net/http: reverseproxy: forward 1xx responses

Support for 1xx responses has recently been merged in
net/http (golang#42597).

As discussed in this CL
(https://go-review.googlesource.com/c/go/+/269997/comments/1ff70bef_c25a829a),
support for forwarding 1xx responses in ReverseProxy has been extracted
in this separate patch.

According to RFC 7231, "a proxy MUST forward 1xx responses unless the
proxy itself requested the generation of the 1xx response".
Consequently, all received 1xx responses are automatically forwarded as long as the
underlying transport supports ClientTrace.Got1xxResponse.

Fixes golang#26088
Fixes golang#51914

src/net/http/httputil/reverseproxy.go

@@ -14,6 +14,7 @@ import (
 	"mime"
 	"net"
 	"net/http"
+	"net/http/httptrace"
 	"net/http/internal/ascii"
 	"net/textproto"
 	"net/url"
@@ -40,6 +41,9 @@ import (
 // To prevent IP spoofing, be sure to delete any pre-existing
 // X-Forwarded-For header coming from the client or
 // an untrusted proxy.
+//
+// 1xx responses are forwarded to the client if the underlying
+// transport supports ClientTrace.Got1xxResponse.
 type ReverseProxy struct {
 	// Director must be a function which modifies
 	// the request into a new request to be sent
@@ -307,6 +311,23 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 		}
 	}
 
+	var headerSet bool
+	trace := &httptrace.ClientTrace{
+		Got1xxResponse: func(code int, header textproto.MIMEHeader) error {
+			h := rw.Header()
+			copyHeader(h, http.Header(header))
+			rw.WriteHeader(code)
+
+			// Clear headers, it's not automatically done by ResponseWriter.WriteHeader() for 1xx responses
+			for k, _ := range h {
+				h.Del(k)
+			}
+
+			return nil
+		},
+	}
+	outreq = outreq.WithContext(httptrace.WithClientTrace(outreq.Context(), trace))
+
 	res, err := transport.RoundTrip(outreq)
 	if err != nil {
 		p.getErrorHandler()(rw, outreq, err)
@@ -332,7 +353,14 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 		return
 	}
 
-	copyHeader(rw.Header(), res.Header)
+	h := rw.Header()
+	if headerSet {
+		for k, _ := range h {
+			h.Del(k)
+		}
+	}
+
+	copyHeader(h, res.Header)
 
 	// The "Trailer" header isn't included in the Transport's response,
 	// at least for *http.Transport. Build it up from Trailer.

src/net/http/httputil/reverseproxy_test.go

@@ -16,7 +16,9 @@ import (
 	"log"
 	"net/http"
 	"net/http/httptest"
+	"net/http/httptrace"
 	"net/http/internal/ascii"
+	"net/textproto"
 	"net/url"
 	"os"
 	"reflect"
@@ -1537,3 +1539,77 @@ func TestJoinURLPath(t *testing.T) {
 		}
 	}
 }
+
+func Test1xxResponses(t *testing.T) {
+	backend := httptest.NewServer(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([]byte("Hello"))
+	}))
+	defer backend.Close()
+	backendURL, err := url.Parse(backend.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+	proxyHandler := NewSingleHostReverseProxy(backendURL)
+	proxyHandler.ErrorLog = log.New(io.Discard, "", 0) // quiet for tests
+	frontend := httptest.NewServer(proxyHandler)
+	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 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)
+
+	res, err := frontendClient.Do(req)
+	if err != nil {
+		t.Fatalf("Get: %v", err)
+	}
+
+	defer res.Body.Close()
+
+	if respCounter != 2 {
+		t.Errorf("Excpected 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"])
+
+	body, _ := io.ReadAll(res.Body)
+	if string(body) != "Hello" {
+		t.Errorf("Read body %q; want Hello", body)
+	}
+}