Use TransactionXmin instead of RecentGlobalXmin in heap_abort_speculative().

There's a very low risk that RecentGlobalXmin could be far enough in
the past to be older than relfrozenxid, or even wrapped
around. Luckily the consequences of that having happened wouldn't be
too bad - the page wouldn't be pruned for a while.

Avoid that risk by using TransactionXmin instead. As that's announced
via MyPgXact->xmin, it is protected against wrapping around (see code
comments for details around relfrozenxid).

Author: Andres Freund
Discussion: https://postgr.es/m/20200328213023.s4eyijhdosuc4vcj@alap3.anarazel.de
Backpatch: 9.5-

src/backend/access/heap/heapam.c

@@ -5581,6 +5581,7 @@ heap_abort_speculative(Relation relation, ItemPointer tid)
 	Page		page;
 	BlockNumber block;
 	Buffer		buffer;
+	TransactionId prune_xid;
 
 	Assert(ItemPointerIsValid(tid));
 
@@ -5623,13 +5624,21 @@ heap_abort_speculative(Relation relation, ItemPointer tid)
 	START_CRIT_SECTION();
 
 	/*
-	 * The tuple will become DEAD immediately.  Flag that this page
-	 * immediately is a candidate for pruning by setting xmin to
-	 * RecentGlobalXmin.  That's not pretty, but it doesn't seem worth
-	 * inventing a nicer API for this.
+	 * The tuple will become DEAD immediately.  Flag that this page is a
+	 * candidate for pruning by setting xmin to TransactionXmin. While not
+	 * immediately prunable, it is the oldest xid we can cheaply determine
+	 * that's safe against wraparound / being older than the table's
+	 * relfrozenxid.  To defend against the unlikely case of a new relation
+	 * having a newer relfrozenxid than our TransactionXmin, use relfrozenxid
+	 * if so (vacuum can't subsequently move relfrozenxid to beyond
+	 * TransactionXmin, so there's no race here).
 	 */
-	Assert(TransactionIdIsValid(RecentGlobalXmin));
-	PageSetPrunable(page, RecentGlobalXmin);
+	Assert(TransactionIdIsValid(TransactionXmin));
+	if (TransactionIdPrecedes(TransactionXmin, relation->rd_rel->relfrozenxid))
+		prune_xid = relation->rd_rel->relfrozenxid;
+	else
+		prune_xid = TransactionXmin;
+	PageSetPrunable(page, prune_xid);
 
 	/* store transaction information of xact deleting the tuple */
 	tp.t_data->t_infomask &= ~(HEAP_XMAX_BITS | HEAP_MOVED);