forked-synapse/synapse/util/caches/deferred_cache.py

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# Copyright 2015, 2016 OpenMarket Ltd
# Copyright 2018 New Vector Ltd
# Copyright 2020 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.
import abc
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import enum
import threading
from typing import (
Callable,
Collection,
Dict,
Generic,
MutableMapping,
Optional,
Set,
Sized,
Tuple,
TypeVar,
Union,
cast,
)
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from prometheus_client import Gauge
from twisted.internet import defer
from twisted.python.failure import Failure
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from synapse.util.async_helpers import ObservableDeferred
from synapse.util.caches.lrucache import LruCache
from synapse.util.caches.treecache import TreeCache, iterate_tree_cache_entry
cache_pending_metric = Gauge(
"synapse_util_caches_cache_pending",
"Number of lookups currently pending for this cache",
["name"],
)
T = TypeVar("T")
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KT = TypeVar("KT")
VT = TypeVar("VT")
class _Sentinel(enum.Enum):
# defining a sentinel in this way allows mypy to correctly handle the
# type of a dictionary lookup.
sentinel = object()
class DeferredCache(Generic[KT, VT]):
"""Wraps an LruCache, adding support for Deferred results.
It expects that each entry added with set() will be a Deferred; likewise get()
will return a Deferred.
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"""
__slots__ = (
"cache",
"thread",
"_pending_deferred_cache",
)
def __init__(
self,
name: str,
max_entries: int = 1000,
tree: bool = False,
iterable: bool = False,
apply_cache_factor_from_config: bool = True,
prune_unread_entries: bool = True,
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):
"""
Args:
name: The name of the cache
max_entries: Maximum amount of entries that the cache will hold
tree: Use a TreeCache instead of a dict as the underlying cache type
iterable: If True, count each item in the cached object as an entry,
rather than each cached object
apply_cache_factor_from_config: Whether cache factors specified in the
config file affect `max_entries`
prune_unread_entries: If True, cache entries that haven't been read recently
will be evicted from the cache in the background. Set to False to
opt-out of this behaviour.
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"""
cache_type = TreeCache if tree else dict
# _pending_deferred_cache maps from the key value to a `CacheEntry` object.
self._pending_deferred_cache: Union[
TreeCache, "MutableMapping[KT, CacheEntry[KT, VT]]"
] = cache_type()
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def metrics_cb() -> None:
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
# a Deferred.
self.cache: LruCache[KT, VT] = LruCache(
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max_size=max_entries,
cache_name=name,
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cache_type=cache_type,
size_callback=(
(lambda d: len(cast(Sized, d)) or 1)
# Argument 1 to "len" has incompatible type "VT"; expected "Sized"
# We trust that `VT` is `Sized` when `iterable` is `True`
if iterable
else None
),
metrics_collection_callback=metrics_cb,
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apply_cache_factor_from_config=apply_cache_factor_from_config,
prune_unread_entries=prune_unread_entries,
)
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self.thread: Optional[threading.Thread] = None
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@property
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
if expected_thread is None:
self.thread = threading.current_thread()
else:
if expected_thread is not threading.current_thread():
raise ValueError(
"Cache objects can only be accessed from the main thread"
)
def get(
self,
key: KT,
callback: Optional[Callable[[], None]] = None,
update_metrics: bool = True,
) -> defer.Deferred:
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"""Looks the key up in the caches.
For symmetry with set(), this method does *not* follow the synapse logcontext
rules: the logcontext will not be cleared on return, and the Deferred will run
its callbacks in the sentinel context. In other words: wrap the result with
make_deferred_yieldable() before `await`ing it.
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Args:
key:
callback: Gets called when the entry in the cache is invalidated
update_metrics: whether to update the cache hit rate metrics
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Returns:
A Deferred which completes with the result. Note that this may later fail
if there is an ongoing set() operation which later completes with a failure.
Raises:
KeyError if the key is not found in the cache
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"""
val = self._pending_deferred_cache.get(key, _Sentinel.sentinel)
if val is not _Sentinel.sentinel:
val.add_invalidation_callback(key, callback)
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if update_metrics:
m = self.cache.metrics
assert m # we always have a name, so should always have metrics
m.inc_hits()
return val.deferred(key)
callbacks = (callback,) if callback else ()
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val2 = self.cache.get(
key, _Sentinel.sentinel, callbacks=callbacks, update_metrics=update_metrics
)
if val2 is _Sentinel.sentinel:
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raise KeyError()
else:
return defer.succeed(val2)
def get_bulk(
self,
keys: Collection[KT],
callback: Optional[Callable[[], None]] = None,
) -> Tuple[Dict[KT, VT], Optional["defer.Deferred[Dict[KT, VT]]"], Collection[KT]]:
"""Bulk lookup of items in the cache.
