Reduce impact of btree page reuse on Hot Standby by fixing off-by-1 error.

WAL records of type XLOG_BTREE_REUSE_PAGE were generated using a
latestRemovedXid one higher than actually needed because xid used was
page opaque->btpo.xact rather than an actually removed xid.
Noticed on an otherwise quiet system by Noah Misch.

Noah Misch and Simon Riggs

Author: simonat2ndQuadrant

src/backend/access/nbtree/nbtpage.c

@@ -560,9 +560,19 @@ _bt_getbuf(Relation rel, BlockNumber blkno, int access)
 					 */
 					if (XLogStandbyInfoActive())
 					{
+						TransactionId latestRemovedXid;
+
 						BTPageOpaque opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 
-						_bt_log_reuse_page(rel, blkno, opaque->btpo.xact);
+						/*
+						 * opaque->btpo.xact is the threshold value not the
+						 * value to measure conflicts against. We must retreat
+						 * by one from it to get the correct conflict xid.
+						 */
+						latestRemovedXid = opaque->btpo.xact;
+						TransactionIdRetreat(latestRemovedXid);
+
+						_bt_log_reuse_page(rel, blkno, latestRemovedXid);
 					}
 
 					/* Okay to use page.  Re-initialize and return it */