Change the relation_open protocol so that we obtain lock on a relation

(table or index) before trying to open its relcache entry.  This fixes
race conditions in which someone else commits a change to the relation's
catalog entries while we are in process of doing relcache load.  Problems
of that ilk have been reported sporadically for years, but it was not
really practical to fix until recently --- for instance, the recent
addition of WAL-log support for in-place updates helped.

Along the way, remove pg_am.amconcurrent: all AMs are now expected to support
concurrent update.

1 files changed, 9 insertions, 12 deletions

diff --git a/src/backend/executor/nodeBitmapIndexscan.c b/src/backend/executor/nodeBitmapIndexscan.c
index 7555b5a22ca..6a0303cddd1 100644
--- a/src/backend/executor/nodeBitmapIndexscan.c
+++ b/src/backend/executor/nodeBitmapIndexscan.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/nodeBitmapIndexscan.c,v 1.19 2006/05/30 14:01:58 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/nodeBitmapIndexscan.c,v 1.20 2006/07/31 20:09:04 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -201,7 +201,7 @@ ExecEndBitmapIndexScan(BitmapIndexScanState *node)
 	 * close the index relation
 	 */
 	index_endscan(indexScanDesc);
-	index_close(indexRelationDesc);
+	index_close(indexRelationDesc, NoLock);
 }
 
 /* ----------------------------------------------------------------
@@ -258,8 +258,14 @@ ExecInitBitmapIndexScan(BitmapIndexScan *node, EState *estate, int eflags)
 
 	/*
 	 * Open the index relation.
+	 *
+	 * If the parent table is one of the target relations of the query, then
+	 * InitPlan already opened and write-locked the index, so we can avoid
+	 * taking another lock here.  Otherwise we need a normal reader's lock.
 	 */
-	indexstate->biss_RelationDesc = index_open(node->indexid);
+	relistarget = ExecRelationIsTargetRelation(estate, node->scan.scanrelid);
+	indexstate->biss_RelationDesc = index_open(node->indexid,
+									relistarget ? NoLock : AccessShareLock);
 
 	/*
 	 * Initialize index-specific scan state
@@ -303,18 +309,9 @@ ExecInitBitmapIndexScan(BitmapIndexScan *node, EState *estate, int eflags)
 
 	/*
 	 * Initialize scan descriptor.
-	 *
-	 * Note we acquire no locks here; the index machinery does its own locks
-	 * and unlocks.  (We rely on having a lock on the parent table to
-	 * ensure the index won't go away!)  Furthermore, if the parent table
-	 * is one of the target relations of the query, then InitPlan already
-	 * opened and write-locked the index, so we can tell the index machinery
-	 * not to bother getting an extra lock.
 	 */
-	relistarget = ExecRelationIsTargetRelation(estate, node->scan.scanrelid);
 	indexstate->biss_ScanDesc =
 		index_beginscan_multi(indexstate->biss_RelationDesc,
-							  !relistarget,
 							  estate->es_snapshot,
 							  indexstate->biss_NumScanKeys,
 							  indexstate->biss_ScanKeys);