fix(mailer): time out when the SMTP server never sends its greeting

pkg/mailer/dialer.go

@@ -2,6 +2,9 @@ package mailer
 
 import (
 	"crypto/tls"
+	"net"
+	"sync"
+	"time"
 
 	"github.com/raystack/frontier/pkg/mailer/mocks"
 	"github.com/stretchr/testify/mock"
@@ -13,6 +16,31 @@ const (
 	defaultFromHeader = "frontier@raystack.org"
 )
 
+var setDialOnce sync.Once
+
+// installGreetingDeadline makes every SMTP connection get a deadline as soon
+// as it opens. gopkg.in/mail.v2 only sets the deadline after it has read the
+// server greeting, so a server that accepts the connection and then stays
+// silent would block the send forever.
+func installGreetingDeadline() {
+	setDialOnce.Do(func() {
+		dial := mail.NetDialTimeout
+		mail.NetDialTimeout = func(network, address string, timeout time.Duration) (net.Conn, error) {
+			conn, err := dial(network, address, timeout)
+			if err != nil {
+				return nil, err
+			}
+			if timeout > 0 {
+				if err := conn.SetDeadline(time.Now().Add(timeout)); err != nil {
+					conn.Close()
+					return nil, err
+				}
+			}
+			return conn, nil
+		}
+	})
+}
+
 type Dialer interface {
 	DialAndSend(m *mail.Message) error
 	FromHeader() string
@@ -32,6 +60,7 @@ type DialerImpl struct {
 
 func NewDialerImpl(SMTPHost string, SMTPPort int, SMTPUser string, SMTPPass string,
 	SMTPInsecure bool, headers map[string]string, tlsPolicy mail.StartTLSPolicy) *DialerImpl {
+	installGreetingDeadline()
 	d := mail.NewDialer(SMTPHost, SMTPPort, SMTPUser, SMTPPass)
 	d.TLSConfig = &tls.Config{
 		InsecureSkipVerify: SMTPInsecure, //nolint:gosec

pkg/mailer/dialer_test.go

@@ -0,0 +1,48 @@
+package mailer
+
+import (
+	"net"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"gopkg.in/mail.v2"
+)
+
+// A server that accepts the connection but never sends its greeting must make
+// the send fail after the dialer timeout instead of blocking forever.
+func TestDialerImpl_DialAndSend_SilentServer(t *testing.T) {
+	ln, err := net.Listen("tcp", "127.0.0.1:0")
+	require.NoError(t, err)
+	defer ln.Close()
+
+	go func() {
+		for {
+			conn, err := ln.Accept()
+			if err != nil {
+				return
+			}
+			defer conn.Close() // held open, never written to
+		}
+	}()
+
+	addr := ln.Addr().(*net.TCPAddr)
+	d := NewDialerImpl("127.0.0.1", addr.Port, "", "", true, nil, mail.NoStartTLS)
+	d.dialer.Timeout = 200 * time.Millisecond
+
+	msg := mail.NewMessage()
+	msg.SetHeader("From", "from@example.com")
+	msg.SetHeader("To", "to@example.com")
+	msg.SetBody("text/plain", "hi")
+
+	done := make(chan error, 1)
+	go func() { done <- d.DialAndSend(msg) }()
+
+	select {
+	case err := <-done:
+		assert.Error(t, err)
+	case <-time.After(3 * time.Second):
+		t.Fatal("DialAndSend still blocked after 3s: no deadline on the greeting read")
+	}
+}