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![Richard van der Hoff](/assets/img/avatar_default.png)
tighten up some of the typing in CachedCall, which is going to be needed when Twisted 21.7 brings better typing on Deferred.
136 lines
5.2 KiB
Python
136 lines
5.2 KiB
Python
# Copyright 2021 The Matrix.org Foundation C.I.C.
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#
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# Licensed under the Apache License, Version 2.0 (the "License");
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# you may not use this file except in compliance with the License.
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# You may obtain a copy of the License at
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#
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# http://www.apache.org/licenses/LICENSE-2.0
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#
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# Unless required by applicable law or agreed to in writing, software
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# distributed under the License is distributed on an "AS IS" BASIS,
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# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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# See the License for the specific language governing permissions and
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# limitations under the License.
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import enum
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from typing import Awaitable, Callable, Generic, Optional, TypeVar, Union
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from twisted.internet.defer import Deferred
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from twisted.python.failure import Failure
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from synapse.logging.context import make_deferred_yieldable, run_in_background
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TV = TypeVar("TV")
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class _Sentinel(enum.Enum):
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sentinel = object()
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class CachedCall(Generic[TV]):
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"""A wrapper for asynchronous calls whose results should be shared
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This is useful for wrapping asynchronous functions, where there might be multiple
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callers, but we only want to call the underlying function once (and have the result
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returned to all callers).
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Similar results can be achieved via a lock of some form, but that typically requires
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more boilerplate (and ends up being less efficient).
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Correctly handles Synapse logcontexts (logs and resource usage for the underlying
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function are logged against the logcontext which is active when get() is first
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called).
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Example usage:
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_cached_val = CachedCall(_load_prop)
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async def handle_request() -> X:
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# We can call this multiple times, but it will result in a single call to
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# _load_prop().
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return await _cached_val.get()
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async def _load_prop() -> X:
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await difficult_operation()
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The implementation is deliberately single-shot (ie, once the call is initiated,
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there is no way to ask for it to be run). This keeps the implementation and
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semantics simple. If you want to make a new call, simply replace the whole
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CachedCall object.
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"""
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__slots__ = ["_callable", "_deferred", "_result"]
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def __init__(self, f: Callable[[], Awaitable[TV]]):
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"""
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Args:
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f: The underlying function. Only one call to this function will be alive
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at once (per instance of CachedCall)
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"""
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self._callable: Optional[Callable[[], Awaitable[TV]]] = f
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self._deferred: Optional[Deferred] = None
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self._result: Union[_Sentinel, TV, Failure] = _Sentinel.sentinel
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async def get(self) -> TV:
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"""Kick off the call if necessary, and return the result"""
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# Fire off the callable now if this is our first time
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if not self._deferred:
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self._deferred = run_in_background(self._callable)
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# we will never need the callable again, so make sure it can be GCed
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self._callable = None
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# once the deferred completes, store the result. We cannot simply leave the
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# result in the deferred, since `awaiting` a deferred destroys its result.
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# (Also, if it's a Failure, GCing the deferred would log a critical error
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# about unhandled Failures)
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def got_result(r):
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self._result = r
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self._deferred.addBoth(got_result)
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# TODO: consider cancellation semantics. Currently, if the call to get()
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# is cancelled, the underlying call will continue (and any future calls
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# will get the result/exception), which I think is *probably* ok, modulo
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# the fact the underlying call may be logged to a cancelled logcontext,
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# and any eventual exception may not be reported.
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# we can now await the deferred, and once it completes, return the result.
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if isinstance(self._result, _Sentinel):
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await make_deferred_yieldable(self._deferred)
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assert not isinstance(self._result, _Sentinel)
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if isinstance(self._result, Failure):
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self._result.raiseException()
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raise AssertionError("unexpected return from Failure.raiseException")
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return self._result
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class RetryOnExceptionCachedCall(Generic[TV]):
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"""A wrapper around CachedCall which will retry the call if an exception is thrown
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This is used in much the same way as CachedCall, but adds some extra functionality
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so that if the underlying function throws an exception, then the next call to get()
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will initiate another call to the underlying function. (Any calls to get() which
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are already pending will raise the exception.)
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"""
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slots = ["_cachedcall"]
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def __init__(self, f: Callable[[], Awaitable[TV]]):
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async def _wrapper() -> TV:
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try:
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return await f()
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except Exception:
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# the call raised an exception: replace the underlying CachedCall to
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# trigger another call next time get() is called
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self._cachedcall = CachedCall(_wrapper)
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raise
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self._cachedcall = CachedCall(_wrapper)
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async def get(self) -> TV:
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return await self._cachedcall.get()
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