Write Content-Encoding header in the response properly when gzip encoding is requested (#15499)

* Write gzip header properly

* changelog

* regression test

api/server/middleware/middleware.go

@@ -127,7 +127,7 @@ func AcceptEncodingHeaderHandler() Middleware {
 			gz := gzip.NewWriter(w)
 			gzipRW := &gzipResponseWriter{gz: gz, ResponseWriter: w}
 			defer func() {
-				if !gzipRW.zipped {
+				if !gzipRW.zip {
 					return
 				}
 				if err := gz.Close(); err != nil {
@@ -143,13 +143,22 @@ func AcceptEncodingHeaderHandler() Middleware {
 type gzipResponseWriter struct {
 	gz *gzip.Writer
 	http.ResponseWriter
-	zipped bool
+	zip bool
+}
+
+func (g *gzipResponseWriter) WriteHeader(statusCode int) {
+	if strings.Contains(g.Header().Get("Content-Type"), api.JsonMediaType) {
+		// Removing the current Content-Length because zipping will change it.
+		g.Header().Del("Content-Length")
+		g.Header().Set("Content-Encoding", "gzip")
+		g.zip = true
+	}
+
+	g.ResponseWriter.WriteHeader(statusCode)
 }
 
 func (g *gzipResponseWriter) Write(b []byte) (int, error) {
-	if strings.Contains(g.Header().Get("Content-Type"), api.JsonMediaType) {
-		g.zipped = true
-		g.Header().Set("Content-Encoding", "gzip")
+	if g.zip {
 		return g.gz.Write(b)
 	}
 	return g.ResponseWriter.Write(b)

api/server/middleware/middleware_test.go

@@ -13,6 +13,23 @@ import (
 	log "github.com/sirupsen/logrus"
 )
 
+// frozenHeaderRecorder allows asserting that response headers were not modified
+// after the call to WriteHeader.
+//
+// Its purpose is to have a regression test for https://github.com/OffchainLabs/prysm/pull/15499.
+type frozenHeaderRecorder struct {
+	*httptest.ResponseRecorder
+	frozenHeader http.Header
+}
+
+func (r *frozenHeaderRecorder) WriteHeader(code int) {
+	if r.frozenHeader != nil {
+		return
+	}
+	r.ResponseRecorder.WriteHeader(code)
+	r.frozenHeader = r.ResponseRecorder.Header().Clone()
+}
+
 func TestNormalizeQueryValuesHandler(t *testing.T) {
 	nextHandler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		_, err := w.Write([]byte("next handler"))
@@ -132,6 +149,7 @@ func TestAcceptEncodingHeaderHandler(t *testing.T) {
 	dummyContent := "Test gzip middleware content"
 	nextHandler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		w.Header().Set("Content-Type", r.Header.Get("Accept"))
+		w.WriteHeader(http.StatusOK)
 		_, err := w.Write([]byte(dummyContent))
 		require.NoError(t, err)
 	})
@@ -145,19 +163,19 @@ func TestAcceptEncodingHeaderHandler(t *testing.T) {
 		expectCompressed bool
 	}{
 		{
-			name:             "Gzip supported",
+			name:             "Accept gzip",
 			accept:           api.JsonMediaType,
 			acceptEncoding:   "gzip",
 			expectCompressed: true,
 		},
 		{
-			name:             "Multiple encodings supported",
+			name:             "Accept multiple encodings",
 			accept:           api.JsonMediaType,
 			acceptEncoding:   "deflate, gzip",
 			expectCompressed: true,
 		},
 		{
-			name:             "Gzip not supported",
+			name:             "Accept unsupported encoding",
 			accept:           api.JsonMediaType,
 			acceptEncoding:   "deflate",
 			expectCompressed: false,
@@ -183,12 +201,12 @@ func TestAcceptEncodingHeaderHandler(t *testing.T) {
 			if tt.acceptEncoding != "" {
 				req.Header.Set("Accept-Encoding", tt.acceptEncoding)
 			}
-			rr := httptest.NewRecorder()
+			rr := &frozenHeaderRecorder{ResponseRecorder: httptest.NewRecorder()}
 
 			handler.ServeHTTP(rr, req)
 
 			if tt.expectCompressed {
-				require.Equal(t, "gzip", rr.Header().Get("Content-Encoding"), "Expected Content-Encoding header to be 'gzip'")
+				require.Equal(t, "gzip", rr.frozenHeader.Get("Content-Encoding"), "Expected Content-Encoding header to be 'gzip'")
 
 				compressedBody := rr.Body.Bytes()
 				require.NotEqual(t, dummyContent, string(compressedBody), "Response body should be compressed and differ from the original")

changelog/radek_fix-gzip.md

@@ -0,0 +1,3 @@
+### Fixed
+
+- Write `Content-Encoding` header in the response properly when gzip encoding is requested.