352 lines
13 KiB
Python
352 lines
13 KiB
Python
# Copyright 2015, 2016 OpenMarket Ltd
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# Copyright 2018 New Vector Ltd
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# Copyright 2020 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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import threading
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from typing import (
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Callable,
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Generic,
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Iterable,
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MutableMapping,
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Optional,
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Sized,
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TypeVar,
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Union,
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cast,
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)
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from prometheus_client import Gauge
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from twisted.internet import defer
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from twisted.python import failure
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from twisted.python.failure import Failure
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from synapse.util.async_helpers import ObservableDeferred
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from synapse.util.caches.lrucache import LruCache
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from synapse.util.caches.treecache import TreeCache, iterate_tree_cache_entry
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cache_pending_metric = Gauge(
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"synapse_util_caches_cache_pending",
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"Number of lookups currently pending for this cache",
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["name"],
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)
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T = TypeVar("T")
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KT = TypeVar("KT")
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VT = TypeVar("VT")
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class _Sentinel(enum.Enum):
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# defining a sentinel in this way allows mypy to correctly handle the
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# type of a dictionary lookup.
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sentinel = object()
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class DeferredCache(Generic[KT, VT]):
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"""Wraps an LruCache, adding support for Deferred results.
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It expects that each entry added with set() will be a Deferred; likewise get()
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will return a Deferred.
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"""
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__slots__ = (
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"cache",
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"thread",
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"_pending_deferred_cache",
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)
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def __init__(
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self,
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name: str,
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max_entries: int = 1000,
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tree: bool = False,
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iterable: bool = False,
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apply_cache_factor_from_config: bool = True,
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prune_unread_entries: bool = True,
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):
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"""
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Args:
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name: The name of the cache
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max_entries: Maximum amount of entries that the cache will hold
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tree: Use a TreeCache instead of a dict as the underlying cache type
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iterable: If True, count each item in the cached object as an entry,
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rather than each cached object
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apply_cache_factor_from_config: Whether cache factors specified in the
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config file affect `max_entries`
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prune_unread_entries: If True, cache entries that haven't been read recently
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will be evicted from the cache in the background. Set to False to
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opt-out of this behaviour.
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"""
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cache_type = TreeCache if tree else dict
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# _pending_deferred_cache maps from the key value to a `CacheEntry` object.
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self._pending_deferred_cache: Union[
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TreeCache, "MutableMapping[KT, CacheEntry]"
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] = cache_type()
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def metrics_cb() -> None:
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cache_pending_metric.labels(name).set(len(self._pending_deferred_cache))
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# cache is used for completed results and maps to the result itself, rather than
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# a Deferred.
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self.cache: LruCache[KT, VT] = LruCache(
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max_size=max_entries,
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cache_name=name,
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cache_type=cache_type,
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size_callback=(
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(lambda d: len(cast(Sized, d)) or 1)
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# Argument 1 to "len" has incompatible type "VT"; expected "Sized"
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# We trust that `VT` is `Sized` when `iterable` is `True`
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if iterable
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else None
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),
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metrics_collection_callback=metrics_cb,
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apply_cache_factor_from_config=apply_cache_factor_from_config,
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prune_unread_entries=prune_unread_entries,
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)
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self.thread: Optional[threading.Thread] = None
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@property
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def max_entries(self) -> int:
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return self.cache.max_size
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def check_thread(self) -> None:
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expected_thread = self.thread
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if expected_thread is None:
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self.thread = threading.current_thread()
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else:
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if expected_thread is not threading.current_thread():
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raise ValueError(
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"Cache objects can only be accessed from the main thread"
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)
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def get(
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self,
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key: KT,
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callback: Optional[Callable[[], None]] = None,
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update_metrics: bool = True,
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) -> defer.Deferred:
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"""Looks the key up in the caches.
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For symmetry with set(), this method does *not* follow the synapse logcontext
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rules: the logcontext will not be cleared on return, and the Deferred will run
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its callbacks in the sentinel context. In other words: wrap the result with
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make_deferred_yieldable() before `await`ing it.
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Args:
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key:
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callback: Gets called when the entry in the cache is invalidated
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update_metrics (bool): whether to update the cache hit rate metrics
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Returns:
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A Deferred which completes with the result. Note that this may later fail
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if there is an ongoing set() operation which later completes with a failure.
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Raises:
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KeyError if the key is not found in the cache
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"""
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callbacks = [callback] if callback else []
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val = self._pending_deferred_cache.get(key, _Sentinel.sentinel)
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if val is not _Sentinel.sentinel:
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val.callbacks.update(callbacks)
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if update_metrics:
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m = self.cache.metrics
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assert m # we always have a name, so should always have metrics
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m.inc_hits()
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return val.deferred.observe()
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val2 = self.cache.get(
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key, _Sentinel.sentinel, callbacks=callbacks, update_metrics=update_metrics
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)
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if val2 is _Sentinel.sentinel:
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raise KeyError()
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else:
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return defer.succeed(val2)
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def get_immediate(
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self, key: KT, default: T, update_metrics: bool = True
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) -> Union[VT, T]:
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"""If we have a *completed* cached value, return it."""
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return self.cache.get(key, default, update_metrics=update_metrics)
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def set(
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self,
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key: KT,
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value: "defer.Deferred[VT]",
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callback: Optional[Callable[[], None]] = None,
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) -> defer.Deferred:
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"""Adds a new entry to the cache (or updates an existing one).
