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commit    080cf32d22317591cd8891c797a4d925cf1dbda2
Author:   Thomas Munro <tmunro@postgresql.org>
Date:     Wed Oct 16 20:59:21 2019 +0000
First released in: 13.0, 9.4.25, 9.5.20, 9.6.16, 10.11, 11.6, 12.1

    Fix bug that could try to freeze running multixacts.
    
    Commits 801c2dc7 and 801c2dc7 made it possible for vacuum to
    try to freeze a multixact that is still running.  That was
    prevented by a check, but raised an error.  Repair.
    
    Back-patch all the way.
    
    Author: Nathan Bossart, Jeremy Schneider
    Reported-by: Jeremy Schneider
    Reviewed-by: Jim Nasby, Thomas Munro
    Discussion: https://postgr.es/m/DAFB8AFF-2F05-4E33-AD7F-FF8B0F760C17%40amazon.com


 src/backend/commands/vacuum.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/src/backend/commands/vacuum.c b/src/backend/commands/vacuum.c
index 35f07f0369c..ed91d0e0e13 100644
--- a/src/backend/commands/vacuum.c
+++ b/src/backend/commands/vacuum.c
@@ -429,6 +429,7 @@ vacuum_set_xid_limits(Relation rel,
 	int			effective_multixact_freeze_max_age;
 	TransactionId limit;
 	TransactionId safeLimit;
+	MultiXactId oldestMxact;
 	MultiXactId mxactLimit;
 	MultiXactId safeMxactLimit;
 
@@ -504,7 +505,8 @@ vacuum_set_xid_limits(Relation rel,
 	Assert(mxid_freezemin >= 0);
 
 	/* compute the cutoff multi, being careful to generate a valid value */
-	mxactLimit = GetOldestMultiXactId() - mxid_freezemin;
+	oldestMxact = GetOldestMultiXactId();
+	mxactLimit = oldestMxact - mxid_freezemin;
 	if (mxactLimit < FirstMultiXactId)
 		mxactLimit = FirstMultiXactId;
 
@@ -518,7 +520,11 @@ vacuum_set_xid_limits(Relation rel,
 		ereport(WARNING,
 				(errmsg("oldest multixact is far in the past"),
 				 errhint("Close open transactions with multixacts soon to avoid wraparound problems.")));
-		mxactLimit = safeMxactLimit;
+		/* Use the safe limit, unless an older mxact is still running */
+		if (MultiXactIdPrecedes(oldestMxact, safeMxactLimit))
+			mxactLimit = oldestMxact;
+		else
+			mxactLimit = safeMxactLimit;
 	}
 
 	*multiXactCutoff = mxactLimit;


[parent: bc3a94dc0005]