Insert CHECK_FOR_INTERRUPTS() calls into btree and hash index scans at the

points where we step right or left to the next page.  This should ensure
reasonable response time to a query cancel request during an unsuccessful
index scan, as seen in recent gripe from Marc Cousin.  It's a bit trickier
than it might seem at first glance, because CHECK_FOR_INTERRUPTS() is a no-op
if executed while holding a buffer lock.  So we have to do it just at the
point where we've dropped one page lock and not yet acquired the next.

Remove CHECK_FOR_INTERRUPTS calls at the top level of btgetbitmap and
hashgetbitmap, since they're pointless given the added checks.

I think that GIST is okay already --- at least, there's a CHECK_FOR_INTERRUPTS
at a plausible-looking place in gistnext().  I don't claim to know GIN well
enough to try to poke it for this, if indeed it has a problem at all.

This is a pre-existing issue, but in view of the lack of prior complaints
I'm not going to risk back-patching.

src/backend/access/hash/hash.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/hash/hash.c,v 1.109 2009/03/24 20:17:11 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/hash/hash.c,v 1.110 2009/05/05 19:36:32 tgl Exp $
  *
  * NOTES
  *	  This file contains only the public interface routines.
@@ -22,7 +22,6 @@
 #include "access/relscan.h"
 #include "catalog/index.h"
 #include "commands/vacuum.h"
-#include "miscadmin.h"
 #include "optimizer/cost.h"
 #include "optimizer/plancat.h"
 #include "storage/bufmgr.h"
@@ -297,8 +296,6 @@ hashgetbitmap(PG_FUNCTION_ARGS)
 	{
 		bool	add_tuple;
 
-		CHECK_FOR_INTERRUPTS();
-
 		/*
 		 * Skip killed tuples if asked to.
 		 */

src/backend/access/hash/hashsearch.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/hash/hashsearch.c,v 1.55 2009/01/01 17:23:35 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/hash/hashsearch.c,v 1.56 2009/05/05 19:36:32 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -16,6 +16,7 @@
 
 #include "access/hash.h"
 #include "access/relscan.h"
+#include "miscadmin.h"
 #include "pgstat.h"
 #include "storage/bufmgr.h"
 #include "utils/rel.h"
@@ -74,6 +75,8 @@ _hash_readnext(Relation rel,
 	blkno = (*opaquep)->hasho_nextblkno;
 	_hash_relbuf(rel, *bufp);
 	*bufp = InvalidBuffer;
+	/* check for interrupts while we're not holding any buffer lock */
+	CHECK_FOR_INTERRUPTS();
 	if (BlockNumberIsValid(blkno))
 	{
 		*bufp = _hash_getbuf(rel, blkno, HASH_READ, LH_OVERFLOW_PAGE);
@@ -94,6 +97,8 @@ _hash_readprev(Relation rel,
 	blkno = (*opaquep)->hasho_prevblkno;
 	_hash_relbuf(rel, *bufp);
 	*bufp = InvalidBuffer;
+	/* check for interrupts while we're not holding any buffer lock */
+	CHECK_FOR_INTERRUPTS();
 	if (BlockNumberIsValid(blkno))
 	{
 		*bufp = _hash_getbuf(rel, blkno, HASH_READ,

src/backend/access/nbtree/nbtree.c

@@ -12,7 +12,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/nbtree/nbtree.c,v 1.168 2009/03/24 20:17:12 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/nbtree/nbtree.c,v 1.169 2009/05/05 19:36:32 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -24,7 +24,6 @@
 #include "catalog/index.h"
 #include "catalog/storage.h"
 #include "commands/vacuum.h"
-#include "miscadmin.h"
 #include "storage/bufmgr.h"
 #include "storage/freespace.h"
 #include "storage/indexfsm.h"
@@ -315,8 +314,6 @@ btgetbitmap(PG_FUNCTION_ARGS)
 		 */
 		if (++so->currPos.itemIndex > so->currPos.lastItem)
 		{
-			CHECK_FOR_INTERRUPTS();
-
 			/* let _bt_next do the heavy lifting */
 			if (!_bt_next(scan, ForwardScanDirection))
 				break;

src/backend/access/nbtree/nbtsearch.c

@@ -8,7 +8,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/nbtree/nbtsearch.c,v 1.119 2009/01/01 17:23:35 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/nbtree/nbtsearch.c,v 1.120 2009/05/05 19:36:32 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -18,6 +18,7 @@
 #include "access/genam.h"
 #include "access/nbtree.h"
 #include "access/relscan.h"
+#include "miscadmin.h"
 #include "pgstat.h"
 #include "storage/bufmgr.h"
 #include "utils/lsyscache.h"
@@ -1126,16 +1127,16 @@ _bt_steppage(IndexScanDesc scan, ScanDirection dir)
 
 		for (;;)
 		{
-			/* if we're at end of scan, release the buffer and return */
+			/* release the previous buffer */
+			_bt_relbuf(rel, so->currPos.buf);
+			so->currPos.buf = InvalidBuffer;
+			/* if we're at end of scan, give up */
 			if (blkno == P_NONE || !so->currPos.moreRight)
-			{
-				_bt_relbuf(rel, so->currPos.buf);
-				so->currPos.buf = InvalidBuffer;
 				return false;
-			}
+			/* check for interrupts while we're not holding any buffer lock */
+			CHECK_FOR_INTERRUPTS();
 			/* step right one page */
-			so->currPos.buf = _bt_relandgetbuf(rel, so->currPos.buf,
-											   blkno, BT_READ);
+			so->currPos.buf = _bt_getbuf(rel, blkno, BT_READ);
 			/* check for deleted page */
 			page = BufferGetPage(so->currPos.buf);
 			opaque = (BTPageOpaque) PageGetSpecialPointer(page);
@@ -1239,7 +1240,10 @@ _bt_walk_left(Relation rel, Buffer buf)
 		obknum = BufferGetBlockNumber(buf);
 		/* step left */
 		blkno = lblkno = opaque->btpo_prev;
-		buf = _bt_relandgetbuf(rel, buf, blkno, BT_READ);
+		_bt_relbuf(rel, buf);
+		/* check for interrupts while we're not holding any buffer lock */
+		CHECK_FOR_INTERRUPTS();
+		buf = _bt_getbuf(rel, blkno, BT_READ);
 		page = BufferGetPage(buf);
 		opaque = (BTPageOpaque) PageGetSpecialPointer(page);