cherry pick pingcap#33677 to release-4.0

Signed-off-by: ti-srebot <ti-srebot@pingcap.com>

ddl/ddl.go

@@ -326,6 +326,8 @@ func (d *ddl) Start(ctxPool *pools.ResourcePool) error {
 	d.wg.Add(1)
 	go d.limitDDLJobs()
 
+	d.sessPool = newSessionPool(ctxPool)
+
 	// If RunWorker is true, we need campaign owner and do DDL job.
 	// Otherwise, we needn't do that.
 	if RunWorker {
@@ -335,7 +337,6 @@ func (d *ddl) Start(ctxPool *pools.ResourcePool) error {
 		}
 
 		d.workers = make(map[workerType]*worker, 2)
-		d.sessPool = newSessionPool(ctxPool)
 		d.delRangeMgr = d.newDeleteRangeManager(ctxPool == nil)
 		d.workers[generalWorker] = newWorker(generalWorker, d.store, d.sessPool, d.delRangeMgr)
 		d.workers[addIdxWorker] = newWorker(addIdxWorker, d.store, d.sessPool, d.delRangeMgr)