Fix race condition in TransactionGroupUpdateXidStatus().

When we cannot immediately acquire XactSLRULock in exclusive mode at
commit time, we add ourselves to a list of processes that need their XIDs
status update. We do this if the clog page where we need to update the
current transaction status is the same as the group leader's clog page,
otherwise, we allow the caller to clear it by itself. Now, when we can't
add ourselves to any group, we were not clearing the current proc if it
has already become a member of some group which was leading to an
assertion failure when the same proc was assigned to another backend after
the current backend exits.

Reported-by: Alexander Lakhin
Bug: 17072
Author: Amit Kapila
Tested-By: Alexander Lakhin
Backpatch-through: 11, where it was introduced
Discussion: https://postgr.es/m/17072-2f8764857ef2c92a@postgresql.org

diff --git a/src/backend/access/transam/clog.c b/src/backend/access/transam/clog.c
index 6fa4713fb4d8af42cb4c8ca689e81ff9f51a8392..3ea16a270a8c641b2096ba98e5d8a11d7dbf7b38 100644 (file)
--- a/src/backend/access/transam/clog.c
+++ b/src/backend/access/transam/clog.c
@@ -451,7 +451,12 @@ TransactionGroupUpdateXidStatus(TransactionId xid, XidStatus status,
        if (nextidx != INVALID_PGPROCNO &&
            ProcGlobal->allProcs[nextidx].clogGroupMemberPage != proc->clogGroupMemberPage)
        {
+           /*
+            * Ensure that this proc is not a member of any clog group that
+            * needs an XID status update.
+            */
            proc->clogGroupMember = false;
+           pg_atomic_write_u32(&proc->clogGroupNext, INVALID_PGPROCNO);
            return false;
        }