Fix off-by-one error in calculating subtrans/multixact truncation point.

If there were no subtransactions (or multixacts) active, we would calculate
the oldestxid == next xid. That's correct, but if next XID happens to be
on the next pg_subtrans (pg_multixact) page, the page does not exist yet,
and SimpleLruTruncate will produce an "apparent wraparound" warning. The
warning is harmless in this case, but looks very alarming to users.

Backpatch to all supported versions. Patch and analysis by Thomas Munro.

src/backend/access/transam/multixact.c

@@ -173,6 +173,8 @@
 #define MULTIXACT_MEMBER_DANGER_THRESHOLD	\
 	(MaxMultiXactOffset - MaxMultiXactOffset / 4)
 
+#define PreviousMultiXactId(xid) \
+	((xid) == FirstMultiXactId ? MaxMultiXactId : (xid) - 1)
 
 /*
  * Links to shared-memory data structures for MultiXact control
@@ -3057,10 +3059,15 @@ TruncateMultiXact(void)
 
 	SlruScanDirectory(MultiXactMemberCtl, SlruScanDirCbRemoveMembers, &range);
 
-	/* Now we can truncate MultiXactOffset */
+	/*
+	 * Now we can truncate MultiXactOffset.  We step back one multixact to
+	 * avoid passing a cutoff page that hasn't been created yet in the rare
+	 * case that oldestMXact would be the first item on a page and oldestMXact
+	 * == nextMXact.  In that case, if we didn't subtract one, we'd trigger
+	 * SimpleLruTruncate's wraparound detection.
+	 */
 	SimpleLruTruncate(MultiXactOffsetCtl,
-					  MultiXactIdToOffsetPage(oldestMXact));
-
+				  MultiXactIdToOffsetPage(PreviousMultiXactId(oldestMXact)));
 
 	/*
 	 * Now, and only now, we can advance the stop point for multixact members.

src/backend/access/transam/subtrans.c

@@ -340,8 +340,13 @@ TruncateSUBTRANS(TransactionId oldestXact)
 
 	/*
 	 * The cutoff point is the start of the segment containing oldestXact. We
-	 * pass the *page* containing oldestXact to SimpleLruTruncate.
+	 * pass the *page* containing oldestXact to SimpleLruTruncate.  We step
+	 * back one transaction to avoid passing a cutoff page that hasn't been
+	 * created yet in the rare case that oldestXact would be the first item on
+	 * a page and oldestXact == next XID.  In that case, if we didn't subtract
+	 * one, we'd trigger SimpleLruTruncate's wraparound detection.
 	 */
+	TransactionIdRetreat(oldestXact);
 	cutoffPage = TransactionIdToPage(oldestXact);
 
 	SimpleLruTruncate(SubTransCtl, cutoffPage);