From 906f27dd73124b7ab91ac2335bf42a95c51b9d7b Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Sun, 15 Jun 2008 16:29:05 +0000
Subject: [PATCH] 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 | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index b3d21875b2..639cd7296e 100644
--- 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);