Flush buffered responses if we have to wait for the next request

Don't wait for the next request as this can take some time, instead
flush the outstanding responses already.

Fixes #1043

header.go

@@ -1494,13 +1494,20 @@ func headerErrorMsg(typ string, err error, b []byte, secureErrorLogMessage bool)
 //
 // io.EOF is returned if r is closed before reading the first header byte.
 func (h *RequestHeader) Read(r *bufio.Reader) error {
+	return h.readLoop(r, true)
+}
+
+// readLoop reads request header from r optionally loops until it has enough data.
+//
+// io.EOF is returned if r is closed before reading the first header byte.
+func (h *RequestHeader) readLoop(r *bufio.Reader, waitForMore bool) error {
 	n := 1
 	for {
 		err := h.tryRead(r, n)
 		if err == nil {
 			return nil
 		}
-		if err != errNeedMore {
+		if !waitForMore || err != errNeedMore {
 			h.resetSkipNormalize()
 			return err
 		}

server.go

@@ -2091,8 +2091,28 @@ func (s *Server) serveConn(c net.Conn) (err error) {
 				ctx.Request.Header.DisableNormalizing()
 				ctx.Response.Header.DisableNormalizing()
 			}
-			// reading Headers
-			if err = ctx.Request.Header.Read(br); err == nil {
+
+			// Reading Headers.
+			//
+			// If we have pipline response in the outgoing buffer,
+			// we only want to try and read the next headers once.
+			// If we have to wait for the next request we flush the
+			// outgoing buffer first so it doesn't have to wait.
+			if bw != nil && bw.Buffered() > 0 {
+				err = ctx.Request.Header.readLoop(br, false)
+				if err == errNeedMore {
+					err = bw.Flush()
+					if err != nil {
+						break
+					}
+
+					err = ctx.Request.Header.Read(br)
+				}
+			} else {
+				err = ctx.Request.Header.Read(br)
+			}
+
+			if err == nil {
 				if onHdrRecv := s.HeaderReceived; onHdrRecv != nil {
 					reqConf := onHdrRecv(&ctx.Request.Header)
 					if reqConf.ReadTimeout > 0 {

server_test.go

@@ -23,6 +23,67 @@ import (
 // Make sure RequestCtx implements context.Context
 var _ context.Context = &RequestCtx{}
 
+func TestServerPipelineFlush(t *testing.T) {
+	t.Parallel()
+
+	s := &Server{
+		Handler: func(ctx *RequestCtx) {
+		},
+	}
+	ln := fasthttputil.NewInmemoryListener()
+
+	go func() {
+		if err := s.Serve(ln); err != nil {
+			t.Errorf("unexpected error: %s", err)
+		}
+	}()
+
+	c, err := ln.Dial()
+	if err != nil {
+		t.Fatalf("unexpected error: %s", err)
+	}
+	if _, err = c.Write([]byte("GET /foo1 HTTP/1.1\r\nHost: google.com\r\n\r\n")); err != nil {
+		t.Fatal(err)
+	}
+
+	// Write a partial request.
+	if _, err = c.Write([]byte("GET /foo1 HTTP/1.1\r\nHost: ")); err != nil {
+		t.Fatal(err)
+	}
+	go func() {
+		// Wait for 100ms to finish the request
+		time.Sleep(time.Millisecond * 100)
+
+		if _, err = c.Write([]byte("google.com\r\n\r\n")); err != nil {
+			t.Error(err)
+		}
+	}()
+
+	start := time.Now()
+	br := bufio.NewReader(c)
+	var resp Response
+
+	if err := resp.Read(br); err != nil {
+		t.Fatalf("unexpected error: %s", err)
+	}
+	if resp.StatusCode() != StatusOK {
+		t.Fatalf("unexpected status code: %d. Expecting %d", resp.StatusCode(), StatusOK)
+	}
+
+	// Since the second request takes 100ms to finish we expect the first one to be flushed earlier.
+	d := time.Since(start)
+	if d > time.Millisecond*10 {
+		t.Fatalf("had to wait for %v", d)
+	}
+
+	if err := resp.Read(br); err != nil {
+		t.Fatalf("unexpected error: %s", err)
+	}
+	if resp.StatusCode() != StatusOK {
+		t.Fatalf("unexpected status code: %d. Expecting %d", resp.StatusCode(), StatusOK)
+	}
+}
+
 func TestServerInvalidHeader(t *testing.T) {
 	t.Parallel()