WIP: Add smarthost support

WORK IN PROGRESS -- WORK IN PROGRESS -- WORK IN PROGRESS

This patch adds support for delivering mail via a smarthost.

In this mode, all accepted mail gets delivered through an SMTP
connection to a specific host, statically configured.

internal/courier/smarthost.go

@@ -0,0 +1,145 @@
+package courier
+
+import (
+	"crypto/tls"
+	"crypto/x509"
+	"net"
+	netsmtp "net/smtp"
+	"net/url"
+	"time"
+
+	"blitiri.com.ar/go/chasquid/internal/expvarom"
+	"blitiri.com.ar/go/chasquid/internal/smtp"
+	"blitiri.com.ar/go/chasquid/internal/trace"
+)
+
+var (
+	// Timeouts for smarthost delivery.
+	shDialTimeout  = 1 * time.Minute
+	shTotalTimeout = 10 * time.Minute
+)
+
+// Exported variables.
+var (
+	shAttempts = expvarom.NewInt("chasquid/smarthostOut/attempts",
+		"count of attempts to deliver via smarthost")
+	shErrors = expvarom.NewMap("chasquid/smarthostOut/errors",
+		"reason", "count of smarthost delivery errors, per reason")
+	shSuccess = expvarom.NewInt("chasquid/smarthostOut/success",
+		"count of successful delivering via smarthost")
+)
+
+// SmartHost delivers remote mail via smarthost relaying.
+type SmartHost struct {
+	HelloDomain string
+	URL         url.URL
+
+	// For testing.
+	rootCAs *x509.CertPool
+}
+
+// Deliver an email. On failures, returns an error, and whether or not it is
+// permanent.
+func (s *SmartHost) Deliver(from string, to string, data []byte) (error, bool) {
+	tr := trace.New("Courier.SmartHost", to)
+	defer tr.Finish()
+	tr.Debugf("%s  ->  %s", from, to)
+	shAttempts.Add(1)
+
+	conn, onTLS, err := s.dial()
+	if err != nil {
+		shErrors.Add("dial", 1)
+		return tr.Errorf("Could not dial %q: %v", s.URL.Host, err), false
+	}
+
+	defer conn.Close()
+	conn.SetDeadline(time.Now().Add(shTotalTimeout))
+
+	host, _, _ := net.SplitHostPort(s.URL.Host)
+
+	c, err := smtp.NewClient(conn, host)
+	if err != nil {
+		shErrors.Add("client", 1)
+		return tr.Errorf("Error creating client: %v", err), false
+	}
+
+	if err = c.Hello(s.HelloDomain); err != nil {
+		shErrors.Add("hello", 1)
+		return tr.Errorf("Error saying hello: %v", err), false
+	}
+
+	if !onTLS {
+		if ok, _ := c.Extension("STARTTLS"); !ok {
+			shErrors.Add("starttls-support", 1)
+			return tr.Errorf("Server does not support STARTTLS"), false
+		}
+
+		config := &tls.Config{
+			ServerName: host,
+			RootCAs:    s.rootCAs,
+		}
+		if err = c.StartTLS(config); err != nil {
+			shErrors.Add("starttls-exchange", 1)
+			return tr.Errorf("Error in STARTTLS: %v", err), false
+		}
+	}
+
+	if s.URL.User != nil {
+		user := s.URL.User.Username()
+		password, _ := s.URL.User.Password()
+		auth := netsmtp.PlainAuth("", user, password, host)
+		if err = c.Auth(auth); err != nil {
+			shErrors.Add("auth", 1)
+			return tr.Errorf("AUTH error: %v", err), false
+		}
+	}
+
+	// smtp.Client.Mail will add the <> for us when the address is empty.
+	if from == "<>" {
+		from = ""
+	}
+
+	if err = c.MailAndRcpt(from, to); err != nil {
+		shErrors.Add("mail", 1)
+		return tr.Errorf("MAIL+RCPT %v", err), smtp.IsPermanent(err)
+	}
+
+	w, err := c.Data()
+	if err != nil {
+		shErrors.Add("data", 1)
+		return tr.Errorf("DATA %v", err), smtp.IsPermanent(err)
+	}
+	_, err = w.Write(data)
+	if err != nil {
+		shErrors.Add("dataw", 1)
+		return tr.Errorf("DATA writing: %v", err), smtp.IsPermanent(err)
+	}
+
+	err = w.Close()
+	if err != nil {
+		shErrors.Add("close", 1)
+		return tr.Errorf("DATA closing %v", err), smtp.IsPermanent(err)
+	}
+
+	_ = c.Quit()
+	tr.Debugf("done")
+	shSuccess.Add(1)
+
+	return nil, false
+}
+
+func (s *SmartHost) dial() (conn net.Conn, onTLS bool, err error) {
+	dialer := &net.Dialer{Timeout: shDialTimeout}
+
+	if s.URL.Scheme == "tls" {
+		onTLS = true
+		config := &tls.Config{
+			RootCAs: s.rootCAs,
+		}
+		conn, err = tls.DialWithDialer(dialer, "tcp", s.URL.Host, config)
+	} else {
+		onTLS = false
+		conn, err = dialer.Dial("tcp", s.URL.Host)
+	}
+	return
+}