Deadlock if closing pool while a new connection is being created

[codestation]

Can reproduce consistently with this:

package email

import (
	"fmt"
	"net/smtp"
	"testing"
	"time"
)

func TestEmail_Deadlock(t *testing.T) {
	e := NewEmail()
	p, err := NewPool("smtp.gmail.com:587", 1, smtp.PlainAuth("", "[email protected]", "password123", "smtp.gmail.com"))
	if err != nil {
		t.Fatal(err)
	}
	err = p.Send(e, 3 * time.Second)
	if err == nil {
		t.Fatal("must have auth error")
	}
	fmt.Println(err)
	p.Close()
}

The main problem is that the created counter and the current clients array isn't in sync. On the makeOne() call the counter is incremented before the connection is ready, and only decremented when it has an error. An example would be:

• Counter increased, created=1
• Tries to create connection
• Tries to send email and times out because it couldn't get a client.
• Calls close() and created > 0 so enters loop and tries to read clients and locks.
• The smtp connection times out and the counter is decreased, created=0
• Stuck forever reading clients channel.