Disable Content-Type auto-detection by default

pkg/middlewares/contenttype/content_type.go

@@ -0,0 +1,46 @@
+package contenttype
+
+import (
+	"context"
+	"net/http"
+
+	"github.com/traefik/traefik/v2/pkg/middlewares"
+)
+
+const (
+	typeName = "ContentType"
+)
+
+// ContentType is a middleware used to activate Content-Type auto-detection.
+type contentType struct {
+	next http.Handler
+	name string
+}
+
+// New creates a new handler.
+func New(ctx context.Context, next http.Handler, name string) (http.Handler, error) {
+	middlewares.GetLogger(ctx, name, typeName).Debug().Msg("Creating middleware")
+	return &contentType{next: next, name: name}, nil
+}
+
+func (c *contentType) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
+	// Re-enable auto-detection.
+	if ct, ok := rw.Header()["Content-Type"]; ok && ct == nil {
+		middlewares.GetLogger(req.Context(), c.name, typeName).
+			Debug().Msg("Enable Content-Type auto-detection.")
+		delete(rw.Header(), "Content-Type")
+	}
+
+	c.next.ServeHTTP(rw, req)
+}
+
+func DisableAutoDetection(next http.Handler) http.HandlerFunc {
+	return func(rw http.ResponseWriter, req *http.Request) {
+		// Prevent Content-Type auto-detection.
+		if _, ok := rw.Header()["Content-Type"]; !ok {
+			rw.Header()["Content-Type"] = nil
+		}
+
+		next.ServeHTTP(rw, req)
+	}
+}

pkg/middlewares/contenttype/content_type_test.go

@@ -0,0 +1,79 @@
+package contenttype
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"github.com/traefik/traefik/v2/pkg/testhelpers"
+)
+
+func TestAutoDetection(t *testing.T) {
+	testCases := []struct {
+		desc            string
+		autoDetect      bool
+		contentType     string
+		wantContentType string
+	}{
+		{
+			desc:            "Keep the Content-Type returned by the server",
+			autoDetect:      false,
+			contentType:     "application/json",
+			wantContentType: "application/json",
+		},
+		{
+			desc:            "Don't auto-detect Content-Type header by default when not set by the server",
+			autoDetect:      false,
+			contentType:     "",
+			wantContentType: "",
+		},
+		{
+			desc:            "Keep the Content-Type returned by the server with auto-detection middleware",
+			autoDetect:      true,
+			contentType:     "application/json",
+			wantContentType: "application/json",
+		},
+		{
+			desc:            "Auto-detect when Content-Type header is not already set by the server with auto-detection middleware",
+			autoDetect:      true,
+			contentType:     "",
+			wantContentType: "text/plain; charset=utf-8",
+		},
+	}
+
+	for _, test := range testCases {
+		test := test
+		t.Run(test.desc, func(t *testing.T) {
+			t.Parallel()
+
+			var next http.Handler
+			next = http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				if test.contentType != "" {
+					w.Header().Set("Content-Type", test.contentType)
+				}
+				w.WriteHeader(http.StatusOK)
+				_, _ = w.Write([]byte("Test"))
+			})
+
+			if test.autoDetect {
+				var err error
+				next, err = New(context.Background(), next, "foo-content-type")
+				require.NoError(t, err)
+			}
+
+			server := httptest.NewServer(
+				DisableAutoDetection(next),
+			)
+			t.Cleanup(server.Close)
+
+			req := testhelpers.MustNewRequest(http.MethodGet, server.URL, nil)
+			res, err := server.Client().Do(req)
+			require.NoError(t, err)
+
+			assert.Equal(t, test.wantContentType, res.Header.Get("Content-Type"))
+		})
+	}
+}