Add a distributed lock (#10269)

This adds a simple best effort locking mechanism that works cross workers.

2 files changed, 336 insertions, 0 deletions

diff --git a/synapse/storage/databases/main/__init__.py b/synapse/storage/databases/main/__init__.py
index 9cce62ae6c..a3fddea042 100644
--- a/synapse/storage/databases/main/__init__.py
+++ b/synapse/storage/databases/main/__init__.py
@@ -46,6 +46,7 @@ from .events_forward_extremities import EventForwardExtremitiesStore
 from .filtering import FilteringStore
 from .group_server import GroupServerStore
 from .keys import KeyStore
+from .lock import LockStore
 from .media_repository import MediaRepositoryStore
 from .metrics import ServerMetricsStore
 from .monthly_active_users import MonthlyActiveUsersStore
@@ -119,6 +120,7 @@ class DataStore(
     CacheInvalidationWorkerStore,
     ServerMetricsStore,
     EventForwardExtremitiesStore,
+    LockStore,
 ):
     def __init__(self, database: DatabasePool, db_conn, hs):
         self.hs = hs
diff --git a/synapse/storage/databases/main/lock.py b/synapse/storage/databases/main/lock.py
new file mode 100644
index 0000000000..e76188328c
--- /dev/null
+++ b/synapse/storage/databases/main/lock.py
@@ -0,0 +1,334 @@
+# Copyright 2021 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 logging
+from types import TracebackType
+from typing import TYPE_CHECKING, Dict, Optional, Tuple, Type
+
+from twisted.internet.interfaces import IReactorCore
+
+from synapse.metrics.background_process_metrics import wrap_as_background_process
+from synapse.storage._base import SQLBaseStore
+from synapse.storage.database import DatabasePool, LoggingTransaction
+from synapse.storage.types import Connection
+from synapse.util import Clock
+from synapse.util.stringutils import random_string
+
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
+
+logger = logging.getLogger(__name__)
+
+
+# How often to renew an acquired lock by updating the `last_renewed_ts` time in
+# the lock table.
+_RENEWAL_INTERVAL_MS = 30 * 1000
+
+# How long before an acquired lock times out.
+_LOCK_TIMEOUT_MS = 2 * 60 * 1000
+
+
+class LockStore(SQLBaseStore):
+    """Provides a best effort distributed lock between worker instances.
+
+    Locks are identified by a name and key. A lock is acquired by inserting into
+    the `worker_locks` table if a) there is no existing row for the name/key or
+    b) the existing row has a `last_renewed_ts` older than `_LOCK_TIMEOUT_MS`.
+
+    When a lock is taken out the instance inserts a random `token`, the instance
+    that holds that token holds the lock until it drops (or times out).
+
+    The instance that holds the lock should regularly update the
+    `last_renewed_ts` column with the current time.
+    """
+
+    def __init__(self, database: DatabasePool, db_conn: Connection, hs: "HomeServer"):
+        super().__init__(database, db_conn, hs)
+
+        self._reactor = hs.get_reactor()
+        self._instance_name = hs.get_instance_id()
+
+        # A map from `(lock_name, lock_key)` to the token of any locks that we
+        # think we currently hold.
+        self._live_tokens: Dict[Tuple[str, str], str] = {}
+
+        # When we shut down we want to remove the locks. Technically this can
+        # lead to a race, as we may drop the lock while we are still processing.
+        # However, a) it should be a small window, b) the lock is best effort
+        # anyway and c) we want to really avoid leaking locks when we restart.
