RegisterDial is not safe to call from multiple goroutines

[zombiezen]

Issue description

As the subject implies, RegisterDial is not safe to call from multiple goroutines.

mysql/driver.go

Line 43 in 1a676ac

func RegisterDial(net string, dial DialFunc) {

Example code

var wg sync.WaitGroup
wg.Add(1)
go func() {
  mysql.RegisterDial("dial1", func(string) (net.Conn, error) { return nil, errors.New("bork") })
  wg.Done()
}()
go func() {
  mysql.RegisterDial("dial2", func(string) (net.Conn, error) { return nil, errors.New("bork") })
  wg.Done()
}()
wg.Wait()
sql.Open("mysql", /* ... */)