Add checks for postgres sequence consistency (#8402)

Author: erikjohnston

changelog.d/8402.misc

@@ -0,0 +1 @@
+Add checks on startup that PostgreSQL sequences are consistent with their associated tables.

docs/postgres.md

@@ -106,6 +106,17 @@ Note that the above may fail with an error about duplicate rows if corruption
 has already occurred, and such duplicate rows will need to be manually removed.
 
 
+## Fixing inconsistent sequences error
+
+Synapse uses Postgres sequences to generate IDs for various tables. A sequence
+and associated table can get out of sync if, for example, Synapse has been
+downgraded and then upgraded again.
+
+To fix the issue shut down Synapse (including any and all workers) and run the
+SQL command included in the error message. Once done Synapse should start
+successfully.
+
+
 ## Tuning Postgres
 
 The default settings should be fine for most deployments. For larger

synapse/storage/databases/main/registration.py

@@ -41,6 +41,9 @@ def __init__(self, database: DatabasePool, db_conn, hs):
         self.config = hs.config
         self.clock = hs.get_clock()
 
+        # Note: we don't check this sequence for consistency as we'd have to
+        # call `find_max_generated_user_id_localpart` each time, which is
+        # expensive if there are many entries.
         self._user_id_seq = build_sequence_generator(
             database.engine, find_max_generated_user_id_localpart, "user_id_seq",
         )

synapse/storage/databases/state/store.py

@@ -99,6 +99,9 @@ def get_max_state_group_txn(txn: Cursor):
         self._state_group_seq_gen = build_sequence_generator(
             self.database_engine, get_max_state_group_txn, "state_group_id_seq"
         )
+        self._state_group_seq_gen.check_consistency(
+            db_conn, table="state_groups", id_column="id"
+        )
 
     @cached(max_entries=10000, iterable=True)
     async def get_state_group_delta(self, state_group):

synapse/storage/util/id_generators.py

@@ -258,6 +258,11 @@ def __init__(
 
         self._sequence_gen = PostgresSequenceGenerator(sequence_name)
 
+        # We check that the table and sequence haven't diverged.
+        self._sequence_gen.check_consistency(
+            db_conn, table=table, id_column=id_column, positive=positive
+        )
+
         # This goes and fills out the above state from the database.
         self._load_current_ids(db_conn, table, instance_column, id_column)
 

synapse/storage/util/sequence.py

@@ -13,11 +13,34 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import abc
+import logging
 import threading
 from typing import Callable, List, Optional
 
-from synapse.storage.engines import BaseDatabaseEngine, PostgresEngine
-from synapse.storage.types import Cursor
+from synapse.storage.engines import (
+    BaseDatabaseEngine,
+    IncorrectDatabaseSetup,
+    PostgresEngine,
+)
+from synapse.storage.types import Connection, Cursor
+
+logger = logging.getLogger(__name__)
+
+
+_INCONSISTENT_SEQUENCE_ERROR = """
+Postgres sequence '%(seq)s' is inconsistent with associated
+table '%(table)s'. This can happen if Synapse has been downgraded and
+then upgraded again, or due to a bad migration.
+
+To fix this error, shut down Synapse (including any and all workers)
+and run the following SQL:
+
+    SELECT setval('%(seq)s', (
+        %(max_id_sql)s
+    ));
+
+See docs/postgres.md for more information.
+"""
 
 
 class SequenceGenerator(metaclass=abc.ABCMeta):
@@ -28,6 +51,19 @@ def get_next_id_txn(self, txn: Cursor) -> int:
         """Gets the next ID in the sequence"""
         ...
 
+    @abc.abstractmethod
+    def check_consistency(
+        self, db_conn: Connection, table: str, id_column: str, positive: bool = True
+    ):
+        """Should be called during start up to test that the current value of
+        the sequence is greater than or equal to the maximum ID in the table.
+
+        This is to handle various cases where the sequence value can get out
+        of sync with the table, e.g. if Synapse gets rolled back to a previous
+        version and the rolled forwards again.
+        """
+        ...
+
 
 class PostgresSequenceGenerator(SequenceGenerator):
     """An implementation of SequenceGenerator which uses a postgres sequence"""
@@ -45,6 +81,50 @@ def get_next_mult_txn(self, txn: Cursor, n: int) -> List[int]:
         )
         return [i for (i,) in txn]
 
