fix #91: use github.com/golang/gddo/httputil to parse Accept header

Gopkg.toml

@@ -2,6 +2,10 @@
   name = "github.com/go-chi/chi"
   version = "^3.3.0"
 
+[[constraint]]
+  name = "github.com/golang/gddo"
+  branch = "master"
+
 [[constraint]]
   name = "github.com/golang/mock"
   branch = "master"
@@ -11,16 +15,16 @@
   branch = "master"
 
 [[constraint]]
-  name = "github.com/rubenv/sql-migrate"
+  name = "github.com/pkg/errors"
   branch = "master"
 
 [[constraint]]
-  name = "github.com/pkg/errors"
-  version = "*"
+  name = "github.com/rubenv/sql-migrate"
+  branch = "master"
 
 [[constraint]]
   name = "github.com/spf13/cobra"
-  version = "*"
+  branch = "master"
 
 [[constraint]]
   name = "github.com/spf13/viper"

server/middleware/encoder.go

@@ -3,35 +3,27 @@ package middleware
 import (
 	"context"
 	"net/http"
-	"strings"
 
+	"github.com/golang/gddo/httputil"
 	"github.com/kamilsk/form-api/transfer/encoding"
 )
 
+// Notes:
+// - related issue https://github.com/golang/go/issues/19307
+
 // Encoder injects required response encoder to the request context.
 func Encoder(next http.HandlerFunc) http.HandlerFunc {
 	return func(rw http.ResponseWriter, req *http.Request) {
-		// Accept: */*
-		// Accept: text/html
-		// Accept: image/*
-		// Accept: text/html, application/xhtml+xml, application/xml; q=0.9, */*; q=0.8
-		accept := fallback(req.Header.Get("Accept"), encoding.XML)
-		contentType := strings.TrimSpace(strings.Split(strings.Split(accept, ";")[0], ",")[0])
+		var contentType string
+		if req.Header.Get("Accept") == "" {
+			contentType = encoding.XML
+		} else {
+			contentType = httputil.NegotiateContentType(req, encoding.Offers(), "")
+		}
 		if !encoding.IsSupported(contentType) {
 			rw.WriteHeader(http.StatusNotAcceptable)
 			return
 		}
 		next(rw, req.WithContext(context.WithValue(req.Context(), EncoderKey{}, encoding.NewEncoder(rw, contentType))))
 	}
 }
-
-func fallback(value string, fallbackValues ...string) string {
-	if value == "" || value == "*/*" {
-		for _, value := range fallbackValues {
-			if value != "" {
-				return value
-			}
-		}
-	}
-	return value
-}

transfer/encoding/encoder.go

@@ -20,14 +20,20 @@ const (
 	XML = "application/xml"
 )
 
+// This var is related to `Offers`.
 var supported = map[string]func(io.Writer) Encoder{
 	HTML: func(stream io.Writer) Encoder { return htmlEncoder{stream} },
 	JSON: func(stream io.Writer) Encoder { return json.NewEncoder(stream) },
 	TEXT: func(stream io.Writer) Encoder { return yamlEncoder{stream, yaml.Marshal} },
 	XML:  func(stream io.Writer) Encoder { return xml.NewEncoder(stream) },
 }
 
-// IsSupported returns true if provided content type is supported by encoder.
+// Offers returns supported content types.
+func Offers() []string {
+	return []string{HTML, JSON, TEXT, XML}
+}
+
+// IsSupported returns true if the provided content type is supported by encoder.
 func IsSupported(contentType string) bool {
 	_, ok := supported[contentType]
 	return ok