implement early hints (#1996)

pjebs · web-flow · commit a05560dd7e07 · 2025-04-21T13:01:35.000+02:00
diff --git a/server.go b/server.go
@@ -623,6 +623,72 @@ type RequestCtx struct {
 	hijackNoResponse bool
 }
 
+// EarlyHints allows the server to hint to the browser what resources a page would need
+// so the browser can preload them while waiting for the server's full response. Only Link
+// headers already written to the response will be transmitted as Early Hints.
+//
+// This is a HTTP/2+ feature but all browsers will either understand it or safely ignore it.
+//
+// NOTE: Older HTTP/1.1 non-browser clients may face compatibility issues.
+//
+// See: https://developer.chrome.com/docs/web-platform/early-hints and
+// https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Link#syntax
+//
+// Example:
+//
+//	func(ctx *fasthttp.RequestCtx) {
+//	   ctx.Response.Header.Add("Link", "<https://fonts.google.com>; rel=preconnect")
+//	   ctx.EarlyHints()
+//	   time.Sleep(5*time.Second) // some time-consuming task
+//	   ctx.SetStatusCode(fasthttp.StatusOK)
+//	   ctx.SetBody([]byte("<html><head></head><body><h1>Hello from Fasthttp</h1></body></html>"))
+//	}
+func (ctx *RequestCtx) EarlyHints() error {
+	links := ctx.Response.Header.PeekAll(b2s(strLink))
+	if len(links) > 0 {
+		c := acquireWriter(ctx)
+		defer releaseWriter(ctx.s, c)
+		_, err := c.Write(strEarlyHints)
+		if err != nil {
+			return err
+		}
+		for _, l := range links {
+			if len(l) == 0 {
+				continue
+			}
+			_, err = c.Write(strLink)
+			if err != nil {
+				return err
+			}
+			_, err = c.Write(strColon)
+			if err != nil {
+				return err
+			}
+			_, err = c.Write(strSpace)
+			if err != nil {
+				return err
+			}
+			_, err = c.Write(l)
+			if err != nil {
+				return err
+			}
+			_, err = c.Write(strCRLF)
+			if err != nil {
+				return err
+			}
+		}
+		_, err = c.Write(strCRLF)
+		if err != nil {
+			return err
+		}
+		err = c.Flush()
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
 // HijackHandler must process the hijacked connection c.
 //
 // If KeepHijackedConns is disabled, which is by default,
diff --git a/server_test.go b/server_test.go
@@ -1919,6 +1919,49 @@ func TestServerExpect100Continue(t *testing.T) {
 	}
 }
 
+func TestServerExpect103EarlyHints(t *testing.T) {
+	t.Parallel()
+
+	s := &Server{
+		NoDefaultContentType:  true,
+		NoDefaultDate:         true,
+		NoDefaultServerHeader: true,
+		Handler: func(ctx *RequestCtx) {
+			ctx.Response.Header.Add("Link", "<https://cdn.com>; rel=preload; as=script")
+			ctx.EarlyHints() //nolint:errcheck
+		},
+	}
+
+	rw := &readWriter{}
+	rw.r.WriteString("GET /foo HTTP/1.1\r\nContent-Length: 5\r\nContent-Type: a/b\r\n\r\n12345")
+
+	if err := s.ServeConn(rw); err != nil {
+		t.Fatalf("Unexpected error from serveConn: %v", err)
+	}
+
+	scanner := bufio.NewScanner(&rw.w)
+	expected := []string{
+		"HTTP/1.1 103 Early Hints",
+		"Link: <https://cdn.com>; rel=preload; as=script",
+		"",
+		"HTTP/1.1 200 OK",
+		"Content-Length: 0",
+		"Link: <https://cdn.com>; rel=preload; as=script",
+	}
+
+	i := 0
+	for scanner.Scan() {
+		if i >= len(expected) {
+			break
+		}
+		line := scanner.Text()
+		if line != expected[i] {
+			t.Fatalf("unexpected data: %s. Expecting %s", line, expected[i])
+		}
+		i++
+	}
+}
+
 func TestServerContinueHandler(t *testing.T) {
 	t.Parallel()
 
diff --git a/strings.go b/strings.go
@@ -25,8 +25,10 @@ var (
 	strColonSpace               = []byte(": ")
 	strCommaSpace               = []byte(", ")
 	strGMT                      = []byte("GMT")
+	strSpace                    = []byte(" ")
 
 	strResponseContinue = []byte("HTTP/1.1 100 Continue\r\n\r\n")
+	strEarlyHints       = []byte("HTTP/1.1 103 Early Hints\r\n")
 
 	strExpect             = []byte(HeaderExpect)
 	strConnection         = []byte(HeaderConnection)
@@ -86,6 +88,7 @@ var (
 	strBoundary            = []byte("boundary")
 	strBytes               = []byte("bytes")
 	strBasicSpace          = []byte("Basic ")
+	strLink                = []byte("Link")
 
 	strApplicationSlash = []byte("application/")
 	strImageSVG         = []byte("image/svg")