+    def check_consistency(
+        self, db_conn: Connection, table: str, id_column: str, positive: bool = True
+    ):
+        txn = db_conn.cursor()
+
+        # First we get the current max ID from the table.
+        table_sql = "SELECT GREATEST(%(agg)s(%(id)s), 0) FROM %(table)s" % {
+            "id": id_column,
+            "table": table,
+            "agg": "MAX" if positive else "-MIN",
+        }
+
+        txn.execute(table_sql)
+        row = txn.fetchone()
+        if not row:
+            # Table is empty, so nothing to do.
+            txn.close()
+            return
+
+        # Now we fetch the current value from the sequence and compare with the
+        # above.
+        max_stream_id = row[0]
+        txn.execute(
+            "SELECT last_value, is_called FROM %(seq)s" % {"seq": self._sequence_name}
+        )
+        last_value, is_called = txn.fetchone()
+        txn.close()
+
+        # If `is_called` is False then `last_value` is actually the value that
+        # will be generated next, so we decrement to get the true "last value".
+        if not is_called:
+            last_value -= 1
+
+        if max_stream_id > last_value:
+            logger.warning(
+                "Postgres sequence %s is behind table %s: %d < %d",
+                last_value,
+                max_stream_id,
+            )
+            raise IncorrectDatabaseSetup(
+                _INCONSISTENT_SEQUENCE_ERROR
+                % {"seq": self._sequence_name, "table": table, "max_id_sql": table_sql}
+            )
+
 
 GetFirstCallbackType = Callable[[Cursor], int]
 
@@ -81,6 +161,12 @@ def get_next_id_txn(self, txn: Cursor) -> int:
             self._current_max_id += 1
             return self._current_max_id
 
+    def check_consistency(
+        self, db_conn: Connection, table: str, id_column: str, positive: bool = True
+    ):
+        # There is nothing to do for in memory sequences
+        pass
+
 
 def build_sequence_generator(
     database_engine: BaseDatabaseEngine,

tests/storage/test_id_generators.py

@@ -12,9 +12,8 @@
 # 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.
-
-
 from synapse.storage.database import DatabasePool
+from synapse.storage.engines import IncorrectDatabaseSetup
 from synapse.storage.util.id_generators import MultiWriterIdGenerator
 
 from tests.unittest import HomeserverTestCase
@@ -59,7 +58,7 @@ def _create(conn):
                 writers=writers,
             )
 
-        return self.get_success(self.db_pool.runWithConnection(_create))
+        return self.get_success_or_raise(self.db_pool.runWithConnection(_create))
 
     def _insert_rows(self, instance_name: str, number: int):
         """Insert N rows as the given instance, inserting with stream IDs pulled
@@ -411,6 +410,23 @@ async def _get_next_async():
         self.get_success(_get_next_async())
         self.assertEqual(id_gen_3.get_persisted_upto_position(), 6)
 
+    def test_sequence_consistency(self):
+        """Test that we error out if the table and sequence diverges.
+        """
+
+        # Prefill with some rows
+        self._insert_row_with_id("master", 3)
+
+        # Now we add a row *without* updating the stream ID
+        def _insert(txn):
+            txn.execute("INSERT INTO foobar VALUES (26, 'master')")
+
+        self.get_success(self.db_pool.runInteraction("_insert", _insert))
+
+        # Creating the ID gen should error
+        with self.assertRaises(IncorrectDatabaseSetup):
+            self._create_id_generator("first")
+
 
 class BackwardsMultiWriterIdGeneratorTestCase(HomeserverTestCase):
     """Tests MultiWriterIdGenerator that produce *negative* stream IDs.

tests/unittest.py

@@ -14,7 +14,6 @@
 # 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 gc
 import hashlib
 import hmac
@@ -28,6 +27,7 @@
 from canonicaljson import json
 
 from twisted.internet.defer import Deferred, ensureDeferred, succeed
+from twisted.python.failure import Failure
 from twisted.python.threadpool import ThreadPool
 from twisted.trial import unittest
 
@@ -476,6 +476,35 @@ def get_failure(self, d, exc):
         self.pump()
         return self.failureResultOf(d, exc)
 
+    def get_success_or_raise(self, d, by=0.0):
+        """Drive deferred to completion and return result or raise exception
+        on failure.
+        """
+
+        if inspect.isawaitable(d):
+            deferred = ensureDeferred(d)
+        if not isinstance(deferred, Deferred):
+            return d
+
+        results = []  # type: list
+        deferred.addBoth(results.append)
+
+        self.pump(by=by)
+
+        if not results:
+            self.fail(
+                "Success result expected on {!r}, found no result instead".format(
+                    deferred
+                )
+            )
+
+        result = results[0]
+
+        if isinstance(result, Failure):
+            result.raiseException()
+
+        return result
+
     def register_user(self, username, password, admin=False):
         """
         Register a user. Requires the Admin API be registered.