postgres.git / summary / log / commit / refs

commit    24aa77ec9549f2ca67220ef7b5d7f2dce5863d31
Author:   Robert Haas <rhaas@postgresql.org>
Date:     Mon May 11 01:34:26 2015 +0000
First released in: 9.5.0, 9.3.7, 9.4.2

    Fix DetermineSafeOldestOffset for the case where there are no mxacts.
    
    Commit b69bf30b9bfacafc733a9ba77c9587cf54d06c0c 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 | 23 +++++++++++++++++------
 1 file changed, 17 insertions(+), 6 deletions(-)

diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c
index 174dfba6057..fc3f525c809 100644
--- a/src/backend/access/transam/multixact.c
+++ b/src/backend/access/transam/multixact.c
@@ -2511,13 +2511,24 @@ 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);


[parent: 3de791ee7667]