From 507cafc2c3d752a45f1fc498055aa009ca4c83dc Mon Sep 17 00:00:00 2001
From: "Olivier Wilkinson (reivilibre)" <olivier@librepush.net>
Date: Mon, 2 Aug 2021 14:19:20 +0100
Subject: [PATCH] Make a single-group transaction function

---
 synapse/storage/databases/state/bg_updates.py | 111 +++++++++++++++++-
 1 file changed, 108 insertions(+), 3 deletions(-)

diff --git a/synapse/storage/databases/state/bg_updates.py b/synapse/storage/databases/state/bg_updates.py
index c2891cb07f..7608bb67a5 100644
--- a/synapse/storage/databases/state/bg_updates.py
+++ b/synapse/storage/databases/state/bg_updates.py
@@ -13,13 +13,17 @@
 # limitations under the License.
 
 import logging
-from typing import Optional
+import typing
+from typing import Dict, List, Optional
 
 from synapse.storage._base import SQLBaseStore
 from synapse.storage.database import DatabasePool
 from synapse.storage.engines import PostgresEngine
 from synapse.storage.state import StateFilter
 
+if typing.TYPE_CHECKING:
+    from synapse.types import StateMap
+
 logger = logging.getLogger(__name__)
 
 
@@ -73,8 +77,8 @@ class StateGroupBackgroundUpdateStore(SQLBaseStore):
             return count
 
     def _get_state_groups_from_groups_txn(
-        self, txn, groups, state_filter: Optional[StateFilter] = None
-    ):
+        self, txn, groups: List[int], state_filter: Optional[StateFilter] = None
+    ) -> Dict[int, "StateMap[str]"]:
         state_filter = state_filter or StateFilter.all()
 
         results = {group: {} for group in groups}
@@ -175,6 +179,107 @@ class StateGroupBackgroundUpdateStore(SQLBaseStore):
 
         return results
 
+    def _get_state_groups_from_group_txn(
+        self, txn, group: int, state_filter: Optional[StateFilter] = None
+    ) -> "StateMap[str]":
+        state_filter = state_filter or StateFilter.all()
+
+        result = {}
+
+        where_clause, where_args = state_filter.make_sql_filter_clause()
+
+        # Unless the filter clause is empty, we're going to append it after an
+        # existing where clause
+        if where_clause:
+            where_clause = " AND (%s)" % (where_clause,)
+
+        if isinstance(self.database_engine, PostgresEngine):
+            # Temporarily disable sequential scans in this transaction. This is
+            # a temporary hack until we can add the right indices in
+            txn.execute("SET LOCAL enable_seqscan=off")
+
+            # The below query walks the state_group tree so that the "state"
+            # table includes all state_groups in the tree. It then joins
+            # against `state_groups_state` to fetch the latest state.
+            # It assumes that previous state groups are always numerically
+            # lesser.
+            # The PARTITION is used to get the event_id in the greatest state
+            # group for the given type, state_key.
+            # This may return multiple rows per (type, state_key), but last_value
+            # should be the same.
+            sql = """
+                WITH RECURSIVE state(state_group) AS (
+                    VALUES(?::bigint)
+                    UNION ALL
+                    SELECT prev_state_group FROM state_group_edges e, state s
+                    WHERE s.state_group = e.state_group
+                )
+                SELECT DISTINCT ON (type, state_key)
+                    type, state_key, event_id
+                FROM state_groups_state
+                WHERE state_group IN (
+                    SELECT state_group FROM state
+                ) %s
+                ORDER BY type, state_key, state_group DESC
+            """
+
+            args = [group]
+            args.extend(where_args)
+
+            txn.execute(sql % (where_clause,), args)
+            for row in txn:
+                typ, state_key, event_id = row
+                key = (typ, state_key)
+                result[key] = event_id
+        else:
+            max_entries_returned = state_filter.max_entries_returned()
+
+            # We don't use WITH RECURSIVE on sqlite3 as there are distributions
+            # that ship with an sqlite3 version that doesn't support it (e.g. wheezy)
+            next_group = group
+
+            while next_group:
+                # We did this before by getting the list of group ids, and
+                # then passing that list to sqlite to get latest event for
+                # each (type, state_key). However, that was terribly slow
+                # without the right indices (which we can't add until
+                # after we finish deduping state, which requires this func)
+                args = [next_group]
+                args.extend(where_args)
+
+                txn.execute(
+                    "SELECT type, state_key, event_id FROM state_groups_state"
+                    " WHERE state_group = ? " + where_clause,
+                    args,
+                )
+                result.update(
+                    ((typ, state_key), event_id)
+                    for typ, state_key, event_id in txn
+                    if (typ, state_key) not in result
+                )
+
+                # If the number of entries in the (type,state_key)->event_id dict
+                # matches the number of (type,state_keys) types we were searching
+                # for, then we must have found them all, so no need to go walk
+                # further down the tree... UNLESS our types filter contained
+                # wildcards (i.e. Nones) in which case we have to do an exhaustive
+                # search
+                if (
+                    max_entries_returned is not None
+                    and len(result) == max_entries_returned
+                ):
+                    break
+
+                next_group = self.db_pool.simple_select_one_onecol_txn(
+                    txn,
+                    table="state_group_edges",
+                    keyvalues={"state_group": next_group},
+                    retcol="prev_state_group",
+                    allow_none=True,
+                )
+
+        return result
+
 
 class StateBackgroundUpdateStore(StateGroupBackgroundUpdateStore):