ from typing import (
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Optional,
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Pattern,
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Tuple,
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TypeVar,
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Union,
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)
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@ -92,6 +93,66 @@ HTML_ERROR_TEMPLATE = """<!DOCTYPE html>
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HTTP_STATUS_REQUEST_CANCELLED = 499
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F = TypeVar("F", bound=Callable[..., Any])
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_cancellable_method_names = frozenset(
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{
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# `RestServlet`, `BaseFederationServlet` and `BaseFederationServerServlet`
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# methods
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"on_GET",
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"on_PUT",
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"on_POST",
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"on_DELETE",
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# `_AsyncResource`, `DirectServeHtmlResource` and `DirectServeJsonResource`
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# methods
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"_async_render_GET",
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"_async_render_PUT",
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"_async_render_POST",
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"_async_render_DELETE",
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"_async_render_OPTIONS",
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# `ReplicationEndpoint` methods
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"_handle_request",
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}
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)
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def cancellable(method: F) -> F:
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"""Marks a servlet method as cancellable.
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Methods with this decorator will be cancelled if the client disconnects before we
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finish processing the request.
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During cancellation, `Deferred.cancel()` will be invoked on the `Deferred` wrapping
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the method. The `cancel()` call will propagate down to the `Deferred` that is
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currently being waited on. That `Deferred` will raise a `CancelledError`, which will
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propagate up, as per normal exception handling.
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Before applying this decorator to a new endpoint, you MUST recursively check
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that all `await`s in the function are on `async` functions or `Deferred`s that
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handle cancellation cleanly, otherwise a variety of bugs may occur, ranging from
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premature logging context closure, to stuck requests, to database corruption.
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Usage:
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class SomeServlet(RestServlet):
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@cancellable
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async def on_GET(self, request: SynapseRequest) -> ...:
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...
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"""
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if method.__name__ not in _cancellable_method_names:
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raise ValueError(
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"@cancellable decorator can only be applied to servlet methods."
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)
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method.cancellable = True # type: ignore[attr-defined]
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return method
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def is_method_cancellable(method: Callable[..., Any]) -> bool:
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"""Checks whether a servlet method has the `@cancellable` flag."""
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return getattr(method, "cancellable", False)
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def return_json_error(f: failure.Failure, request: SynapseRequest) -> None:
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"""Sends a JSON error response to clients."""
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