Fix race conditions associated with SPGiST redirection tuples.

The correct test for whether a redirection tuple is removable is whether
tuple's xid < RecentGlobalXmin, not OldestXmin; the previous coding
failed to protect index searches being done in concurrent transactions that
have no XID.  This mirrors the recent fix in btree's page recycling logic
made in commit d3abbbebe52eb1e59e621c880ad57df9d40d13f2.

Also, WAL-log the newest XID of any removed redirection tuple on an index
page, and apply ResolveRecoveryConflictWithSnapshot during InHotStandby WAL
replay.  This protects against concurrent Hot Standby transactions possibly
needing to see the redirection tuple(s).

Per my query of 2012-03-12 and subsequent discussion.

1 files changed, 11 insertions, 9 deletions

diff --git a/src/backend/access/spgist/spgvacuum.c b/src/backend/access/spgist/spgvacuum.c
index 27b55170cb4..26bbd656c10 100644
--- a/src/backend/access/spgist/spgvacuum.c
+++ b/src/backend/access/spgist/spgvacuum.c
@@ -24,7 +24,6 @@
 #include "storage/bufmgr.h"
 #include "storage/indexfsm.h"
 #include "storage/lmgr.h"
-#include "storage/procarray.h"
 #include "utils/snapmgr.h"
 
 
@@ -49,7 +48,6 @@ typedef struct spgBulkDeleteState
 	SpGistState spgstate;		/* for SPGiST operations that need one */
 	spgVacPendingItem *pendingList;		/* TIDs we need to (re)visit */
 	TransactionId myXmin;		/* for detecting newly-added redirects */
-	TransactionId OldestXmin;	/* for deciding a redirect is obsolete */
 	BlockNumber lastFilledBlock;	/* last non-deletable block */
 } spgBulkDeleteState;
 
@@ -491,8 +489,7 @@ vacuumLeafRoot(spgBulkDeleteState *bds, Relation index, Buffer buffer)
  * Unlike the routines above, this works on both leaf and inner pages.
  */
 static void
-vacuumRedirectAndPlaceholder(Relation index, Buffer buffer,
-							 TransactionId OldestXmin)
+vacuumRedirectAndPlaceholder(Relation index, Buffer buffer)
 {
 	Page		page = BufferGetPage(buffer);
 	SpGistPageOpaque opaque = SpGistPageGetOpaque(page);
@@ -509,6 +506,7 @@ vacuumRedirectAndPlaceholder(Relation index, Buffer buffer,
 	xlrec.node = index->rd_node;
 	xlrec.blkno = BufferGetBlockNumber(buffer);
 	xlrec.nToPlaceholder = 0;
+	xlrec.newestRedirectXid = InvalidTransactionId;
 
 	START_CRIT_SECTION();
 
@@ -526,13 +524,18 @@ vacuumRedirectAndPlaceholder(Relation index, Buffer buffer,
 		dt = (SpGistDeadTuple) PageGetItem(page, PageGetItemId(page, i));
 
 		if (dt->tupstate == SPGIST_REDIRECT &&
-			TransactionIdPrecedes(dt->xid, OldestXmin))
+			TransactionIdPrecedes(dt->xid, RecentGlobalXmin))
 		{
 			dt->tupstate = SPGIST_PLACEHOLDER;
 			Assert(opaque->nRedirection > 0);
 			opaque->nRedirection--;
 			opaque->nPlaceholder++;
 
+			/* remember newest XID among the removed redirects */
+			if (!TransactionIdIsValid(xlrec.newestRedirectXid) ||
+				TransactionIdPrecedes(xlrec.newestRedirectXid, dt->xid))
+				xlrec.newestRedirectXid = dt->xid;
+
 			ItemPointerSetInvalid(&dt->pointer);
 
 			itemToPlaceholder[xlrec.nToPlaceholder] = i;
@@ -640,13 +643,13 @@ spgvacuumpage(spgBulkDeleteState *bds, BlockNumber blkno)
 		else
 		{
 			vacuumLeafPage(bds, index, buffer, false);
-			vacuumRedirectAndPlaceholder(index, buffer, bds->OldestXmin);
+			vacuumRedirectAndPlaceholder(index, buffer);
 		}
 	}
 	else
 	{
 		/* inner page */
-		vacuumRedirectAndPlaceholder(index, buffer, bds->OldestXmin);
+		vacuumRedirectAndPlaceholder(index, buffer);
 	}
 
 	/*
@@ -723,7 +726,7 @@ spgprocesspending(spgBulkDeleteState *bds)
 			/* deal with any deletable tuples */
 			vacuumLeafPage(bds, index, buffer, true);
 			/* might as well do this while we are here */
-			vacuumRedirectAndPlaceholder(index, buffer, bds->OldestXmin);
+			vacuumRedirectAndPlaceholder(index, buffer);
 
 			SpGistSetLastUsedPage(index, buffer);
 
@@ -806,7 +809,6 @@ spgvacuumscan(spgBulkDeleteState *bds)
 	initSpGistState(&bds->spgstate, index);
 	bds->pendingList = NULL;
 	bds->myXmin = GetActiveSnapshot()->xmin;
-	bds->OldestXmin = GetOldestXmin(true, false);
 	bds->lastFilledBlock = SPGIST_LAST_FIXED_BLKNO;
 
 	/*