Returns:
A 3-tuple of:
1. a dict of key/value of items already cached;
2. a deferred that resolves to a dict of key/value of items
we're already fetching; and
3. a collection of keys that don't appear in the previous two.
"""
# The cached results
cached = {}
# List of pending deferreds
pending = []
# Dict that gets filled out when the pending deferreds complete
pending_results = {}
# List of keys that aren't in either cache
missing = []
callbacks = (callback,) if callback else ()
for key in keys:
# Check if its in the main cache.
immediate_value = self.cache.get(
key,
_Sentinel.sentinel,
callbacks=callbacks,
)
if immediate_value is not _Sentinel.sentinel:
cached[key] = immediate_value
continue
# Check if its in the pending cache
pending_value = self._pending_deferred_cache.get(key, _Sentinel.sentinel)
if pending_value is not _Sentinel.sentinel:
pending_value.add_invalidation_callback(key, callback)
def completed_cb(value: VT, key: KT) -> VT:
pending_results[key] = value
return value
# Add a callback to fill out `pending_results` when that completes
d = pending_value.deferred(key).addCallback(completed_cb, key)
pending.append(d)
continue
# Not in either cache
missing.append(key)
# If we've got pending deferreds, squash them into a single one that
# returns `pending_results`.
pending_deferred = None
if pending:
pending_deferred = defer.gatherResults(
pending, consumeErrors=True
).addCallback(lambda _: pending_results)
return (cached, pending_deferred, missing)
def get_immediate(
self, key: KT, default: T, update_metrics: bool = True
) -> Union[VT, T]:
"""If we have a *completed* cached value, return it."""
return self.cache.get(key, default, update_metrics=update_metrics)
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def set(
self,
key: KT,
value: "defer.Deferred[VT]",
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callback: Optional[Callable[[], None]] = None,
) -> defer.Deferred:
"""Adds a new entry to the cache (or updates an existing one).
The given `value` *must* be a Deferred.
First any existing entry for the same key is invalidated. Then a new entry
is added to the cache for the given key.
Until the `value` completes, calls to `get()` for the key will also result in an
incomplete Deferred, which will ultimately complete with the same result as
`value`.
If `value` completes successfully, subsequent calls to `get()` will then return
a completed deferred with the same result. If it *fails*, the cache is
invalidated and subequent calls to `get()` will raise a KeyError.
If another call to `set()` happens before `value` completes, then (a) any
invalidation callbacks registered in the interim will be called, (b) any
`get()`s in the interim will continue to complete with the result from the
*original* `value`, (c) any future calls to `get()` will complete with the
result from the *new* `value`.
It is expected that `value` does *not* follow the synapse logcontext rules - ie,
if it is incomplete, it runs its callbacks in the sentinel context.
Args:
key: Key to be set
value: a deferred which will complete with a result to add to the cache
callback: An optional callback to be called when the entry is invalidated
"""
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self.check_thread()
self._pending_deferred_cache.pop(key, None)
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# XXX: why don't we invalidate the entry in `self.cache` yet?
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# otherwise, we'll add an entry to the _pending_deferred_cache for now,
# and add callbacks to add it to the cache properly later.
entry = CacheEntrySingle[KT, VT](value)
entry.add_invalidation_callback(key, callback)
self._pending_deferred_cache[key] = entry
deferred = entry.deferred(key).addCallbacks(
self._completed_callback,
self._error_callback,
callbackArgs=(entry, key),
errbackArgs=(entry, key),
)
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# we return a new Deferred which will be called before any subsequent observers.
return deferred
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def start_bulk_input(
self,
keys: Collection[KT],
callback: Optional[Callable[[], None]] = None,
) -> "CacheMultipleEntries[KT, VT]":
"""Bulk set API for use when fetching multiple keys at once from the DB.
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Called *before* starting the fetch from the DB, and the caller *must*
call either `complete_bulk(..)` or `error_bulk(..)` on the return value.
"""
entry = CacheMultipleEntries[KT, VT]()
entry.add_global_invalidation_callback(callback)
for key in keys:
self._pending_deferred_cache[key] = entry
return entry
def _completed_callback(
self, value: VT, entry: "CacheEntry[KT, VT]", key: KT
) -> VT:
"""Called when a deferred is completed."""
# We check if the current entry matches the entry associated with the
# deferred. If they don't match then it got invalidated.
current_entry = self._pending_deferred_cache.pop(key, None)
if current_entry is not entry:
if current_entry:
self._pending_deferred_cache[key] = current_entry
return value
self.cache.set(key, value, entry.get_invalidation_callbacks(key))
return value
def _error_callback(
self,
failure: Failure,
entry: "CacheEntry[KT, VT]",
key: KT,
) -> Failure:
"""Called when a deferred errors."""