+        hs.get_reactor().addSystemEventTrigger(
+            "before",
+            "shutdown",
+            self._on_shutdown,
+        )
+
+    @wrap_as_background_process("LockStore._on_shutdown")
+    async def _on_shutdown(self) -> None:
+        """Called when the server is shutting down"""
+        logger.info("Dropping held locks due to shutdown")
+
+        for (lock_name, lock_key), token in self._live_tokens.items():
+            await self._drop_lock(lock_name, lock_key, token)
+
+        logger.info("Dropped locks due to shutdown")
+
+    async def try_acquire_lock(self, lock_name: str, lock_key: str) -> Optional["Lock"]:
+        """Try to acquire a lock for the given name/key. Will return an async
+        context manager if the lock is successfully acquired, which *must* be
+        used (otherwise the lock will leak).
+        """
+
+        now = self._clock.time_msec()
+        token = random_string(6)
+
+        if self.db_pool.engine.can_native_upsert:
+
+            def _try_acquire_lock_txn(txn: LoggingTransaction) -> bool:
+                # We take out the lock if either a) there is no row for the lock
+                # already or b) the existing row has timed out.
+                sql = """
+                    INSERT INTO worker_locks (lock_name, lock_key, instance_name, token, last_renewed_ts)
+                    VALUES (?, ?, ?, ?, ?)
+                    ON CONFLICT (lock_name, lock_key)
+                    DO UPDATE
+                        SET
+                            token = EXCLUDED.token,
+                            instance_name = EXCLUDED.instance_name,
+                            last_renewed_ts = EXCLUDED.last_renewed_ts
+                        WHERE
+                            worker_locks.last_renewed_ts < ?
+                """
+                txn.execute(
+                    sql,
+                    (
+                        lock_name,
+                        lock_key,
+                        self._instance_name,
+                        token,
+                        now,
+                        now - _LOCK_TIMEOUT_MS,
+                    ),
+                )
+
+                # We only acquired the lock if we inserted or updated the table.
+                return bool(txn.rowcount)
+
+            did_lock = await self.db_pool.runInteraction(
+                "try_acquire_lock",
+                _try_acquire_lock_txn,
+                # We can autocommit here as we're executing a single query, this
+                # will avoid serialization errors.
+                db_autocommit=True,
+            )
+            if not did_lock:
+                return None
+
+        else:
+            # If we're on an old SQLite we emulate the above logic by first
+            # clearing out any existing stale locks and then upserting.
+
+            def _try_acquire_lock_emulated_txn(txn: LoggingTransaction) -> bool:
+                sql = """
+                    DELETE FROM worker_locks
+                    WHERE
+                        lock_name = ?
+                        AND lock_key = ?
+                        AND last_renewed_ts < ?
+                """
+                txn.execute(
+                    sql,
+                    (lock_name, lock_key, now - _LOCK_TIMEOUT_MS),
+                )
+
+                inserted = self.db_pool.simple_upsert_txn_emulated(
+                    txn,
+                    table="worker_locks",
+                    keyvalues={
+                        "lock_name": lock_name,
+                        "lock_key": lock_key,
+                    },
+                    values={},
+                    insertion_values={
+                        "token": token,
+                        "last_renewed_ts": self._clock.time_msec(),
+                        "instance_name": self._instance_name,
+                    },
+                )
+
+                return inserted
+
+            did_lock = await self.db_pool.runInteraction(
+                "try_acquire_lock_emulated", _try_acquire_lock_emulated_txn
+            )
+
+            if not did_lock:
+                return None
+
+        self._live_tokens[(lock_name, lock_key)] = token
+
+        return Lock(
+            self._reactor,
+            self._clock,
+            self,
+            lock_name=lock_name,
+            lock_key=lock_key,
+            token=token,
+        )
+
+    async def _is_lock_still_valid(
+        self, lock_name: str, lock_key: str, token: str
+    ) -> bool:
+        """Checks whether this instance still holds the lock."""
