Resetting connection properly when the underlying TCP connection breaks. Fixes #997

mailer/mailer.go

@@ -146,7 +146,7 @@ func sendMail(ctx context.Context, dialer Dialer, ms []Mail) {
 	}
 	defer sender.Close()
 	message := gomail.NewMessage()
-	for _, m := range ms {
+	for i, m := range ms {
 		select {
 		case <-ctx.Done():
 			return
@@ -187,8 +187,16 @@ func sendMail(ctx context.Context, dialer Dialer, ms []Mail) {
 					continue
 				}
 			} else {
-				m.Error(err)
+				// This likely indicates that something happened to the underlying
-				sender.Reset()
+				// connection. We'll try to reconnect and, if that fails, we'll
+				// error out the remaining emails.
+				origErr := err
+				sender, err = dialHost(ctx, dialer)
+				if err != nil {
+					errorMail(err, ms[i:])
+					break
+				}
+				m.Backoff(origErr)
 				continue
 			}
 		}

mailer/mailer_test.go

@@ -263,16 +263,21 @@ func (ms *MailerSuite) TestUnknownError() {
 
 	message := messages[0].(*mockMessage)
 
-	// Check that the first message did not perform a backoff
+	// If we get an unexpected error, this means that it's likely the
-	expectedBackoffCount := 0
+	// underlying connection dropped. When this happens, we expect the
+	// connection to be re-established (see #997).
+	// In this case, we're successfully reestablishing the connection
+	// so we expect the backoff to occur.
+	expectedBackoffCount := 1
 	backoffCount := message.backoffCount
 	if backoffCount != expectedBackoffCount {
-		ms.T().Fatalf("Did not receive expected backoff. Got backoffCount %d, Expected %d", backoffCount, expectedCount)
+		ms.T().Fatalf("Did not receive expected backoff. Got backoffCount %d, Expected %d", backoffCount, expectedBackoffCount)
 	}
 
-	// Check that there was a reset performed on the sender
+	// Check that the underlying connection was reestablished
-	if sender.resetCount != expectedCount {
+	expectedDialCount := 2
-		ms.T().Fatalf("Did not receive expected reset. Got resetCount %d, expected %d", sender.resetCount, expectedCount)
+	if dialer.dialCount != expectedDialCount {
+		ms.T().Fatalf("Did not receive expected dial count. Got %d expected %d", dialer.dialCount, expectedDialCount)
 	}
 
 	// Check that the email errored out appropriately

mailer/mockmailer.go

@@ -147,6 +147,7 @@ func (mm *mockMessage) GetDialer() (Dialer, error) {
 
 func (mm *mockMessage) Backoff(reason error) error {
 	mm.backoffCount++
+	mm.err = reason
 	return nil
 }