Create a mysql.Connect API.

[methane]

Description

This is a counter-proposal to #1042.

You can create a custom Connector object using this new API:

import (
    "database/sql"
    "database/sql/driver"
    "github.com/go-sql-driver/mysql"
)

type CustomConnector struct {
    cfg *mysql.Config
}

func (cc *CustomConnector) Connect(ctx Context) (driver.Conn, error) {
    cc.cfg.Host = getCurrentMySQLHost()
    cfg.User, cfg.Password = getCurrentMySQLAuth()
    return mysql.Connect(ctx, cc.cfg)
}

func (cc *CustomConnector) Driver() driver.Driver {
    return nil
}

Checklist

• Code compiles correctly
• Created tests which fail without the change (if possible)
• All tests passing
• Extended the README / documentation, if necessary
• Added myself / the copyright holder to the AUTHORS file

[sagikazarmark]

Why use a pointer here if it's sensitive to concurrent changes?

[methane]

Because shallow copy of the config object is not a right way to copy config object.

[dolmen]

This patch extends the API surface while it doesn't seem necessary (at least the description doesn't tell why it is necessary).

As a counter example I implemented years ago a driver that wraps the MySQL driver to load creadentials from the ~/.mylogin.cnf without needing this extended API.
See my packages:

• github.com/dolmen-go/mylogin-driver
• github.com/dolmen-go/mylogin-driver/register

[dolmen]

If the need is to avoid formatting+parsing of DSNs, the mysql.NewConnector API already exists and the connector can be used with sql.OpenDB. So we don't need to extend the API.

[methane]

If the need is to avoid formatting+parsing of DSNs, the mysql.NewConnector API already exists and the connector can be used with sql.OpenDB. So we don't need to extend the API.

Of course, I know it. I am the who proposed it first.

Please read this issue and issue #1042 carefully. The need is not avoiding to parse DSN, but change the configuration (e.g. credential, server IP address, etc) on the fly.

My main intension is to promote custom connector, instead of adding some dynamic (callback) options to the config object.

Adding new API to help custom connector is not my main point. It is just a little helper function. I'm OK to reject this PR if people don't think this helper function is not worth enough.