TiDB will not wait for auto-commit statements during graceful shutdown

[YangKeao]

Bug Report

In the following logic, conn.getCtx().GetSessionVars().InTxn() is only true for explictly transactions. Therefore, the DrainClients will not wait for auto-commit statements, and results in leaking locks.

// DrainClients drain all connections in drainWait.
// After drainWait duration, we kill all connections still not quit explicitly and wait for cancelWait.
func (s *Server) DrainClients(drainWait time.Duration, cancelWait time.Duration) {
...
	go func() {
		defer close(allDone)
		for _, conn := range conns {
			if !conn.getCtx().GetSessionVars().InTxn() {
				continue
			}
			select {
			case <-conn.quit:
			case <-quitWaitingForConns:
				return
			}
		}
	}()
...
}

It'd be better to also wait for all auto-commit statements to finish 🤔 .

[YangKeao]

It's introduced by #44953