gzhttp: Handle informational headers (#815)

* Handle informational headers
* review: gate 1xx responses forward to go1.20+

gzhttp/compress.go

@@ -302,7 +302,15 @@ func (w *GzipResponseWriter) startPlain() error {
 }
 
 // WriteHeader just saves the response code until close or GZIP effective writes.
+// In the specific case of 1xx status codes, WriteHeader is directly calling the wrapped ResponseWriter.
 func (w *GzipResponseWriter) WriteHeader(code int) {
+	// Handle informational headers
+	// This is gated to not forward 1xx responses on builds prior to go1.20.
+	if shouldWrite1xxResponses() && code >= 100 && code <= 199 {
+		w.ResponseWriter.WriteHeader(code)
+		return
+	}
+
 	if w.code == 0 {
 		w.code = code
 	}

gzhttp/compress_go119.go

@@ -0,0 +1,9 @@
+//go:build !go1.20
+// +build !go1.20
+
+package gzhttp
+
+// shouldWrite1xxResponses indicates whether the current build supports writes of 1xx status codes.
+func shouldWrite1xxResponses() bool {
+	return false
+}

gzhttp/compress_go120.go

@@ -0,0 +1,9 @@
+//go:build go1.20
+// +build go1.20
+
+package gzhttp
+
+// shouldWrite1xxResponses indicates whether the current build supports writes of 1xx status codes.
+func shouldWrite1xxResponses() bool {
+	return true
+}

gzhttp/compress_test.go

@@ -2,12 +2,15 @@ package gzhttp
 
 import (
 	"bytes"
+	"context"
 	"fmt"
 	"io"
 	"math/rand"
 	"net"
 	"net/http"
 	"net/http/httptest"
+	"net/http/httptrace"
+	"net/textproto"
 	"net/url"
 	"os"
 	"strconv"
@@ -1796,3 +1799,87 @@ func TestGzipHandlerNilContentType(t *testing.T) {
 
 	assertEqual(t, "", res.Header().Get("Content-Type"))
 }
+
+// This test is an adapted version of net/http/httputil.Test1xxResponses test.
+func Test1xxResponses(t *testing.T) {
+	// do not test 1xx responses on builds prior to go1.20.
+	if !shouldWrite1xxResponses() {
+		return
+	}
+
+	wrapper, _ := NewWrapper()
+	handler := wrapper(http.HandlerFunc(
+		func(w http.ResponseWriter, r *http.Request) {
+			h := w.Header()
+			h.Add("Link", "</style.css>; rel=preload; as=style")
+			h.Add("Link", "</script.js>; rel=preload; as=script")
+			w.WriteHeader(http.StatusEarlyHints)
+
+			h.Add("Link", "</foo.js>; rel=preload; as=script")
+			w.WriteHeader(http.StatusProcessing)
+
+			w.Write(testBody)
+		},
+	))
+
+	frontend := httptest.NewServer(handler)
+	defer frontend.Close()
+	frontendClient := frontend.Client()
+
+	checkLinkHeaders := func(t *testing.T, expected, got []string) {
+		t.Helper()
+
+		if len(expected) != len(got) {
+			t.Errorf("Expected %d link headers; got %d", len(expected), len(got))
+		}
+
+		for i := range expected {
+			if i >= len(got) {
+				t.Errorf("Expected %q link header; got nothing", expected[i])
+
+				continue
+			}
+
+			if expected[i] != got[i] {
+				t.Errorf("Expected %q link header; got %q", expected[i], got[i])
+			}
+		}
+	}
+
+	var respCounter uint8
+	trace := &httptrace.ClientTrace{
+		Got1xxResponse: func(code int, header textproto.MIMEHeader) error {
+			switch code {
+			case http.StatusEarlyHints:
+				checkLinkHeaders(t, []string{"</style.css>; rel=preload; as=style", "</script.js>; rel=preload; as=script"}, header["Link"])
+			case http.StatusProcessing:
+				checkLinkHeaders(t, []string{"</style.css>; rel=preload; as=style", "</script.js>; rel=preload; as=script", "</foo.js>; rel=preload; as=script"}, header["Link"])
+			default:
+				t.Error("Unexpected 1xx response")
+			}
+
+			respCounter++
+
+			return nil
+		},
+	}
+	req, _ := http.NewRequestWithContext(httptrace.WithClientTrace(context.Background(), trace), "GET", frontend.URL, nil)
+	req.Header.Set("Accept-Encoding", "gzip")
+
+	res, err := frontendClient.Do(req)
+	if err != nil {
+		t.Fatalf("Get: %v", err)
+	}
+
+	defer res.Body.Close()
+
+	if respCounter != 2 {
+		t.Errorf("Expected 2 1xx responses; got %d", respCounter)
+	}
+	checkLinkHeaders(t, []string{"</style.css>; rel=preload; as=style", "</script.js>; rel=preload; as=script", "</foo.js>; rel=preload; as=script"}, res.Header["Link"])
+
+	assertEqual(t, "gzip", res.Header.Get("Content-Encoding"))
+
+	body, _ := io.ReadAll(res.Body)
+	assertEqual(t, gzipStrLevel(testBody, gzip.DefaultCompression), body)
+}