Store state groups separately from events (#2784)

* Split state group persist into seperate storage func

* Add per database engine code for state group id gen

* Move store_state_group to StateReadStore

This allows other workers to use it, and so resolve state.

* Hook up store_state_group

* Fix tests

* Rename _store_mult_state_groups_txn

* Rename StateGroupReadStore

* Remove redundant _have_persisted_state_group_txn

* Update comments

* Comment compute_event_context

* Set start val for state_group_id_seq

... otherwise we try to recreate old state groups

* Update comments

* Don't store state for outliers

* Update comment

* Update docstring as state groups are ints

synapse/storage/schema/delta/47/state_group_seq.py

@@ -0,0 +1,37 @@
+# Copyright 2018 New Vector Ltd
+#
+# 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.engines import PostgresEngine
+
+
+def run_create(cur, database_engine, *args, **kwargs):
+    if isinstance(database_engine, PostgresEngine):
+        # if we already have some state groups, we want to start making new
+        # ones with a higher id.
+        cur.execute("SELECT max(id) FROM state_groups")
+        row = cur.fetchone()
+
+        if row[0] is None:
+            start_val = 1
+        else:
+            start_val = row[0] + 1
+
+        cur.execute(
+            "CREATE SEQUENCE state_group_id_seq START WITH %s",
+            (start_val, ),
+        )
+
+
+def run_upgrade(*args, **kwargs):
+    pass