diff --git a/synapse/util/caches/cached_call.py b/synapse/util/caches/cached_call.py
new file mode 100644
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+++ b/synapse/util/caches/cached_call.py
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+# -*- coding: utf-8 -*-
+# Copyright 2021 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 Awaitable, Callable, Generic, Optional, TypeVar, Union
+
+from twisted.internet.defer import Deferred
+from twisted.python.failure import Failure
+
+from synapse.logging.context import make_deferred_yieldable, run_in_background
+
+TV = TypeVar("TV")
+
+
+class CachedCall(Generic[TV]):
+ """A wrapper for asynchronous calls whose results should be shared
+
+ This is useful for wrapping asynchronous functions, where there might be multiple
+ callers, but we only want to call the underlying function once (and have the result
+ returned to all callers).
+
+ Similar results can be achieved via a lock of some form, but that typically requires
+ more boilerplate (and ends up being less efficient).
+
+ Correctly handles Synapse logcontexts (logs and resource usage for the underlying
+ function are logged against the logcontext which is active when get() is first
+ called).
+
+ Example usage:
+
+ _cached_val = CachedCall(_load_prop)
+
+ async def handle_request() -> X:
+ # We can call this multiple times, but it will result in a single call to
+ # _load_prop().
+ return await _cached_val.get()
+
+ async def _load_prop() -> X:
+ await difficult_operation()
+
+
+ The implementation is deliberately single-shot (ie, once the call is initiated,
+ there is no way to ask for it to be run). This keeps the implementation and
+ semantics simple. If you want to make a new call, simply replace the whole
+ CachedCall object.
+ """
+
+ __slots__ = ["_callable", "_deferred", "_result"]
+
+ def __init__(self, f: Callable[[], Awaitable[TV]]):
+ """
+ Args:
+ f: The underlying function. Only one call to this function will be alive
+ at once (per instance of CachedCall)
+ """
+ self._callable = f # type: Optional[Callable[[], Awaitable[TV]]]
+ self._deferred = None # type: Optional[Deferred]
+ self._result = None # type: Union[None, Failure, TV]
+
+ async def get(self) -> TV:
+ """Kick off the call if necessary, and return the result"""
+
+ # Fire off the callable now if this is our first time
+ if not self._deferred:
+ self._deferred = run_in_background(self._callable)
+
+ # we will never need the callable again, so make sure it can be GCed
+ self._callable = None
+
+ # once the deferred completes, store the result. We cannot simply leave the
+ # result in the deferred, since if it's a Failure, GCing the deferred
+ # would then log a critical error about unhandled Failures.
+ def got_result(r):
+ self._result = r
+
+ self._deferred.addBoth(got_result)
+
+ # TODO: consider cancellation semantics. Currently, if the call to get()
+ # is cancelled, the underlying call will continue (and any future calls
+ # will get the result/exception), which I think is *probably* ok, modulo
+ # the fact the underlying call may be logged to a cancelled logcontext,
+ # and any eventual exception may not be reported.
+
+ # we can now await the deferred, and once it completes, return the result.
+ await make_deferred_yieldable(self._deferred)
+
+ # I *think* this is the easiest way to correctly raise a Failure without having
+ # to gut-wrench into the implementation of Deferred.
+ d = Deferred()
+ d.callback(self._result)
+ return await d
+
+
+class RetryOnExceptionCachedCall(Generic[TV]):
+ """A wrapper around CachedCall which will retry the call if an exception is thrown
+
+ This is used in much the same way as CachedCall, but adds some extra functionality
+ so that if the underlying function throws an exception, then the next call to get()
+ will initiate another call to the underlying function. (Any calls to get() which
+ are already pending will raise the exception.)
+ """
+
+ slots = ["_cachedcall"]
+
+ def __init__(self, f: Callable[[], Awaitable[TV]]):
+ async def _wrapper() -> TV:
+ try:
+ return await f()
+ except Exception:
+ # the call raised an exception: replace the underlying CachedCall to
+ # trigger another call next time get() is called
+ self._cachedcall = CachedCall(_wrapper)
+ raise
+
+ self._cachedcall = CachedCall(_wrapper)
+
+ async def get(self) -> TV:
+ return await self._cachedcall.get()
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