Commit() function can not distinguish whether the transaction has been sent

[syuanyuan708]

Issue description

when I call func (tx *mysqlTx) Commit() (err error) ，if connection has problem it return an ErrInvalidConn

but if current connection is fine it call exec

and in exec it call readResultSetHeaderPacket after the transaction sent to db successfully, and readResultSetHeaderPacket call readPacket() and it also may lead to ErrInvalidConn

thus I can't distinguish whether the transaction has been sent, it lead to one transaction sent twice and data not correct. Is it possible to add a new error to distinguish this two cases?

Example code

If possible, please enter some example code here to reproduce the issue.

err = tx.Commit(ctx)
if err != nil {
xlog.Errorf(ctx, "%s commit transaction failed, table:%v err:%v", fun, table, err) ...

Error log

***err:invalid connection 

Configuration

Driver version (or git SHA):

*Go version: go1.12 linux/amd64

*Server version: tidb

*Server OS: redhat7.2

[shogo82148]

duplicated by #1226