+        last_renewed_ts = await self.db_pool.simple_select_one_onecol(
+            table="worker_locks",
+            keyvalues={
+                "lock_name": lock_name,
+                "lock_key": lock_key,
+                "token": token,
+            },
+            retcol="last_renewed_ts",
+            allow_none=True,
+            desc="is_lock_still_valid",
+        )
+        return (
+            last_renewed_ts is not None
+            and self._clock.time_msec() - _LOCK_TIMEOUT_MS < last_renewed_ts
+        )
+
+    async def _renew_lock(self, lock_name: str, lock_key: str, token: str) -> None:
+        """Attempt to renew the lock if we still hold it."""
+        await self.db_pool.simple_update(
+            table="worker_locks",
+            keyvalues={
+                "lock_name": lock_name,
+                "lock_key": lock_key,
+                "token": token,
+            },
+            updatevalues={"last_renewed_ts": self._clock.time_msec()},
+            desc="renew_lock",
+        )
+
+    async def _drop_lock(self, lock_name: str, lock_key: str, token: str) -> None:
+        """Attempt to drop the lock, if we still hold it"""
+        await self.db_pool.simple_delete(
+            table="worker_locks",
+            keyvalues={
+                "lock_name": lock_name,
+                "lock_key": lock_key,
+                "token": token,
+            },
+            desc="drop_lock",
+        )
+
+        self._live_tokens.pop((lock_name, lock_key), None)
+
+
+class Lock:
+    """An async context manager that manages an acquired lock, ensuring it is
+    regularly renewed and dropping it when the context manager exits.
+
+    The lock object has an `is_still_valid` method which can be used to
+    double-check the lock is still valid, if e.g. processing work in a loop.
+
+    For example:
+
+        lock = await self.store.try_acquire_lock(...)
+        if not lock:
+            return
+
+        async with lock:
+            for item in work:
+                await process(item)
+
+                if not await lock.is_still_valid():
+                    break
+    """
+
+    def __init__(
+        self,
+        reactor: IReactorCore,
+        clock: Clock,
+        store: LockStore,
+        lock_name: str,
+        lock_key: str,
+        token: str,
+    ) -> None:
+        self._reactor = reactor
+        self._clock = clock
+        self._store = store
+        self._lock_name = lock_name
+        self._lock_key = lock_key
+
+        self._token = token
+
+        self._looping_call = clock.looping_call(
+            self._renew, _RENEWAL_INTERVAL_MS, store, lock_name, lock_key, token
+        )
+
+        self._dropped = False
+
+    @staticmethod
+    @wrap_as_background_process("Lock._renew")
+    async def _renew(
+        store: LockStore,
+        lock_name: str,
+        lock_key: str,
+        token: str,
+    ) -> None:
+        """Renew the lock.
+
+        Note: this is a static method, rather than using self.*, so that we
+        don't end up with a reference to `self` in the reactor, which would stop
+        this from being cleaned up if we dropped the context manager.
+        """
+        await store._renew_lock(lock_name, lock_key, token)
+
+    async def is_still_valid(self) -> bool:
+        """Check if the lock is still held by us"""
+        return await self._store._is_lock_still_valid(
+            self._lock_name, self._lock_key, self._token
+        )
+
+    async def __aenter__(self) -> None:
+        if self._dropped:
+            raise Exception("Cannot reuse a Lock object")
+
+    async def __aexit__(
+        self,
+        _exctype: Optional[Type[BaseException]],
+        _excinst: Optional[BaseException],
+        _exctb: Optional[TracebackType],
+    ) -> bool:
+        if self._looping_call.running:
+            self._looping_call.stop()
+
+        await self._store._drop_lock(self._lock_name, self._lock_key, self._token)
+        self._dropped = True
+
+        return False
+
+    def __del__(self) -> None:
+        if not self._dropped:
+            # We should not be dropped without the lock being released (unless
+            # we're shutting down), but if we are then let's at least stop
+            # renewing the lock.
+            if self._looping_call.running:
+                self._looping_call.stop()
+
+            if self._reactor.running:
+                logger.error(
+                    "Lock for (%s, %s) dropped without being released",
+                    self._lock_name,
+                    self._lock_key,
+                )