diff --git a/tests/storage/test_id_generators.py b/tests/storage/test_id_generators.py
new file mode 100644
index 0000000000..55e9ecf264
--- /dev/null
+++ b/tests/storage/test_id_generators.py
@@ -0,0 +1,184 @@
+# -*- coding: utf-8 -*-
+# 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.
+
+
+from synapse.storage.database import Database
+from synapse.storage.util.id_generators import MultiWriterIdGenerator
+
+from tests.unittest import HomeserverTestCase
+from tests.utils import USE_POSTGRES_FOR_TESTS
+
+
+class MultiWriterIdGeneratorTestCase(HomeserverTestCase):
+ if not USE_POSTGRES_FOR_TESTS:
+ skip = "Requires Postgres"
+
+ def prepare(self, reactor, clock, hs):
+ self.store = hs.get_datastore()
+ self.db = self.store.db # type: Database
+
+ self.get_success(self.db.runInteraction("_setup_db", self._setup_db))
+
+ def _setup_db(self, txn):
+ txn.execute("CREATE SEQUENCE foobar_seq")
+ txn.execute(
+ """
+ CREATE TABLE foobar (
+ stream_id BIGINT NOT NULL,
+ instance_name TEXT NOT NULL,
+ data TEXT
+ );
+ """
+ )
+
+ def _create_id_generator(self, instance_name="master") -> MultiWriterIdGenerator:
+ def _create(conn):
+ return MultiWriterIdGenerator(
+ conn,
+ self.db,
+ instance_name=instance_name,
+ table="foobar",
+ instance_column="instance_name",
+ id_column="stream_id",
+ sequence_name="foobar_seq",
+ )
+
+ return self.get_success(self.db.runWithConnection(_create))
+
+ def _insert_rows(self, instance_name: str, number: int):
+ def _insert(txn):
+ for _ in range(number):
+ txn.execute(
+ "INSERT INTO foobar VALUES (nextval('foobar_seq'), ?)",
+ (instance_name,),
+ )
+
+ self.get_success(self.db.runInteraction("test_single_instance", _insert))
+
+ def test_empty(self):
+ """Test an ID generator against an empty database gives sensible
+ current positions.
+ """
+
+ id_gen = self._create_id_generator()
+
+ # The table is empty so we expect an empty map for positions
+ self.assertEqual(id_gen.get_positions(), {})
+
+ def test_single_instance(self):
+ """Test that reads and writes from a single process are handled
+ correctly.
+ """
+
+ # Prefill table with 7 rows written by 'master'
+ self._insert_rows("master", 7)
+
+ id_gen = self._create_id_generator()
+
+ self.assertEqual(id_gen.get_positions(), {"master": 7})
+ self.assertEqual(id_gen.get_current_token("master"), 7)
+
+ # Try allocating a new ID gen and check that we only see position
+ # advanced after we leave the context manager.
+
+ async def _get_next_async():
+ with await id_gen.get_next() as stream_id:
+ self.assertEqual(stream_id, 8)
+
+ self.assertEqual(id_gen.get_positions(), {"master": 7})
+ self.assertEqual(id_gen.get_current_token("master"), 7)
+
+ self.get_success(_get_next_async())
+
+ self.assertEqual(id_gen.get_positions(), {"master": 8})
+ self.assertEqual(id_gen.get_current_token("master"), 8)
+
+ def test_multi_instance(self):
+ """Test that reads and writes from multiple processes are handled
+ correctly.
+ """
+ self._insert_rows("first", 3)
+ self._insert_rows("second", 4)
+
+ first_id_gen = self._create_id_generator("first")
+ second_id_gen = self._create_id_generator("second")
+
+ self.assertEqual(first_id_gen.get_positions(), {"first": 3, "second": 7})
+ self.assertEqual(first_id_gen.get_current_token("first"), 3)
+ self.assertEqual(first_id_gen.get_current_token("second"), 7)
+
+ # Try allocating a new ID gen and check that we only see position
+ # advanced after we leave the context manager.
+
+ async def _get_next_async():
+ with await first_id_gen.get_next() as stream_id:
+ self.assertEqual(stream_id, 8)
+
+ self.assertEqual(
+ first_id_gen.get_positions(), {"first": 3, "second": 7}
+ )
+
+ self.get_success(_get_next_async())
+
+ self.assertEqual(first_id_gen.get_positions(), {"first": 8, "second": 7})
+
+ # However the ID gen on the second instance won't have seen the update
+ self.assertEqual(second_id_gen.get_positions(), {"first": 3, "second": 7})
+
+ # ... but calling `get_next` on the second instance should give a unique
+ # stream ID
+
+ async def _get_next_async():
+ with await second_id_gen.get_next() as stream_id:
+ self.assertEqual(stream_id, 9)
+
+ self.assertEqual(
+ second_id_gen.get_positions(), {"first": 3, "second": 7}
+ )
+
+ self.get_success(_get_next_async())
+
+ self.assertEqual(second_id_gen.get_positions(), {"first": 3, "second": 9})
+
+ # If the second ID gen gets told about the first, it correctly updates
+ second_id_gen.advance("first", 8)
+ self.assertEqual(second_id_gen.get_positions(), {"first": 8, "second": 9})
+
+ def test_get_next_txn(self):
+ """Test that the `get_next_txn` function works correctly.
+ """
+
+ # Prefill table with 7 rows written by 'master'
+ self._insert_rows("master", 7)
+
+ id_gen = self._create_id_generator()
+
+ self.assertEqual(id_gen.get_positions(), {"master": 7})
+ self.assertEqual(id_gen.get_current_token("master"), 7)
+
+ # Try allocating a new ID gen and check that we only see position
+ # advanced after we leave the context manager.
+
+ def _get_next_txn(txn):
+ stream_id = id_gen.get_next_txn(txn)
+ self.assertEqual(stream_id, 8)
+
+ self.assertEqual(id_gen.get_positions(), {"master": 7})
+ self.assertEqual(id_gen.get_current_token("master"), 7)
+
+ self.get_success(self.db.runInteraction("test", _get_next_txn))
+
+ self.assertEqual(id_gen.get_positions(), {"master": 8})
+ self.assertEqual(id_gen.get_current_token("master"), 8)
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