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The given `value` *must* be a Deferred.
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First any existing entry for the same key is invalidated. Then a new entry
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is added to the cache for the given key.
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Until the `value` completes, calls to `get()` for the key will also result in an
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incomplete Deferred, which will ultimately complete with the same result as
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`value`.
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If `value` completes successfully, subsequent calls to `get()` will then return
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a completed deferred with the same result. If it *fails*, the cache is
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invalidated and subequent calls to `get()` will raise a KeyError.
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If another call to `set()` happens before `value` completes, then (a) any
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invalidation callbacks registered in the interim will be called, (b) any
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`get()`s in the interim will continue to complete with the result from the
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*original* `value`, (c) any future calls to `get()` will complete with the
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result from the *new* `value`.
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It is expected that `value` does *not* follow the synapse logcontext rules - ie,
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if it is incomplete, it runs its callbacks in the sentinel context.
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Args:
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key: Key to be set
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value: a deferred which will complete with a result to add to the cache
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callback: An optional callback to be called when the entry is invalidated
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"""
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if not isinstance(value, defer.Deferred):
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raise TypeError("not a Deferred")
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callbacks = [callback] if callback else []
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self.check_thread()
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existing_entry = self._pending_deferred_cache.pop(key, None)
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if existing_entry:
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existing_entry.invalidate()
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# XXX: why don't we invalidate the entry in `self.cache` yet?
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# we can save a whole load of effort if the deferred is ready.
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if value.called:
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result = value.result
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if not isinstance(result, failure.Failure):
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self.cache.set(key, cast(VT, result), callbacks)
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return value
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# otherwise, we'll add an entry to the _pending_deferred_cache for now,
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# and add callbacks to add it to the cache properly later.
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observable = ObservableDeferred(value, consumeErrors=True)
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observer = observable.observe()
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entry = CacheEntry(deferred=observable, callbacks=callbacks)
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self._pending_deferred_cache[key] = entry
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def compare_and_pop() -> bool:
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"""Check if our entry is still the one in _pending_deferred_cache, and
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if so, pop it.
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Returns true if the entries matched.
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"""
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existing_entry = self._pending_deferred_cache.pop(key, None)
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if existing_entry is entry:
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return True
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# oops, the _pending_deferred_cache has been updated since
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# we started our query, so we are out of date.
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#
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# Better put back whatever we took out. (We do it this way
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# round, rather than peeking into the _pending_deferred_cache
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# and then removing on a match, to make the common case faster)
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if existing_entry is not None:
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self._pending_deferred_cache[key] = existing_entry
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return False
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def cb(result: VT) -> None:
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if compare_and_pop():
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self.cache.set(key, result, entry.callbacks)
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else:
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# we're not going to put this entry into the cache, so need
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# to make sure that the invalidation callbacks are called.
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# That was probably done when _pending_deferred_cache was
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# updated, but it's possible that `set` was called without
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# `invalidate` being previously called, in which case it may
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# not have been. Either way, let's double-check now.
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entry.invalidate()
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def eb(_fail: Failure) -> None:
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compare_and_pop()
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entry.invalidate()
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# once the deferred completes, we can move the entry from the
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# _pending_deferred_cache to the real cache.
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#
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observer.addCallbacks(cb, eb)
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# we return a new Deferred which will be called before any subsequent observers.
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return observable.observe()
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def prefill(
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self, key: KT, value: VT, callback: Optional[Callable[[], None]] = None
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) -> None:
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callbacks = [callback] if callback else []
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self.cache.set(key, value, callbacks=callbacks)
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def invalidate(self, key: KT) -> None:
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"""Delete a key, or tree of entries
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If the cache is backed by a regular dict, then "key" must be of
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the right type for this cache
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If the cache is backed by a TreeCache, then "key" must be a tuple, but
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may be of lower cardinality than the TreeCache - in which case the whole
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subtree is deleted.
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"""
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self.check_thread()
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self.cache.del_multi(key)
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# if we have a pending lookup for this key, remove it from the
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# _pending_deferred_cache, which will (a) stop it being returned
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# for future queries and (b) stop it being persisted as a proper entry
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# in self.cache.
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entry = self._pending_deferred_cache.pop(key, None)
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# run the invalidation callbacks now, rather than waiting for the
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# deferred to resolve.
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if entry:
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# _pending_deferred_cache.pop should either return a CacheEntry, or, in the
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# case of a TreeCache, a dict of keys to cache entries. Either way calling
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# iterate_tree_cache_entry on it will do the right thing.
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for entry in iterate_tree_cache_entry(entry):
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entry.invalidate()
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def invalidate_all(self) -> None:
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self.check_thread()
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self.cache.clear()
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for entry in self._pending_deferred_cache.values():
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entry.invalidate()
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self._pending_deferred_cache.clear()
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class CacheEntry:
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__slots__ = ["deferred", "callbacks", "invalidated"]
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def __init__(
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self, deferred: ObservableDeferred, callbacks: Iterable[Callable[[], None]]
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):
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self.deferred = deferred
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self.callbacks = set(callbacks)
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self.invalidated = False
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def invalidate(self) -> None:
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if not self.invalidated:
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self.invalidated = True
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for callback in self.callbacks:
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callback()
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self.callbacks.clear()
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