Avoid emitting a bogus WAL record when recycling an all-zero btree page.

Commit fafa374f2 caused _bt_getbuf() to possibly emit a WAL record for
a page that it was about to recycle.  However, it failed to distinguish
all-zero pages from dead pages, which is important because only the
latter have valid btpo.xact values, or indeed any special space at all.
Recycling an all-zero page with XLogStandbyInfoActive() enabled therefore
led to an Assert failure, or to emission of a WAL record containing a
bogus cutoff XID, which might lead to unnecessary query cancellations
on hot standby servers.

Per reports from Antonin Houska and 自己.  Amit Kapila was first to
propose this fix, and Robert Haas, myself, and Kyotaro Horiguchi
reviewed it at various times.

This is an old bug, so back-patch to all supported branches.

Discussion: https://postgr.es/m/2628.1474272158@localhost
Discussion: https://postgr.es/m/48875502.f4a0.1635f0c27b0.Coremail.zoulx1982@163.com

src/backend/access/nbtree/nbtpage.c

@@ -782,9 +782,14 @@ _bt_getbuf(Relation rel, BlockNumber blkno, int access)
 					/*
 					 * If we are generating WAL for Hot Standby then create a
 					 * WAL record that will allow us to conflict with queries
-					 * running on standby.
+					 * running on standby, in case they have snapshots older
+					 * than btpo.xact.  This can only apply if the page does
+					 * have a valid btpo.xact value, ie not if it's new.  (We
+					 * must check that because an all-zero page has no special
+					 * space.)
 					 */
-					if (XLogStandbyInfoActive() && RelationNeedsWAL(rel))
+					if (XLogStandbyInfoActive() && RelationNeedsWAL(rel) &&
+						!PageIsNew(page))
 					{
 						BTPageOpaque opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 
@@ -897,7 +902,10 @@ _bt_pageinit(Page page, Size size)
  *	_bt_page_recyclable() -- Is an existing page recyclable?
  *
  * This exists to make sure _bt_getbuf and btvacuumscan have the same
- * policy about whether a page is safe to re-use.
+ * policy about whether a page is safe to re-use.  But note that _bt_getbuf
+ * knows enough to distinguish the PageIsNew condition from the other one.
+ * At some point it might be appropriate to redesign this to have a three-way
+ * result value.
  */
 bool
 _bt_page_recyclable(Page page)