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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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Sean Quah 2022-05-10 14:05:22 +01:00 committed by GitHub
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changelog.d/12586.misc Normal file
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@ -0,0 +1 @@
Add `@cancellable` decorator, for use on endpoint methods that can be cancelled when clients disconnect.

61
synapse/http/server.py
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@ -33,6 +33,7 @@ from typing import (
Optional,
Pattern,
Tuple,
TypeVar,
Union,
)
@ -92,6 +93,66 @@ HTML_ERROR_TEMPLATE = """<!DOCTYPE html>
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."""