Non-obvious connection error when tls is enabled on DSN without port

[artyom]

Issue description

I believe the fix for issue #668 introduced a regression: now if DSN specifies host without a port along with a tls=true argument, connection fails with non-obvious message from crypto/tls package:

tls: either ServerName or InsecureSkipVerify must be specified in the tls.Config

Whereas it should either succeed provided that certificate is valid or fail with the proper error: x509: certificate signed by unknown authority.

Here's the relevant code:

mysql/dsn.go

Lines 524 to 527 in cd4cb90

	host, _, err := net.SplitHostPort(cfg.Addr)
	if err == nil {
	cfg.tls.ServerName = host
	}

As cfg.Addr holds only hostname without port, net.SplitHostPort returns non-nil error and cfg.tls.ServerName left blank leading to error tls: either ServerName or InsecureSkipVerify must be specified in the tls.Config returned from crypto/tls code:

https://github.com/golang/go/blob/21672b36eb0ad3bf1e2220b247900b2c63664464/src/crypto/tls/handshake_client.go#L32-L35

Example code

package main

import (
	"database/sql"
	"flag"
	"fmt"
	"os"

	_ "github.com/go-sql-driver/mysql"
)

func main() {
	dsn := "user:password@tcp(hostname)/?tls=true"
	flag.StringVar(&dsn, "dsn", dsn, "https://github.com/go-sql-driver/mysql#dsn-data-source-name")
	flag.Parse()
	if err := run(dsn); err != nil {
		fmt.Fprintln(os.Stderr, err)
		os.Exit(1)
	}
}

func run(dsn string) error {
	db, err := sql.Open("mysql", dsn)
	if err != nil {
		return err
	}
	defer db.Close()
	return db.Ping()
}

Run this adjusting -dsn flag value as necessary — set host without port as address.

Error log

tls: either ServerName or InsecureSkipVerify must be specified in the tls.Config 

Configuration

Driver version (or git SHA):

949221881fa8e37a73d073dd3e33d857909d909e

Go version: run go version in your console

go version devel +9a13f8e11c Tue Nov 28 06:47:50 2017 +0000 darwin/amd64

Server version: E.g. MySQL 5.6, MariaDB 10.0.20

5.6.10 MySQL Community Server (GPL)

Server OS: E.g. Debian 8.1 (Jessie), Windows 10

Debian GNU/Linux 8.8 (jessie)

[methane]

I believe the fix for issue #668 introduced a regression: now if DSN specifies host without a port along with a tls=true argument, connection fails with non-obvious message from crypto/tls package:

#668 is not wrong. See #564, it is broken at first.

[methane]

Before #668, hostname without port is not accepted.
After #668, port can be omitted. But cfg.Addr has port always after parsing DSN.