Fix DetermineSafeOldestOffset for the case where there are no mxacts.

Commit b69bf30b failed to take into
account the possibility that there might be no multixacts in existence
at all.

Report by Thomas Munro; patch by me.

src/backend/access/transam/multixact.c

@@ -2492,13 +2492,24 @@ DetermineSafeOldestOffset(MultiXactId oldestMXact)
 		return;
 
 	/*
-	 * We determine the safe upper bound for offsets of new xacts by reading
-	 * the offset of the oldest multixact, and going back one segment.  This
-	 * way, the sequence of multixact member segments will always have a
-	 * one-segment hole at a minimum.  We start spewing warnings a few
-	 * complete segments before that.
+	 * Determine the offset of the oldest multixact.  Normally, we can read
+	 * the offset from the multixact itself, but there's an important special
+	 * case: if there are no multixacts in existence at all, oldestMXact
+	 * obviously can't point to one.  It will instead point to the multixact
+	 * ID that will be assigned the next time one is needed.
 	 */
-	oldestOffset = find_multixact_start(oldestMXact);
+	LWLockAcquire(MultiXactGenLock, LW_SHARED);
+	if (MultiXactState->nextMXact == oldestMXact)
+	{
+		oldestOffset = MultiXactState->nextOffset;
+		LWLockRelease(MultiXactGenLock);
+	}
+	else
+	{
+		LWLockRelease(MultiXactGenLock);
+		oldestOffset = find_multixact_start(oldestMXact);
+	}
+
 	/* move back to start of the corresponding segment */
 	oldestOffset -= oldestOffset %
 		(MULTIXACT_MEMBERS_PER_PAGE * SLRU_PAGES_PER_SEGMENT);