Implement couriers

This patch introduces the couriers, which the queue uses to deliver mail.

We have a local courier (using procmail), a remote courier (uses SMTP), and a
router courier that decides which of the two to use based on a list of local
domains.

There are still a few things pending, but they all have their basic
functionality working and tested.

internal/courier/smtp.go

@@ -0,0 +1,128 @@
+package courier
+
+import (
+	"crypto/tls"
+	"net"
+	"net/smtp"
+	"time"
+
+	"github.com/golang/glog"
+
+	"blitiri.com.ar/go/chasquid/internal/trace"
+)
+
+var (
+	// Timeouts for SMTP delivery.
+	smtpDialTimeout  = 1 * time.Minute
+	smtpTotalTimeout = 10 * time.Minute
+
+	// Port for outgoing SMTP.
+	// Tests can override this.
+	smtpPort = "25"
+
+	// Fake MX records, used for testing only.
+	fakeMX = map[string]string{}
+)
+
+// SMTP delivers remote mail via outgoing SMTP.
+type SMTP struct {
+}
+
+func (s *SMTP) Deliver(from string, to string, data []byte) error {
+	tr := trace.New("goingSMTP", "Deliver")
+	defer tr.Finish()
+	tr.LazyPrintf("%s  ->  %s", from, to)
+
+	mx, err := lookupMX(domainOf(to))
+	if err != nil {
+		return tr.Errorf("Could not find mail server: %v", err)
+	}
+	tr.LazyPrintf("MX: %s", mx)
+
+	// Do we use insecure TLS?
+	// Set as fallback when retrying.
+	insecure := false
+
+retry:
+	conn, err := net.DialTimeout("tcp", mx+":"+smtpPort, smtpDialTimeout)
+	if err != nil {
+		return tr.Errorf("Could not dial: %v", err)
+	}
+	conn.SetDeadline(time.Now().Add(smtpTotalTimeout))
+
+	c, err := smtp.NewClient(conn, mx)
+	if err != nil {
+		return tr.Errorf("Error creating client: %v", err)
+	}
+
+	// TODO: Keep track of hosts and MXs that we've successfully done TLS
+	// against, and enforce it.
+	if ok, _ := c.Extension("STARTTLS"); ok {
+		config := &tls.Config{
+			ServerName:         mx,
+			InsecureSkipVerify: insecure,
+		}
+		err = c.StartTLS(config)
+		if err != nil {
+			// Unfortunately, many servers use self-signed certs, so if we
+			// fail verification we just try again without validating.
+			if insecure {
+				return tr.Errorf("TLS error: %v", err)
+			}
+
+			insecure = true
+			tr.LazyPrintf("TLS error, retrying insecurely")
+			goto retry
+		}
+
+		if config.InsecureSkipVerify {
+			tr.LazyPrintf("Insecure - self-signed certificate")
+		} else {
+			tr.LazyPrintf("Secure - using TLS")
+		}
+	} else {
+		tr.LazyPrintf("Insecure - not using TLS")
+	}
+
+	if err = c.Mail(from); err != nil {
+		return tr.Errorf("MAIL %v", err)
+	}
+
+	if err = c.Rcpt(to); err != nil {
+		return tr.Errorf("RCPT TO %v", err)
+	}
+
+	w, err := c.Data()
+	if err != nil {
+		return tr.Errorf("DATA %v", err)
+	}
+	_, err = w.Write(data)
+	if err != nil {
+		return tr.Errorf("DATA writing: %v", err)
+	}
+
+	err = w.Close()
+	if err != nil {
+		return tr.Errorf("DATA closing %v", err)
+	}
+
+	c.Quit()
+
+	return nil
+}
+
+func lookupMX(domain string) (string, error) {
+	if v, ok := fakeMX[domain]; ok {
+		return v, nil
+	}
+
+	mxs, err := net.LookupMX(domain)
+	if err != nil {
+		return "", err
+	} else if len(mxs) == 0 {
+		glog.Infof("domain %q has no MX, falling back to A", domain)
+		return domain, nil
+	}
+
+	return mxs[0].Host, nil
+}