encode: Respect Cache-Control no-transform (#5257)

* encode: respect Cache-Control HTTP header no-transform

* encode: switch to strings.Contains

modules/caddyhttp/encode/encode.go

@@ -117,14 +117,20 @@ func (enc *Encode) Validate() error {
 	return nil
 }
 
+func isEncodeAllowed(h http.Header) bool {
+	return !strings.Contains(h.Get("Cache-Control"), "no-transform")
+}
+
 func (enc *Encode) ServeHTTP(w http.ResponseWriter, r *http.Request, next caddyhttp.Handler) error {
-	for _, encName := range AcceptedEncodings(r, enc.Prefer) {
-		if _, ok := enc.writerPools[encName]; !ok {
-			continue // encoding not offered
+	if isEncodeAllowed(r.Header) {
+		for _, encName := range AcceptedEncodings(r, enc.Prefer) {
+			if _, ok := enc.writerPools[encName]; !ok {
+				continue // encoding not offered
+			}
+			w = enc.openResponseWriter(encName, w)
+			defer w.(*responseWriter).Close()
+			break
 		}
-		w = enc.openResponseWriter(encName, w)
-		defer w.(*responseWriter).Close()
-		break
 	}
 	return next.ServeHTTP(w, r)
 }
@@ -281,7 +287,7 @@ func (rw *responseWriter) Close() error {
 
 // init should be called before we write a response, if rw.buf has contents.
 func (rw *responseWriter) init() {
-	if rw.Header().Get("Content-Encoding") == "" &&
+	if rw.Header().Get("Content-Encoding") == "" && isEncodeAllowed(rw.Header()) &&
 		rw.config.Match(rw) {
 
 		rw.w = rw.config.writerPools[rw.encodingName].Get().(Encoder)

modules/caddyhttp/encode/encode_test.go

@@ -261,3 +261,50 @@ func TestValidate(t *testing.T) {
 
 	}
 }
+
+func TestIsEncodeAllowed(t *testing.T) {
+	testCases := []struct {
+		name     string
+		headers  http.Header
+		expected bool
+	}{
+		{
+			name:     "Without any headers",
+			headers:  http.Header{},
+			expected: true,
+		},
+		{
+			name: "Without Cache-Control HTTP header",
+			headers: http.Header{
+				"Accept-Encoding": {"gzip"},
+			},
+			expected: true,
+		},
+		{
+			name: "Cache-Control HTTP header ending with no-transform directive",
+			headers: http.Header{
+				"Accept-Encoding": {"gzip"},
+				"Cache-Control":   {"no-cache; no-transform"},
+			},
+			expected: false,
+		},
+		{
+			name: "With Cache-Control HTTP header no-transform as Cache-Extension value",
+			headers: http.Header{
+				"Accept-Encoding": {"gzip"},
+				"Cache-Control":   {`no-store; no-cache; community="no-transform"`},
+			},
+			expected: false,
+		},
+	}
+
+	for _, test := range testCases {
+		t.Run(test.name, func(t *testing.T) {
+			if result := isEncodeAllowed(test.headers); result != test.expected {
+				t.Errorf("The headers given to the isEncodeAllowed should return %t, %t given.",
+					result,
+					test.expected)
+			}
+		})
+	}
+}