Emit nbtree vacuum cycle id in nbtree xlog through FPIs

NBTree needs a vacuum cycle ID on pages of whom the split resulted in a new
right page that is located before the original page, or who were split from
such split pages in the current vacuum cycle. By WAL-logging the cycle_id
and restoring it in recovery, we assure vacuum doesn't fail to clean up the
earlier pages.

This fixes the PostgreSQL side of neondatabase/neon#9929.

src/backend/access/nbtree/nbtinsert.c

@@ -1981,7 +1981,31 @@ _bt_split(Relation rel, Relation heaprel, BTScanInsert itup_key, Buffer buf,
 		XLogBeginInsert();
 		XLogRegisterData((char *) &xlrec, SizeOfBtreeSplit);
 
-		XLogRegisterBuffer(0, buf, REGBUF_STANDARD);
+		/*
+		 * NEON: If we split to earlier pages during a btree vacuum cycle,
+		 * then we have to include the cycle ID in the WAL record. The
+		 * easiest method to do that is to force an image, which happens to
+		 * be relatively cheap, as the data already contained in the record is
+		 * enough to populate the new right page.
+		 *
+		 * We MUST log an FPI when:
+		 *  - The right page's blckno < the left page's blckno
+		 *  - The right page might be 'C' in a page spit chain B > C > A after
+		 *    B split B > A => B > C > A; or B > C > D > A, etc. (as indicated
+		 *    by the presense of a cycle ID).
+		 */
+		if (lopaque->btpo_cycleid == 0 || (rightpagenumber > origpagenumber &&
+										   lopaque->btpo_cycleid != old_cycleid))
+		{
+			/* no cycle ID is required */
+			XLogRegisterBuffer(0, buf, REGBUF_STANDARD);
+		}
+		else
+		{
+			/* cycle ID is required */
+			XLogRegisterBuffer(0, buf, REGBUF_FORCE_IMAGE | REGBUF_STANDARD);
+		}
+
 		XLogRegisterBuffer(1, rbuf, REGBUF_WILL_INIT);
 		/* Log original right sibling, since we've changed its prev-pointer */
 		if (!isrightmost)

src/backend/access/nbtree/nbtxlog.c

@@ -433,6 +433,22 @@ btree_xlog_split(bool newitemonleft, XLogReaderState *record)
 		PageSetLSN(origpage, lsn);
 		MarkBufferDirty(buf);
 	}
+	else
+	{
+		/*
+		 * btree split FPIs may contain important cycle IDs on the original
+		 * page's FPI; make sure we correctly transfer this over
+		 */
+		Page	opage;
+		BTPageOpaque oopaque;
+
+		Assert(BufferIsValid(buf));
+
+		opage = BufferGetPage(buf);
+		oopaque = BTPageGetOpaque(opage);
+
+		ropaque->btpo_cycleid = oopaque->btpo_cycleid;
+	}
 
 	/* Fix left-link of the page to the right of the new right sibling */
 	if (spagenumber != P_NONE)