Allow users to HTML template error messages

config/config.go

@@ -261,6 +261,8 @@ type Config struct {
 	HTTPRewrite HTTPRewrite `yaml:"http_rewrite,omitempty"`
 
 	EmptyPoolResponseCode503 bool `yaml:"empty_pool_response_code_503,omitempty"`
+
+	HTMLErrorTemplateFile string `yaml:"html_error_template_file,omitempty"`
 }
 
 var defaultConfig = Config{

config/config_test.go

@@ -604,6 +604,17 @@ backends:
 			Expect(err).ToNot(HaveOccurred())
 			Expect(config.DisableHTTP).To(BeTrue())
 		})
+
+		It("defaults HTMLErrorTemplateFile to empty", func() {
+			Expect(config.HTMLErrorTemplateFile).To(Equal(""))
+		})
+
+		It("sets HTMLErrorTemplateFile", func() {
+			var b = []byte(`html_error_template_file: "/path/to/file"`)
+			err := config.Initialize(b)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(config.HTMLErrorTemplateFile).To(Equal("/path/to/file"))
+		})
 	})
 
 	Describe("Process", func() {

errorwriter/error_writer.go

@@ -0,0 +1,123 @@
+package errorwriter
+
+import (
+	"bytes"
+	"fmt"
+	"html/template"
+	"io/ioutil"
+	"net/http"
+
+	"code.cloudfoundry.org/gorouter/logger"
+	"github.com/uber-go/zap"
+)
+
+type ErrorWriter interface {
+	WriteError(
+		rw http.ResponseWriter,
+		code int,
+		message string,
+		logger logger.Logger,
+	)
+}
+
+type plaintextErrorWriter struct{}
+
+func NewPlaintextErrorWriter() ErrorWriter {
+	return &plaintextErrorWriter{}
+}
+
+// WriteStatus attempts to template an error message.
+func (ew *plaintextErrorWriter) WriteError(
+	rw http.ResponseWriter,
+	code int,
+	message string,
+	logger logger.Logger,
+) {
+	body := fmt.Sprintf("%d %s: %s", code, http.StatusText(code), message)
+
+	if code != http.StatusNotFound {
+		logger.Info("status", zap.String("body", body))
+	}
+
+	if code > 299 {
+		rw.Header().Del("Connection")
+	}
+
+	rw.Header().Set("Content-Type", "text/plain; charset=utf-8")
+	rw.Header().Set("X-Content-Type-Options", "nosniff")
+
+	rw.WriteHeader(code)
+	fmt.Fprintln(rw, body)
+}
+
+type htmlErrorWriter struct {
+	tpl *template.Template
+}
+
+type htmlErrorWriterContext struct {
+	Status     int
+	StatusText string
+	Message    string
+	Header     http.Header
+}
+
+func NewHTMLErrorWriterFromFile(path string) (ErrorWriter, error) {
+	ew := &htmlErrorWriter{}
+
+	bytes, err := ioutil.ReadFile(path)
+	if err != nil {
+		return nil, fmt.Errorf("Could not read HTML error template file: %s", err)
+	}
+
+	tpl, err := template.New("error-message").Parse(string(bytes))
+	if err != nil {
+		return nil, err
+	}
+	ew.tpl = tpl
+
+	return ew, nil
+}
+
+// WriteStatus attempts to template an error message.
+// If the template cannot be rendered then text will be sent instead
+// and the error will be returned even though the response has been sent
+func (ew *htmlErrorWriter) WriteError(
+	rw http.ResponseWriter,
+	code int,
+	message string,
+	logger logger.Logger,
+) {
+	body := fmt.Sprintf("%d %s: %s", code, http.StatusText(code), message)
+
+	if code != http.StatusNotFound {
+		logger.Info("status", zap.String("body", body))
+	}
+
+	if code > 299 {
+		rw.Header().Del("Connection")
+	}
+
+	tplContext := htmlErrorWriterContext{
+		Status:     code,
+		StatusText: http.StatusText(code),
+		Message:    message,
+		Header:     rw.Header(),
+	}
+	rw.Header().Set("Content-Type", "text/html; charset=utf-8")
+
+	var respBytes []byte
+	var rendered bytes.Buffer
+	if err := ew.tpl.Execute(&rendered, &tplContext); err != nil {
+		logger.Error("render-error-failed", zap.Error(err))
+		rw.Header().Set("Content-Type", "text/plain; charset=utf-8")
+		rw.Header().Set("X-Content-Type-Options", "nosniff")
+		respBytes = []byte(body)
+	} else {
+		rw.Header().Set("Content-Type", "text/html; charset=utf-8")
+		rw.Header().Set("X-Content-Type-Options", "nosniff")
+		respBytes = rendered.Bytes()
+	}
+
+	rw.WriteHeader(code)
+	rw.Write(respBytes)
+}