net/http: allow sending 1xx responses

src/net/http/clientserver_test.go

@@ -9,6 +9,7 @@ package http_test
 import (
 	"bytes"
 	"compress/gzip"
+	"context"
 	"crypto/rand"
 	"crypto/sha1"
 	"crypto/tls"
@@ -19,7 +20,9 @@ import (
 	"net"
 	. "net/http"
 	"net/http/httptest"
+	"net/http/httptrace"
 	"net/http/httputil"
+	"net/textproto"
 	"net/url"
 	"os"
 	"reflect"
@@ -1616,3 +1619,95 @@ func testIdentityTransferEncoding(t *testing.T, h2 bool) {
 		t.Errorf("got response body = %q; want %q", got, want)
 	}
 }
+
+func TestEarlyHintsRequest_h1(t *testing.T) { testEarlyHintsRequest(t, h1Mode) }
+func TestEarlyHintsRequest_h2(t *testing.T) { testEarlyHintsRequest(t, h2Mode) }
+func testEarlyHintsRequest(t *testing.T, h2 bool) {
+	defer afterTest(t)
+	if h2 {
+		t.Skip("Waiting for H2 support to be merged: https://go-review.googlesource.com/c/net/+/406494")
+	}
+
+	var wg sync.WaitGroup
+	wg.Add(1)
+	cst := newClientServerTest(t, h2, HandlerFunc(func(w ResponseWriter, r *Request) {
+		h := w.Header()
+
+		h.Add("Content-Length", "123") // must be ignored
+		h.Add("Link", "</style.css>; rel=preload; as=style")
+		h.Add("Link", "</script.js>; rel=preload; as=script")
+		w.WriteHeader(StatusEarlyHints)
+
+		wg.Wait()
+
+		h.Add("Link", "</foo.js>; rel=preload; as=script")
+		w.WriteHeader(StatusEarlyHints)
+
+		w.Write([]byte("Hello"))
+	}))
+	defer cst.close()
+
+	checkLinkHeaders := func(t *testing.T, expected, got []string) {
+		t.Helper()
+
+		if len(expected) != len(got) {
+			t.Errorf("got %d expected %d", len(got), len(expected))
+		}
+
+		for i := range expected {
+			if expected[i] != got[i] {
+				t.Errorf("got %q expected %q", got[i], expected[i])
+			}
+		}
+	}
+
+	checkExcludedHeaders := func(t *testing.T, header textproto.MIMEHeader) {
+		t.Helper()
+
+		for _, h := range []string{"Content-Length", "Transfer-Encoding"} {
+			if v, ok := header[h]; ok {
+				t.Errorf("%s is %q; must not be sent", h, v)
+			}
+		}
+	}
+
+	var respCounter uint8
+	trace := &httptrace.ClientTrace{
+		Got1xxResponse: func(code int, header textproto.MIMEHeader) error {
+			switch respCounter {
+			case 0:
+				checkLinkHeaders(t, []string{"</style.css>; rel=preload; as=style", "</script.js>; rel=preload; as=script"}, header["Link"])
+				checkExcludedHeaders(t, header)
+
+				wg.Done()
+			case 1:
+				checkLinkHeaders(t, []string{"</style.css>; rel=preload; as=style", "</script.js>; rel=preload; as=script", "</foo.js>; rel=preload; as=script"}, header["Link"])
+				checkExcludedHeaders(t, header)
+
+			default:
+				t.Error("Unexpected 1xx response")
+			}
+
+			respCounter++
+
+			return nil
+		},
+	}
+	req, _ := NewRequestWithContext(httptrace.WithClientTrace(context.Background(), trace), "GET", cst.ts.URL, nil)
+
+	res, err := cst.c.Do(req)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer res.Body.Close()
+
+	checkLinkHeaders(t, []string{"</style.css>; rel=preload; as=style", "</script.js>; rel=preload; as=script", "</foo.js>; rel=preload; as=script"}, res.Header["Link"])
+	if cl := res.Header.Get("Content-Length"); cl != "123" {
+		t.Errorf("Content-Length is %q; want 123", cl)
+	}
+
+	body, _ := io.ReadAll(res.Body)
+	if string(body) != "Hello" {
+		t.Errorf("Read body %q; want Hello", body)
+	}
+}