# We check if the current entry matches the entry associated with the
# deferred. If they don't match then it got invalidated.
current_entry = self._pending_deferred_cache.pop(key, None)
if current_entry is not entry:
if current_entry:
self._pending_deferred_cache[key] = current_entry
return failure
for cb in entry.get_invalidation_callbacks(key):
cb()
return failure
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def prefill(
self, key: KT, value: VT, callback: Optional[Callable[[], None]] = None
) -> None:
callbacks = (callback,) if callback else ()
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self.cache.set(key, value, callbacks=callbacks)
self._pending_deferred_cache.pop(key, None)
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def invalidate(self, key: KT) -> None:
"""Delete a key, or tree of entries
If the cache is backed by a regular dict, then "key" must be of
the right type for this cache
If the cache is backed by a TreeCache, then "key" must be a tuple, but
may be of lower cardinality than the TreeCache - in which case the whole
subtree is deleted.
"""
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self.check_thread()
self.cache.del_multi(key)
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# if we have a pending lookup for this key, remove it from the
# _pending_deferred_cache, which will (a) stop it being returned for
# future queries and (b) stop it being persisted as a proper entry
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# in self.cache.
entry = self._pending_deferred_cache.pop(key, None)
if entry:
# _pending_deferred_cache.pop should either return a CacheEntry, or, in the
# case of a TreeCache, a dict of keys to cache entries. Either way calling
# iterate_tree_cache_entry on it will do the right thing.
for iter_entry in iterate_tree_cache_entry(entry):
for cb in iter_entry.get_invalidation_callbacks(key):
cb()
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def invalidate_all(self) -> None:
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self.check_thread()
self.cache.clear()
for key, entry in self._pending_deferred_cache.items():
for cb in entry.get_invalidation_callbacks(key):
cb()
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self._pending_deferred_cache.clear()
class CacheEntry(Generic[KT, VT], metaclass=abc.ABCMeta):
"""Abstract class for entries in `DeferredCache[KT, VT]`"""
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@abc.abstractmethod
def deferred(self, key: KT) -> "defer.Deferred[VT]":
"""Get a deferred that a caller can wait on to get the value at the
given key"""
...
@abc.abstractmethod
def add_invalidation_callback(
self, key: KT, callback: Optional[Callable[[], None]]
) -> None:
"""Add an invalidation callback"""
...
@abc.abstractmethod
def get_invalidation_callbacks(self, key: KT) -> Collection[Callable[[], None]]:
"""Get all invalidation callbacks"""
...
class CacheEntrySingle(CacheEntry[KT, VT]):
"""An implementation of `CacheEntry` wrapping a deferred that results in a
single cache entry.
"""
__slots__ = ["_deferred", "_callbacks"]
def __init__(self, deferred: "defer.Deferred[VT]") -> None:
self._deferred = ObservableDeferred(deferred, consumeErrors=True)
self._callbacks: Set[Callable[[], None]] = set()
def deferred(self, key: KT) -> "defer.Deferred[VT]":
return self._deferred.observe()
def add_invalidation_callback(
self, key: KT, callback: Optional[Callable[[], None]]
) -> None:
if callback is None:
return
self._callbacks.add(callback)
def get_invalidation_callbacks(self, key: KT) -> Collection[Callable[[], None]]:
return self._callbacks
class CacheMultipleEntries(CacheEntry[KT, VT]):
"""Cache entry that is used for bulk lookups and insertions."""
__slots__ = ["_deferred", "_callbacks", "_global_callbacks"]
def __init__(self) -> None:
self._deferred: Optional[ObservableDeferred[Dict[KT, VT]]] = None
self._callbacks: Dict[KT, Set[Callable[[], None]]] = {}
self._global_callbacks: Set[Callable[[], None]] = set()
def deferred(self, key: KT) -> "defer.Deferred[VT]":
if not self._deferred:
self._deferred = ObservableDeferred(defer.Deferred(), consumeErrors=True)
return self._deferred.observe().addCallback(lambda res: res.get(key))
def add_invalidation_callback(
self, key: KT, callback: Optional[Callable[[], None]]
) -> None:
if callback is None:
return
self._callbacks.setdefault(key, set()).add(callback)
def get_invalidation_callbacks(self, key: KT) -> Collection[Callable[[], None]]:
return self._callbacks.get(key, set()) | self._global_callbacks
def add_global_invalidation_callback(
self, callback: Optional[Callable[[], None]]
) -> None:
"""Add a callback for when any keys get invalidated."""
if callback is None:
return
self._global_callbacks.add(callback)
def complete_bulk(
self,
cache: DeferredCache[KT, VT],
result: Dict[KT, VT],
) -> None:
"""Called when there is a result"""
for key, value in result.items():
cache._completed_callback(value, self, key)
if self._deferred:
self._deferred.callback(result)
def error_bulk(
self, cache: DeferredCache[KT, VT], keys: Collection[KT], failure: Failure
) -> None:
"""Called when bulk lookup failed."""
for key in keys:
cache._error_callback(failure, self, key)
if self._deferred:
self._deferred.errback(failure)