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

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.

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);