From 3e87d79e1c6ef894387ee2f24e008dfb8f5f853f Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Fri, 25 Sep 2020 09:58:32 +0100
Subject: [PATCH] Fix schema delta for servers that have not backfilled (#8396)
Fixes #8395.
---
changelog.d/8396.feature | 1 +
.../schema/delta/58/14events_instance_name.sql.postgres | 4 +++-
synapse/storage/util/id_generators.py | 6 +++++-
3 files changed, 9 insertions(+), 2 deletions(-)
create mode 100644 changelog.d/8396.feature
diff --git a/changelog.d/8396.feature b/changelog.d/8396.feature
new file mode 100644
index 0000000000..b363e929ea
--- /dev/null
+++ b/changelog.d/8396.feature
@@ -0,0 +1 @@
+Add experimental support for sharding event persister.
diff --git a/synapse/storage/databases/main/schema/delta/58/14events_instance_name.sql.postgres b/synapse/storage/databases/main/schema/delta/58/14events_instance_name.sql.postgres
index 97c1e6a0c5..c31f9af82a 100644
--- a/synapse/storage/databases/main/schema/delta/58/14events_instance_name.sql.postgres
+++ b/synapse/storage/databases/main/schema/delta/58/14events_instance_name.sql.postgres
@@ -21,6 +21,8 @@ SELECT setval('events_stream_seq', (
CREATE SEQUENCE IF NOT EXISTS events_backfill_stream_seq;
+-- If the server has never backfilled a room then doing `-MIN(...)` will give
+-- a negative result, hence why we do `GREATEST(...)`
SELECT setval('events_backfill_stream_seq', (
- SELECT COALESCE(-MIN(stream_ordering), 1) FROM events
+ SELECT GREATEST(COALESCE(-MIN(stream_ordering), 1), 1) FROM events
));
diff --git a/synapse/storage/util/id_generators.py b/synapse/storage/util/id_generators.py
index 727fcc521c..4269eaf918 100644
--- a/synapse/storage/util/id_generators.py
+++ b/synapse/storage/util/id_generators.py
@@ -287,8 +287,12 @@ class MultiWriterIdGenerator:
min_stream_id = min(self._current_positions.values(), default=None)
if min_stream_id is None:
+ # We add a GREATEST here to ensure that the result is always
+ # positive. (This can be a problem for e.g. backfill streams where
+ # the server has never backfilled).
sql = """
- SELECT COALESCE(%(agg)s(%(id)s), 1) FROM %(table)s
+ SELECT GREATEST(COALESCE(%(agg)s(%(id)s), 1), 1)
+ FROM %(table)s
""" % {
"id": id_column,
"table": table,
--
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