Fix incorrect math in DetermineSafeOldestOffset.

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 b69bf30b9bfacafc733a9ba77c9587cf54d06c0c 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
index 928f9fe5d6cc9847b2c7c01ddfded292a3e9135c..cbb69f5e10cfab5742d9d5c2df35f5e179afffdb 100644 (file)
--- 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 */