enhance graceful stop by closing the connection after finishing the ongoing txn

[july2993]

Enhancement

ref

tidb/server/server.go

Line 737 in 27348d6

func (s *Server) GracefulDown(ctx context.Context, done chan struct{}) {

Instead of checking all connections in tick(1s) and notifying it to close if the status is not in txn, we can make the connection in func (cc *clientConn) Run(ctx context.Context) to detect if we are going to shutdown and close the connection after executing the ongoing txn.

The current implementation may be hard to close the connection if the connection is actively executing txns.
And there race cases between ShutdownOrNotify and clientConn.Run, like:

1. status is connStatusDispatching after reading a "start transaction" statement but sessionVar.InTxn() is still false.
2. status is set as connStatusWaitShutdown in ShutdownOrNotify()
3. we will close the connection directly after dispatching the "start transaction" statement.