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# Copyright 2015, 2016 OpenMarket Ltd
#
# 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 enum
import logging
import threading
from typing import Any, Dict, Generic, Iterable, Optional, Set, TypeVar
import attr
from synapse.util.caches.lrucache import LruCache
logger = logging.getLogger(__name__)
# The type of the cache keys.
KT = TypeVar("KT")
# The type of the dictionary keys.
DKT = TypeVar("DKT")
# The type of the dictionary values.
DV = TypeVar("DV")
# This class can't be generic because it uses slots with attrs.
# See: https://github.com/python-attrs/attrs/issues/313
@attr.s(slots=True)
class DictionaryEntry: # should be: Generic[DKT, DV].
"""Returned when getting an entry from the cache
Attributes:
full: Whether the cache has the full or dict or just some keys.
If not full then not all requested keys will necessarily be present
in `value`
known_absent: Keys that were looked up in the dict and were not
there.
value: The full or partial dict value
"""
full = attr.ib(type=bool)
known_absent = attr.ib(type=Set[Any]) # should be: Set[DKT]
value = attr.ib(type=Dict[Any, Any]) # should be: Dict[DKT, DV]
def __len__(self) -> int:
return len(self.value)
class _Sentinel(enum.Enum):
# defining a sentinel in this way allows mypy to correctly handle the
# type of a dictionary lookup.
sentinel = object()
class DictionaryCache(Generic[KT, DKT, DV]):
"""Caches key -> dictionary lookups, supporting caching partial dicts, i.e.
fetching a subset of dictionary keys for a particular key.
"""
def __init__(self, name: str, max_entries: int = 1000):
self.cache: LruCache[KT, DictionaryEntry] = LruCache(
max_size=max_entries, cache_name=name, size_callback=len
)
self.name = name
self.sequence = 0
self.thread: Optional[threading.Thread] = None
def check_thread(self) -> None:
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, dict_keys: Optional[Iterable[DKT]] = None
) -> DictionaryEntry:
"""Fetch an entry out of the cache
Args:
key
dict_keys: If given a set of keys then return only those keys
that exist in the cache.
Returns:
DictionaryEntry
"""
entry = self.cache.get(key, _Sentinel.sentinel)
if entry is not _Sentinel.sentinel:
if dict_keys is None:
return DictionaryEntry(
entry.full, entry.known_absent, dict(entry.value)
)
else:
return DictionaryEntry(
entry.full,
entry.known_absent,
{k: entry.value[k] for k in dict_keys if k in entry.value},
)
return DictionaryEntry(False, set(), {})
def invalidate(self, key: KT) -> None:
self.check_thread()
# Increment the sequence number so that any SELECT statements that
# raced with the INSERT don't update the cache (SYN-369)
self.sequence += 1
self.cache.pop(key, None)
def invalidate_all(self) -> None:
self.check_thread()
self.sequence += 1
self.cache.clear()
def update(
self,
sequence: int,
key: KT,
value: Dict[DKT, DV],
fetched_keys: Optional[Set[DKT]] = None,
) -> None:
"""Updates the entry in the cache
Args:
sequence
key
value: The value to update the cache with.
fetched_keys: All of the dictionary keys which were
fetched from the database.
If None, this is the complete value for key K. Otherwise, it
is used to infer a list of keys which we know don't exist in
the full dict.
"""
self.check_thread()
if self.sequence == sequence:
# Only update the cache if the caches sequence number matches the
# number that the cache had before the SELECT was started (SYN-369)
if fetched_keys is None:
self._insert(key, value, set())
else:
self._update_or_insert(key, value, fetched_keys)
def _update_or_insert(
self, key: KT, value: Dict[DKT, DV], known_absent: Set[DKT]
) -> None:
# We pop and reinsert as we need to tell the cache the size may have
# changed
entry: DictionaryEntry = self.cache.pop(key, DictionaryEntry(False, set(), {}))
entry.value.update(value)
entry.known_absent.update(known_absent)
self.cache[key] = entry
def _insert(self, key: KT, value: Dict[DKT, DV], known_absent: Set[DKT]) -> None:
self.cache[key] = DictionaryEntry(True, known_absent, value)
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