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

[syuanyuan708]

it happend in my project when db connection is instable and an item in db is updated twice, does anyone encountered similar situations please?

[zihengCat]

Use newer go version (Go 1.13+) and try again.

[zihengCat]

@syuanyuan708
It's no need to distinguish error types. Driver handles error internally and user does not care about it.
When error occurred in a transaction, call Rollback() and return.

	err := tx.Commit()
	if err != nil {
		tx.Rollback()
		return
	}

[syuanyuan708]

@zihengCat
ok, I will try to update Go 1.13+ and test it, thanks for your explain.

[Weltloose]

@zihengCat After reading the code and debugging, I found that after successfully sending "Commit" message to server and failing in receiving response from server(e.g. io timeout), tx.Commit() returns "invalid connection" error, and then tx.Rollback() just do nothing for tx.done==1. So is it possible that commit has been executed in server but indicates a rollback to users?

[methane]

I think database/sql should provide explicit guideline for this issue.