Fix corruption due to vacuum_defer_cleanup_age underflowing 64bit xids

When vacuum_defer_cleanup_age is bigger than the current xid, including the
epoch, the subtraction of vacuum_defer_cleanup_age would lead to a wrapped
around xid. While that normally is not a problem, the subsequent conversion to
a 64bit xid results in a 64bit-xid very far into the future. As that xid is
used as a horizon to detect whether rows versions are old enough to be
removed, that allows removal of rows that are still visible (i.e. corruption).

If vacuum_defer_cleanup_age was never changed from the default, there is no
chance of this bug occurring.

This bug was introduced in dc7420c.  A lesser version of it exists in
12-13, introduced by fb5344c, affecting only GiST.

The 12-13 version of the issue can, in rare cases, lead to pages in a gist
index getting recycled too early, potentially causing index entries to be
found multiple times.

The fix is fairly simple - don't allow vacuum_defer_cleanup_age to retreat
further than FirstNormalTransactionId.

Patches to make similar bugs easier to find, by adding asserts to the 64bit
xid infrastructure, have been proposed, but are not suitable for backpatching.

Currently there are no tests for vacuum_defer_cleanup_age. A patch introducing
infrastructure to make writing a test easier has been posted to the list.

Reported-by: Michail Nikolaev <michail.nikolaev@gmail.com>
Reviewed-by: Matthias van de Meent <boekewurm+postgres@gmail.com>
Author: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/20230108002923.cyoser3ttmt63bfn@awork3.anarazel.de
Backpatch: 12-, but impact/fix is smaller for 12-13

src/backend/utils/time/snapmgr.c

@@ -983,24 +983,36 @@ GetFullRecentGlobalXmin(void)
 	uint32		nextxid_epoch;
 	TransactionId nextxid_xid;
 	uint32		epoch;
+	TransactionId horizon = RecentGlobalXmin;
 
-	Assert(TransactionIdIsNormal(RecentGlobalXmin));
+	Assert(TransactionIdIsNormal(horizon));
 
 	/*
 	 * Compute the epoch from the next XID's epoch. This relies on the fact
 	 * that RecentGlobalXmin must be within the 2 billion XID horizon from the
 	 * next XID.
+	 *
+	 * Need to be careful to prevent wrapping around during epoch 0, otherwise
+	 * we would generate an xid far into the future when converting to a
+	 * FullTransactionId. This can happen because RecentGlobalXmin can be held
+	 * back via vacuum_defer_cleanup_age.
 	 */
 	nextxid_full = ReadNextFullTransactionId();
 	nextxid_epoch = EpochFromFullTransactionId(nextxid_full);
 	nextxid_xid = XidFromFullTransactionId(nextxid_full);
 
-	if (RecentGlobalXmin > nextxid_xid)
+	if (horizon <= nextxid_xid)
+		epoch = nextxid_epoch;
+	else if (nextxid_epoch > 0)
 		epoch = nextxid_epoch - 1;
 	else
-		epoch = nextxid_epoch;
+	{
+		/* don't wrap around */
+		epoch = 0;
+		horizon = FirstNormalTransactionId;
+	}
 
-	return FullTransactionIdFromEpochAndXid(epoch, RecentGlobalXmin);
+	return FullTransactionIdFromEpochAndXid(epoch, horizon);
 }
 
 /*