src/net/http/serve_test.go

@@ -3873,7 +3873,7 @@ func testServerReaderFromOrder(t *testing.T, h2 bool) {
 
 // Issue 6157, Issue 6685
 func TestCodesPreventingContentTypeAndBody(t *testing.T) {
-	for _, code := range []int{StatusNotModified, StatusNoContent, StatusContinue} {
+	for _, code := range []int{StatusNotModified, StatusNoContent} {
 		ht := newHandlerTest(HandlerFunc(func(w ResponseWriter, r *Request) {
 			if r.URL.Path == "/header" {
 				w.Header().Set("Content-Length", "123")
@@ -6725,3 +6725,35 @@ func testMaxBytesHandler(t *testing.T, maxSize, requestSize int64) {
 		t.Errorf("expected echo of size %d; got %d", handlerN, buf.Len())
 	}
 }
+
+func TestEarlyHints(t *testing.T) {
+	ht := newHandlerTest(HandlerFunc(func(w ResponseWriter, r *Request) {
+		h := w.Header()
+		h.Add("Link", "</style.css>; rel=preload; as=style")
+		h.Add("Link", "</script.js>; rel=preload; as=script")
+		w.WriteHeader(StatusEarlyHints)
+
+		h.Add("Link", "</foo.js>; rel=preload; as=script")
+		w.WriteHeader(StatusEarlyHints)
+
+		w.Write([]byte("stuff"))
+	}))
+
+	got := ht.rawResponse("GET / HTTP/1.1\nHost: golang.org")
+	expected := "HTTP/1.1 103 Early Hints\r\nLink: </style.css>; rel=preload; as=style\r\nLink: </script.js>; rel=preload; as=script\r\n\r\nHTTP/1.1 103 Early Hints\r\nLink: </style.css>; rel=preload; as=style\r\nLink: </script.js>; rel=preload; as=script\r\nLink: </foo.js>; rel=preload; as=script\r\n\r\nHTTP/1.1 200 OK\r\nLink: </style.css>; rel=preload; as=style\r\nLink: </script.js>; rel=preload; as=script\r\nLink: </foo.js>; rel=preload; as=script\r\nDate: " // dynamic content expected
+	if !strings.Contains(got, expected) {
+		t.Errorf("unexpected response; got %q; should start by %q", got, expected)
+	}
+}
+func TestProcessing(t *testing.T) {
+	ht := newHandlerTest(HandlerFunc(func(w ResponseWriter, r *Request) {
+		w.WriteHeader(StatusProcessing)
+		w.Write([]byte("stuff"))
+	}))
+
+	got := ht.rawResponse("GET / HTTP/1.1\nHost: golang.org")
+	expected := "HTTP/1.1 102 Processing\r\n\r\nHTTP/1.1 200 OK\r\nDate: " // dynamic content expected
+	if !strings.Contains(got, expected) {
+		t.Errorf("unexpected response; got %q; should start by %q", got, expected)
+	}
+}

src/net/http/server.go

@@ -98,8 +98,8 @@ type ResponseWriter interface {
 	// Handlers can set HTTP trailers.
 	//
 	// Changing the header map after a call to WriteHeader (or
-	// Write) has no effect unless the modified headers are
-	// trailers.
+	// Write) has no effect unless the HTTP status code was of the
+	// 1xx class or the modified headers are trailers.
 	//
 	// There are two ways to set Trailers. The preferred way is to
 	// predeclare in the headers which trailers you will later
@@ -144,13 +144,18 @@ type ResponseWriter interface {
 	// If WriteHeader is not called explicitly, the first call to Write
 	// will trigger an implicit WriteHeader(http.StatusOK).
 	// Thus explicit calls to WriteHeader are mainly used to
-	// send error codes.
+	// send error codes or 1xx informational responses.
 	//
 	// The provided code must be a valid HTTP 1xx-5xx status code.
-	// Only one header may be written. Go does not currently
-	// support sending user-defined 1xx informational headers,
-	// with the exception of 100-continue response header that the
-	// Server sends automatically when the Request.Body is read.
+	// Any number of 1xx headers may be written, followed by at most
+	// one 2xx-5xx header. 1xx headers are sent immediately, but 2xx-5xx
+	// headers may be buffered. Use the Flusher interface to send
+	// buffered data. The header map is cleared when 2xx-5xx headers are
+	// sent, but not with 1xx headers.
+	//
+	// The server will automatically send a 100 (Continue) header
+	// on the first read from the request body if the request has
+	// an "Expect: 100-continue" header.
 	WriteHeader(statusCode int)
 }
 
@@ -420,7 +425,7 @@ type response struct {
 	req              *Request // request for this response
 	reqBody          io.ReadCloser
 	cancelCtx        context.CancelFunc // when ServeHTTP exits
-	wroteHeader      bool               // reply header has been (logically) written
+	wroteHeader      bool               // a non-1xx header has been (logically) written
 	wroteContinue    bool               // 100 Continue response was written
 	wants10KeepAlive bool               // HTTP/1.0 w/ Connection "keep-alive"
 	wantsClose       bool               // HTTP request has Connection "close"
@@ -1100,8 +1105,7 @@ func checkWriteHeaderCode(code int) {
 	// Issue 22880: require valid WriteHeader status codes.
 	// For now we only enforce that it's three digits.
 	// In the future we might block things over 599 (600 and above aren't defined
-	// at https://httpwg.org/specs/rfc7231.html#status.codes)
-	// and we might block under 200 (once we have more mature 1xx support).
+	// at https://httpwg.org/specs/rfc7231.html#status.codes).
 	// But for now any three digits.
 	//
 	// We used to send "HTTP/1.1 000 0" on the wire in responses but there's
@@ -1144,6 +1148,26 @@ func (w *response) WriteHeader(code int) {
 		return
 	}
 	checkWriteHeaderCode(code)
+
+	// Handle informational headers
+	if code >= 100 && code <= 199 {
+		// Prevent a potential race with an automatically-sent 100 Continue triggered by Request.Body.Read()
+		if code == 100 && w.canWriteContinue.isSet() {
+			w.writeContinueMu.Lock()
+			w.canWriteContinue.setFalse()
+			w.writeContinueMu.Unlock()
+		}
+
+		writeStatusLine(w.conn.bufw, w.req.ProtoAtLeast(1, 1), code, w.statusBuf[:])
+
+		// Per RFC 8297 we must not clear the current header map
+		w.handlerHeader.WriteSubset(w.conn.bufw, excludedHeadersNoBody)
+		w.conn.bufw.Write(crlf)
+		w.conn.bufw.Flush()
+
+		return
+	}
+
 	w.wroteHeader = true
 	w.status = code
 

src/net/http/transfer.go

@@ -468,6 +468,7 @@ func bodyAllowedForStatus(status int) bool {
 var (
 	suppressedHeaders304    = []string{"Content-Type", "Content-Length", "Transfer-Encoding"}
 	suppressedHeadersNoBody = []string{"Content-Length", "Transfer-Encoding"}
+	excludedHeadersNoBody   = map[string]bool{"Content-Length": true, "Transfer-Encoding": true}
 )
 
 func suppressedHeaders(status int) []string {