From e704cc2a48c6adc5d3da79a49ed02961edfc3b4a Mon Sep 17 00:00:00 2001
From: Kokokokoka <vasiliy.boytsov@gmail.com>
Date: Fri, 24 Sep 2021 12:19:51 +0300
Subject: [PATCH] In `_purge_history_txn`, ensure that txn.fetchall has
elements before accessing rows (#10690)
This change adds a check for row existence before accessing row element, this should fix issue #10669
Signed-off-by: Vasya Boytsov vasiliy.boytsov@phystech.edu
---
changelog.d/10690.bugfix | 1 +
.../storage/databases/main/purge_events.py | 22 +++++++++++--------
2 files changed, 14 insertions(+), 9 deletions(-)
create mode 100644 changelog.d/10690.bugfix
diff --git a/changelog.d/10690.bugfix b/changelog.d/10690.bugfix
new file mode 100644
index 0000000000..059eea7464
--- /dev/null
+++ b/changelog.d/10690.bugfix
@@ -0,0 +1 @@
+Fix a long-standing bug that caused an `AssertionError` when purging history in certain rooms. Contributed by @Kokokokoka.
diff --git a/synapse/storage/databases/main/purge_events.py b/synapse/storage/databases/main/purge_events.py
index bccff5e5b9..3eb30944bf 100644
--- a/synapse/storage/databases/main/purge_events.py
+++ b/synapse/storage/databases/main/purge_events.py
@@ -102,15 +102,19 @@ class PurgeEventsStore(StateGroupWorkerStore, CacheInvalidationWorkerStore):
(room_id,),
)
rows = txn.fetchall()
- max_depth = max(row[1] for row in rows)
-
- if max_depth < token.topological:
- # We need to ensure we don't delete all the events from the database
- # otherwise we wouldn't be able to send any events (due to not
- # having any backwards extremities)
- raise SynapseError(
- 400, "topological_ordering is greater than forward extremeties"
- )
+ # if we already have no forwards extremities (for example because they were
+ # cleared out by the `delete_old_current_state_events` background database
+ # update), then we may as well carry on.
+ if rows:
+ max_depth = max(row[1] for row in rows)
+
+ if max_depth < token.topological:
+ # We need to ensure we don't delete all the events from the database
+ # otherwise we wouldn't be able to send any events (due to not
+ # having any backwards extremities)
+ raise SynapseError(
+ 400, "topological_ordering is greater than forward extremities"
+ )
logger.info("[purge] looking for events to delete")
--
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