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

diff --git a/src/backend/commands/vacuum.c b/src/backend/commands/vacuum.c
index 35f07f0369cc45e4bf4f4052d46df13cc4955a29..ed91d0e0e13bd30a00688ef751c74ec9e8e59b18 100644 (file)
--- 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;