Compress data on flush when compression is not started

Co-authored-by: Romain <rtribotte@users.noreply.github.com>

docs/content/middlewares/http/compress.md

@@ -179,9 +179,15 @@ http:
 _Optional, Default=1024_
 
 `minResponseBodyBytes` specifies the minimum amount of bytes a response body must have to be compressed.
-
 Responses smaller than the specified values will not be compressed.
 
+!!! tip "Streaming"
+
+    When data is sent to the client on flush, the `minResponseBodyBytes` configuration is ignored and the data is compressed.
+    This is particularly the case when data is streamed to the client when using `Transfer-encoding: chunked` response.
+
+When chunked data is sent to the client on flush, it will be compressed by default even if the received data has not reached  
+
 ```yaml tab="Docker & Swarm"
 labels:
   - "traefik.http.middlewares.test-compress.compress.minresponsebodybytes=1200"

pkg/middlewares/compress/compress_test.go

@@ -609,6 +609,27 @@ func TestMinResponseBodyBytes(t *testing.T) {
 func Test1xxResponses(t *testing.T) {
 	fakeBody := generateBytes(100000)
 
+	testCases := []struct {
+		desc     string
+		encoding string
+	}{
+		{
+			desc:     "gzip",
+			encoding: gzipName,
+		},
+		{
+			desc:     "brotli",
+			encoding: brotliName,
+		},
+		{
+			desc:     "zstd",
+			encoding: zstdName,
+		},
+	}
+	for _, test := range testCases {
+		t.Run(test.desc, func(t *testing.T) {
+			t.Parallel()
+
 			next := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 				h := w.Header()
 				h.Add("Link", "</style.css>; rel=preload; as=style")
@@ -671,7 +692,7 @@ func Test1xxResponses(t *testing.T) {
 				},
 			}
 			req, _ := http.NewRequestWithContext(httptrace.WithClientTrace(context.Background(), trace), http.MethodGet, server.URL, nil)
-	req.Header.Add(acceptEncodingHeader, gzipName)
+			req.Header.Add(acceptEncodingHeader, test.encoding)
 
 			res, err := frontendClient.Do(req)
 			assert.NoError(t, err)
@@ -683,9 +704,11 @@ func Test1xxResponses(t *testing.T) {
 			}
 			checkLinkHeaders(t, []string{"</style.css>; rel=preload; as=style", "</script.js>; rel=preload; as=script", "</foo.js>; rel=preload; as=script"}, res.Header["Link"])
 
-	assert.Equal(t, gzipName, res.Header.Get(contentEncodingHeader))
+			assert.Equal(t, test.encoding, res.Header.Get(contentEncodingHeader))
 			body, _ := io.ReadAll(res.Body)
 			assert.NotEqualValues(t, body, fakeBody)
+		})
+	}
 }
 
 func BenchmarkCompressGzip(b *testing.B) {

pkg/middlewares/compress/compression_handler.go

@@ -192,13 +192,18 @@ func (r *responseWriter) Header() http.Header {
 }
 
 func (r *responseWriter) WriteHeader(statusCode int) {
-	if r.statusCodeSet {
+	// Handle informational headers
+	// This is gated to not forward 1xx responses on builds prior to go1.20.
+	if statusCode >= 100 && statusCode <= 199 {
+		r.rw.WriteHeader(statusCode)
 		return
 	}
 
+	if !r.statusCodeSet {
 		r.statusCode = statusCode
 		r.statusCodeSet = true
 	}
+}
 
 func (r *responseWriter) Write(p []byte) (int, error) {
 	// i.e. has write ever been called at least once with non nil data.
@@ -319,11 +324,16 @@ func (r *responseWriter) Flush() {
 	}
 
 	// Here, nothing was ever written either to rw or to bw (since we're still
-	// waiting to decide whether to compress), so we do not need to flush anything.
-	// Note that we diverge with klauspost's gzip behavior, where they instead
-	// force compression and flush whatever was in the buffer in this case.
+	// waiting to decide whether to compress), so to be aligned with klauspost's
+	// gzip behavior we force the compression and flush whatever was in the buffer in this case.
 	if !r.compressionStarted {
-		return
+		r.rw.Header().Del(contentLength)
+
+		r.rw.Header().Set(contentEncoding, r.compressionWriter.ContentEncoding())
+		r.rw.WriteHeader(r.statusCode)
+		r.headersSent = true
+
+		r.compressionStarted = true
 	}
 
 	// Conversely, we here know that something was already written to bw (or is

pkg/middlewares/compress/compression_handler_test.go

@@ -498,6 +498,73 @@ func Test_FlushAfterAllWrites(t *testing.T) {
 	}
 }
 
+func Test_FlushForceCompress(t *testing.T) {
+	testCases := []struct {
+		desc           string
+		cfg            Config
+		algo           string
+		readerBuilder  func(io.Reader) (io.Reader, error)
+		acceptEncoding string
+	}{
+		{
+			desc: "brotli",
+			cfg:  Config{MinSize: 1024, MiddlewareName: "Test"},
+			algo: brotliName,
+			readerBuilder: func(reader io.Reader) (io.Reader, error) {
+				return brotli.NewReader(reader), nil
+			},
+			acceptEncoding: "br",
+		},
+		{
+			desc: "zstd",
+			cfg:  Config{MinSize: 1024, MiddlewareName: "Test"},
+			algo: zstdName,
+			readerBuilder: func(reader io.Reader) (io.Reader, error) {
+				return zstd.NewReader(reader)
+			},
+			acceptEncoding: "zstd",
+		},
+	}
+
+	for _, test := range testCases {
+		t.Run(test.desc, func(t *testing.T) {
+			t.Parallel()
+
+			next := http.HandlerFunc(func(rw http.ResponseWriter, req *http.Request) {
+				rw.WriteHeader(http.StatusOK)
+
+				_, err := rw.Write(smallTestBody)
+				require.NoError(t, err)
+
+				rw.(http.Flusher).Flush()
+			})
+
+			srv := httptest.NewServer(mustNewCompressionHandler(t, test.cfg, test.algo, next))
+			defer srv.Close()
+
+			req, err := http.NewRequest(http.MethodGet, srv.URL, http.NoBody)
+			require.NoError(t, err)
+
+			req.Header.Set(acceptEncoding, test.acceptEncoding)
+
+			res, err := http.DefaultClient.Do(req)
+			require.NoError(t, err)
+
+			defer res.Body.Close()
+
+			assert.Equal(t, http.StatusOK, res.StatusCode)
+			assert.Equal(t, test.acceptEncoding, res.Header.Get(contentEncoding))
+
+			reader, err := test.readerBuilder(res.Body)
+			require.NoError(t, err)
+
+			got, err := io.ReadAll(reader)
+			require.NoError(t, err)
+			assert.Equal(t, smallTestBody, got)
+		})
+	}
+}
+
 func Test_ExcludedContentTypes(t *testing.T) {
 	testCases := []struct {
 		desc                 string