Fix race condition in TransactionGroupUpdateXidStatus().

When we cannot immediately acquire CLogControlLock 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 726ef87e6e24aa211deee72f11e41ad508cb6bfc..0a473d2661655f0a0bef8be4dfa4aa4099394d74 100644 (file)
--- a/src/backend/access/transam/clog.c
+++ b/src/backend/access/transam/clog.c
@@ -450,7 +450,12 @@ 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;
                }