Fix race condition in reading commit timestamps

If a user requests the commit timestamp for a transaction old enough
that its data is concurrently being truncated away by vacuum at just the
right time, they would receive an ugly internal file-not-found error
message from slru.c rather than the expected NULL return value.

In a primary server, the window for the race is very small: the lookup
has to occur exactly between the two calls by vacuum, and there's not a
lot that happens between them (mostly just a multixact truncate).  In a
standby server, however, the window is larger because the truncation is
executed as soon as the WAL record for it is replayed, but the advance
of the oldest-Xid is not executed until the next checkpoint record.

To fix in the primary, simply reverse the order of operations in
vac_truncate_clog.  To fix in the standby, augment the WAL truncation
record so that the standby is aware of the new oldest-XID value and can
apply the update immediately.  WAL version bumped because of this.

No backpatch, because of the low importance of the bug and its rarity.

Author: Craig Ringer
Reviewed-By: Petr Jelínek, Peter Eisentraut
Discussion: https://postgr.es/m/CAMsr+YFhVtRQT1VAwC+WGbbxZZRzNou=N9Ed-FrCqkwQ8H8oJQ@mail.gmail.com

src/backend/commands/vacuum.c

@@ -1152,6 +1152,15 @@ vac_truncate_clog(TransactionId frozenXID,
 	if (bogus)
 		return;
 
+	/*
+	 * Advance the oldest value for commit timestamps before truncating, so
+	 * that if a user requests a timestamp for a transaction we're truncating
+	 * away right after this point, they get NULL instead of an ugly "file not
+	 * found" error from slru.c.  This doesn't matter for xact/multixact
+	 * because they are not subject to arbitrary lookups from users.
+	 */
+	AdvanceOldestCommitTsXid(frozenXID);
+
 	/*
 	 * Truncate CLOG, multixact and CommitTs to the oldest computed value.
 	 */
@@ -1167,7 +1176,6 @@ vac_truncate_clog(TransactionId frozenXID,
 	 */
 	SetTransactionIdLimit(frozenXID, oldestxid_datoid);
 	SetMultiXactIdLimit(minMulti, minmulti_datoid);
-	AdvanceOldestCommitTsXid(frozenXID);
 }