Check syscache result in AlterStatistics

The syscache lookup may return NULL even for valid OID, for example due
to a concurrent DROP STATISTICS, so a HeapTupleIsValid is necessary.
Without it, it may fail with a segfault.

Reported by Alexander Lakhin, patch by me. Backpatch to 13, where ALTER
STATISTICS ... SET STATISTICS was introduced.

Backpatch-through: 13
Discussion: https://postgr.es/m/17372-bf3b6e947e35ae77%40postgresql.org

diff --git a/src/backend/commands/statscmds.c b/src/backend/commands/statscmds.c
index edc4c4810433bfedcd9b0da4b0a0e6222fc19cce..59e531b55d8c718fdbe61107de48c451099b00a3 100644 (file)
--- a/src/backend/commands/statscmds.c
+++ b/src/backend/commands/statscmds.c
@@ -677,6 +677,8 @@ AlterStatistics(AlterStatsStmt *stmt)
    rel = table_open(StatisticExtRelationId, RowExclusiveLock);
 
    oldtup = SearchSysCache1(STATEXTOID, ObjectIdGetDatum(stxoid));
+   if (!HeapTupleIsValid(oldtup))
+       elog(ERROR, "cache lookup failed for extended statistics object %u", stxoid);
 
    /* Must be owner of the existing statistics object */
    if (!pg_statistics_object_ownercheck(stxoid, GetUserId()))