Fix incorrect math in DetermineSafeOldestOffset.

robertmhaas · robertmhaas · commit 7be47c56af3d · 2015-05-07T11:19:31.000-04:00
The old formula didn't have enough parentheses, so it would do the wrong thing, and it used / rather than % to find a remainder. The effect of these oversights is that the stop point chosen by the logic introduced in commit b69bf30 might be rather meaningless. Thomas Munro, reviewed by Kevin Grittner, with a whitespace tweak by me.
diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c
@@ -2495,7 +2495,8 @@ DetermineSafeOldestOffset(MultiXactId oldestMXact)
 	 */
 	oldestOffset = find_multixact_start(oldestMXact);
 	/* move back to start of the corresponding segment */
-	oldestOffset -= oldestOffset / MULTIXACT_MEMBERS_PER_PAGE * SLRU_PAGES_PER_SEGMENT;
+	oldestOffset -= oldestOffset %
+		(MULTIXACT_MEMBERS_PER_PAGE * SLRU_PAGES_PER_SEGMENT);
 
 	LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
 	/* always leave one segment before the wraparound point */
