[FIX] Implement error handling for SendGrid responses and validate email content

commo.go

@@ -34,6 +34,7 @@ package commo
 import (
 	"context"
 	"errors"
+	"fmt"
 	"html/template"
 
 	"github.com/jordan-wright/email"
@@ -166,12 +167,21 @@ func sendSendGrid(e *Email) (err error) {
 	}
 
 	if rep.StatusCode < 200 || rep.StatusCode >= 300 {
-		return errors.New(rep.Body)
+		return sendGridResponseError(rep.StatusCode, rep.Body)
 	}
 
 	return nil
 }
 
+// sendGridResponseError returns a non-retryable error for SendGrid 4xx responses.
+func sendGridResponseError(statusCode int, body string) error {
+	err := fmt.Errorf("sendgrid: status %d: %s", statusCode, body)
+	if statusCode >= 400 && statusCode < 500 {
+		return backoff.NoRetry(err)
+	}
+	return err
+}
+
 func sendMock(*Email) (err error) {
 	return errors.New("not implemented")
 }

email.go

@@ -80,6 +80,10 @@ func (e *Email) ToSMTP() (msg *email.Email, err error) {
 		return nil, err
 	}
 
+	if len(msg.Text) == 0 || len(msg.HTML) == 0 {
+		return nil, ErrEmptyContent
+	}
+
 	return msg, nil
 }
 
@@ -107,6 +111,10 @@ func (e *Email) ToSendGrid() (msg *sgmail.SGMailV3, err error) {
 		return nil, err
 	}
 
+	if text == "" || html == "" {
+		return nil, ErrEmptyContent
+	}
+
 	msg.AddContent(
 		sgmail.NewContent("text/plain", text),
 		sgmail.NewContent("text/html", html),

email_test.go

@@ -1,6 +1,7 @@
 package commo_test
 
 import (
+	"html/template"
 	"testing"
 
 	"github.com/stretchr/testify/require"
@@ -100,3 +101,21 @@ func TestEmailValidate(t *testing.T) {
 	})
 
 }
+
+// Ensures that an error is returned when the email content is empty.
+func TestToSendGridEmptyContent(t *testing.T) {
+	commo.WithTemplates(map[string]*template.Template{
+		"empty.txt":  template.Must(template.New("empty.txt").Parse("")),
+		"empty.html": template.Must(template.New("empty.html").Parse("")),
+	})
+
+	email := &commo.Email{
+		Sender:   "admin@server.com",
+		To:       []string{"test@example.com"},
+		Subject:  "subject",
+		Template: "empty",
+	}
+
+	_, err := email.ToSendGrid()
+	require.ErrorIs(t, err, commo.ErrEmptyContent)
+}

errors.go

@@ -10,6 +10,7 @@ var (
 	ErrMissingTemplate    = errors.New("missing email template name")
 	ErrNotInitialized     = errors.New("email sending method has not been configured")
 	ErrTemplatesNotLoaded = errors.New("templates have not been loaded yet")
+	ErrEmptyContent       = errors.New("email must have non-empty text and html content")
 )
 
 var (

export_test.go

@@ -0,0 +1,4 @@
+package commo
+
+// SendGridResponseError exposes sendGridResponseError for commo_test.
+var SendGridResponseError = sendGridResponseError

render.go

@@ -29,7 +29,7 @@ func RenderString(name string, data any) (text, html string, err error) {
 	)
 
 	if tb, hb, err = Render(name, data); err != nil {
-		return "", "", nil
+		return "", "", err
 	}
 
 	return string(tb), string(hb), nil

render_test.go

@@ -1,6 +1,7 @@
 package commo_test
 
 import (
+	"html/template"
 	"path/filepath"
 	"strings"
 	"testing"
@@ -25,6 +26,18 @@ func TestRenderUnknown(t *testing.T) {
 	require.EqualError(t, err, "could not find \"foo.txt\" in templates", "expected unknown template")
 }
 
+// Ensures that an error is returned when the template is not found.
+func TestRenderStringPropagatesError(t *testing.T) {
+	templates := map[string]*template.Template{
+		"broken.txt":  template.Must(template.New("broken.txt").Parse(`{{call .Fail}}`)),
+		"broken.html": template.Must(template.New("broken.html").Parse(`{{call .Fail}}`)),
+	}
+	commo.WithTemplates(templates)
+
+	_, _, err := commo.RenderString("broken", nil)
+	require.Error(t, err)
+}
+
 func allEmailTemplates(t *testing.T) []string {
 	paths := make(map[string]struct{})
 	ls, err := filepath.Glob("templates/*.*")

sendgrid_test.go

@@ -1,11 +1,14 @@
 package commo_test
 
 import (
+	"context"
+	"errors"
 	"testing"
 
 	sgmail "github.com/sendgrid/sendgrid-go/helpers/mail"
 	"github.com/stretchr/testify/require"
 	"go.rtnl.ai/commo"
+	"go.rtnl.ai/x/backoff"
 )
 
 func TestNewSGEmail(t *testing.T) {
@@ -103,3 +106,37 @@ func TestNewSGEmails(t *testing.T) {
 		}
 	})
 }
+
+// Ensures that Sendgrid errors are handled correctly.
+func TestSendGridResponseError(t *testing.T) {
+	t.Run("4xx is not retried", func(t *testing.T) {
+		err := commo.SendGridResponseError(400, `{"errors":[{"message":"bad"}]}`)
+		var stop *backoff.NoRetryError
+		require.ErrorAs(t, err, &stop)
+
+		attempts := 0
+		_, err = backoff.Retry(context.Background(), func() (any, error) {
+			attempts++
+			return nil, commo.SendGridResponseError(400, "bad request")
+		})
+		require.Error(t, err)
+		require.Equal(t, 1, attempts)
+	})
+
+	t.Run("5xx is retried", func(t *testing.T) {
+		err := commo.SendGridResponseError(503, "unavailable")
+		var stop *backoff.NoRetryError
+		require.False(t, errors.As(err, &stop))
+
+		attempts := 0
+		_, err = backoff.Retry(context.Background(), func() (any, error) {
+			attempts++
+			if attempts < 3 {
+				return nil, commo.SendGridResponseError(503, "unavailable")
+			}
+			return nil, nil
+		}, backoff.WithMaxTries(3))
+		require.NoError(t, err)
+		require.Equal(t, 3, attempts)
+	})
+}