Fix missing _add_persisted_position (#8179)

This was forgotten in #8164.

changelog.d/8179.misc

@@ -0,0 +1 @@
+Add functions to `MultiWriterIdGen` used by events stream.

synapse/storage/util/id_generators.py

@@ -343,6 +343,8 @@ class MultiWriterIdGenerator:
                 curr = self._current_positions.get(self._instance_name, 0)
                 self._current_positions[self._instance_name] = max(curr, next_id)
 
+            self._add_persisted_position(next_id)
+
     def get_current_token(self) -> int:
         """Returns the maximum stream id such that all stream ids less than or
         equal to it have been successfully persisted.

tests/storage/test_id_generators.py

@@ -58,6 +58,10 @@ class MultiWriterIdGeneratorTestCase(HomeserverTestCase):
         return self.get_success(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
+        from the postgres sequence.
+        """
+
         def _insert(txn):
             for _ in range(number):
                 txn.execute(
@@ -65,7 +69,20 @@ class MultiWriterIdGeneratorTestCase(HomeserverTestCase):
                     (instance_name,),
                 )
 
-        self.get_success(self.db_pool.runInteraction("test_single_instance", _insert))
+        self.get_success(self.db_pool.runInteraction("_insert_rows", _insert))
+
+    def _insert_row_with_id(self, instance_name: str, stream_id: int):
+        """Insert one row as the given instance with given stream_id, updating
+        the postgres sequence position to match.
+        """
+
+        def _insert(txn):
+            txn.execute(
+                "INSERT INTO foobar VALUES (?, ?)", (stream_id, instance_name,),
+            )
+            txn.execute("SELECT setval('foobar_seq', ?)", (stream_id,))
+
+        self.get_success(self.db_pool.runInteraction("_insert_row_with_id", _insert))
 
     def test_empty(self):
         """Test an ID generator against an empty database gives sensible
@@ -188,11 +205,17 @@ class MultiWriterIdGeneratorTestCase(HomeserverTestCase):
         positions.
         """
 
-        self._insert_rows("first", 3)
-        self._insert_rows("second", 5)
+        # The following tests are a bit cheeky in that we notify about new
+        # positions via `advance` without *actually* advancing the postgres
+        # sequence.
+
+        self._insert_row_with_id("first", 3)
+        self._insert_row_with_id("second", 5)
 
         id_gen = self._create_id_generator("first")
 
+        self.assertEqual(id_gen.get_positions(), {"first": 3, "second": 5})
+
         # Min is 3 and there is a gap between 5, so we expect it to be 3.
         self.assertEqual(id_gen.get_persisted_upto_position(), 3)
 
@@ -218,3 +241,26 @@ class MultiWriterIdGeneratorTestCase(HomeserverTestCase):
         id_gen.advance("first", 11)
         id_gen.advance("second", 15)
         self.assertEqual(id_gen.get_persisted_upto_position(), 11)
+
+    def test_get_persisted_upto_position_get_next(self):
+        """Test that `get_persisted_upto_position` correctly tracks updates to
+        positions when `get_next` is called.
+        """
+
+        self._insert_row_with_id("first", 3)
+        self._insert_row_with_id("second", 5)
+
+        id_gen = self._create_id_generator("first")
+
+        self.assertEqual(id_gen.get_positions(), {"first": 3, "second": 5})
+
+        self.assertEqual(id_gen.get_persisted_upto_position(), 3)
+        with self.get_success(id_gen.get_next()) as stream_id:
+            self.assertEqual(stream_id, 6)
+            self.assertEqual(id_gen.get_persisted_upto_position(), 3)
+
+        self.assertEqual(id_gen.get_persisted_upto_position(), 6)
+
+        # We assume that so long as `get_next` does correctly advance the
+        # `persisted_upto_position` in this case, then it will be correct in the
+        # other cases that are tested above (since they'll hit the same code).