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99 lines
3.4 KiB
Python
99 lines
3.4 KiB
Python
# -*- coding: utf-8 -*-
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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 abc
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import threading
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from typing import Callable, Optional
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from synapse.storage.engines import BaseDatabaseEngine, PostgresEngine
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from synapse.storage.types import Cursor
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class SequenceGenerator(metaclass=abc.ABCMeta):
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"""A class which generates a unique sequence of integers"""
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@abc.abstractmethod
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def get_next_id_txn(self, txn: Cursor) -> int:
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"""Gets the next ID in the sequence"""
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...
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class PostgresSequenceGenerator(SequenceGenerator):
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"""An implementation of SequenceGenerator which uses a postgres sequence"""
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def __init__(self, sequence_name: str):
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self._sequence_name = sequence_name
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def get_next_id_txn(self, txn: Cursor) -> int:
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txn.execute("SELECT nextval(?)", (self._sequence_name,))
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return txn.fetchone()[0]
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GetFirstCallbackType = Callable[[Cursor], int]
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class LocalSequenceGenerator(SequenceGenerator):
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"""An implementation of SequenceGenerator which uses local locking
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This only works reliably if there are no other worker processes generating IDs at
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the same time.
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"""
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def __init__(self, get_first_callback: GetFirstCallbackType):
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"""
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Args:
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get_first_callback: a callback which is called on the first call to
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get_next_id_txn; should return the curreent maximum id
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"""
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# the callback. this is cleared after it is called, so that it can be GCed.
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self._callback = get_first_callback # type: Optional[GetFirstCallbackType]
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# The current max value, or None if we haven't looked in the DB yet.
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self._current_max_id = None # type: Optional[int]
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self._lock = threading.Lock()
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def get_next_id_txn(self, txn: Cursor) -> int:
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# We do application locking here since if we're using sqlite then
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# we are a single process synapse.
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with self._lock:
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if self._current_max_id is None:
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assert self._callback is not None
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self._current_max_id = self._callback(txn)
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self._callback = None
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self._current_max_id += 1
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return self._current_max_id
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def build_sequence_generator(
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database_engine: BaseDatabaseEngine,
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get_first_callback: GetFirstCallbackType,
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sequence_name: str,
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) -> SequenceGenerator:
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"""Get the best impl of SequenceGenerator available
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This uses PostgresSequenceGenerator on postgres, and a locally-locked impl on
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sqlite.
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Args:
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database_engine: the database engine we are connected to
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get_first_callback: a callback which gets the next sequence ID. Used if
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we're on sqlite.
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sequence_name: the name of a postgres sequence to use.
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"""
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if isinstance(database_engine, PostgresEngine):
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return PostgresSequenceGenerator(sequence_name)
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else:
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return LocalSequenceGenerator(get_first_callback)
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