Make DROP INDEX lock the parent table before locking the index. This behavior

tglsfdc · tglsfdc · commit 906f27dd7312 · 2008-06-15T16:29:05.000Z
is necessary to avoid deadlock against ordinary queries, but we'd broken it
with recent changes that made the DROP machinery lock the index before
arriving at index_drop.  Per intermittent buildfarm failures.
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/tablecmds.c,v 1.257 2008/06/15 01:25:53 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/tablecmds.c,v 1.258 2008/06/15 16:29:05 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -676,6 +676,28 @@ RemoveRelations(DropStmt *drop)
 			continue;
 		}
 
+		/*
+		 * In DROP INDEX, attempt to acquire lock on the parent table before
+		 * locking the index.  index_drop() will need this anyway, and since
+		 * regular queries lock tables before their indexes, we risk deadlock
+		 * if we do it the other way around.  No error if we don't find a
+		 * pg_index entry, though --- that most likely means it isn't an
+		 * index, and we'll fail below.
+		 */
+		if (relkind == RELKIND_INDEX)
+		{
+			tuple = SearchSysCache(INDEXRELID,
+								   ObjectIdGetDatum(relOid),
+								   0, 0, 0);
+			if (HeapTupleIsValid(tuple))
+			{
+				Form_pg_index index = (Form_pg_index) GETSTRUCT(tuple);
+
+				LockRelationOid(index->indrelid, AccessExclusiveLock);
+				ReleaseSysCache(tuple);
+			}
+		}
+
 		/* Get the lock before trying to fetch the syscache entry */
 		LockRelationOid(relOid, AccessExclusiveLock);
 
