In SPGiST replay, do conflict resolution before modifying the page.

In yesterday's commit 962e0cc7, I added the
ResolveRecoveryConflictWithSnapshot call in the wrong place.  I correctly
put it before spgRedoVacuumRedirect itself would modify the index page ---
but not before RestoreBkpBlocks, so replay of a record with a full-page
image would modify the page before kicking off any conflicting HS
transactions.  Oops.

src/backend/access/spgist/spgxlog.c

@@ -889,15 +889,6 @@ spgRedoVacuumRedirect(XLogRecPtr lsn, XLogRecord *record)
 	ptr += sizeof(spgxlogVacuumRedirect);
 	itemToPlaceholder = (OffsetNumber *) ptr;
 
-	/*
-	 * If any redirection tuples are being removed, make sure there are no
-	 * live Hot Standby transactions that might need to see them.  This code
-	 * behaves similarly to btree's XLOG_BTREE_REUSE_PAGE case.
-	 */
-	if (InHotStandby && TransactionIdIsValid(xldata->newestRedirectXid))
-		ResolveRecoveryConflictWithSnapshot(xldata->newestRedirectXid,
-											xldata->node);
-
 	if (!(record->xl_info & XLR_BKP_BLOCK_1))
 	{
 		buffer = XLogReadBuffer(xldata->node, xldata->blkno, false);
@@ -964,10 +955,33 @@ spg_redo(XLogRecPtr lsn, XLogRecord *record)
 	MemoryContext oldCxt;
 
 	/*
-	 * SP-GiST indexes do not require any conflict processing. NB: If we ever
-	 * implement a similar optimization as we have in b-tree, and remove
-	 * killed tuples outside VACUUM, we'll need to handle that here.
+	 * If we have any conflict processing to do, it must happen before we
+	 * update the page.
+	 */
+	if (InHotStandby)
+	{
+		switch (info)
+		{
+			case XLOG_SPGIST_VACUUM_REDIRECT:
+				{
+					spgxlogVacuumRedirect *xldata =
+					(spgxlogVacuumRedirect *) XLogRecGetData(record);
+
+					/*
+					 * If any redirection tuples are being removed, make sure
+					 * there are no live Hot Standby transactions that might
+					 * need to see them.
 					 */
+					if (TransactionIdIsValid(xldata->newestRedirectXid))
+						ResolveRecoveryConflictWithSnapshot(xldata->newestRedirectXid,
+															xldata->node);
+					break;
+				}
+			default:
+				break;
+		}
+	}
+
 	RestoreBkpBlocks(lsn, record, false);
 
 	oldCxt = MemoryContextSwitchTo(opCtx);