net/http: buffer before chunking

This introduces a buffer between writing from a handler and
writing chunks.  Further, it delays writing the header until
the first full chunk is ready.  In the case where the first
full chunk is also the final chunk (for small responses), that
means we can also compute a Content-Length, which is a nice
side effect for certain benchmarks.

Fixes #2357

R=golang-dev, dave, minux.ma, rsc, adg, balasanjay
CC=golang-dev
https://golang.org/cl/6964043

src/pkg/net/http/header.go

@@ -54,6 +54,16 @@ func (h Header) Write(w io.Writer) error {
 	return h.WriteSubset(w, nil)
 }
 
+func (h Header) clone() Header {
+	h2 := make(Header, len(h))
+	for k, vv := range h {
+		vv2 := make([]string, len(vv))
+		copy(vv2, vv)
+		h2[k] = vv2
+	}
+	return h2
+}
+
 var timeFormats = []string{
 	TimeFormat,
 	time.RFC850,

src/pkg/net/http/serve_test.go

@@ -484,6 +484,7 @@ func TestChunkedResponseHeaders(t *testing.T) {
 
 	ts := httptest.NewServer(HandlerFunc(func(w ResponseWriter, r *Request) {
 		w.Header().Set("Content-Length", "intentional gibberish") // we check that this is deleted
+		w.(Flusher).Flush()
 		fmt.Fprintf(w, "I am a chunked response.")
 	}))
 	defer ts.Close()
@@ -764,6 +765,7 @@ func TestServerUnreadRequestBodyLittle(t *testing.T) {
 			t.Errorf("on request, read buffer length is %d; expected about 100 KB", conn.readBuf.Len())
 		}
 		rw.WriteHeader(200)
+		rw.(Flusher).Flush()
 		if g, e := conn.readBuf.Len(), 0; g != e {
 			t.Errorf("after WriteHeader, read buffer length is %d; want %d", g, e)
 		}
@@ -796,14 +798,16 @@ func TestServerUnreadRequestBodyLarge(t *testing.T) {
 			t.Errorf("on request, read buffer length is %d; expected about 1MB", conn.readBuf.Len())
 		}
 		rw.WriteHeader(200)
+		rw.(Flusher).Flush()
 		if conn.readBuf.Len() < len(body)/2 {
 			t.Errorf("post-WriteHeader, read buffer length is %d; expected about 1MB", conn.readBuf.Len())
 		}
-		if c := rw.Header().Get("Connection"); c != "close" {
-			t.Errorf(`Connection header = %q; want "close"`, c)
-		}
 	}))
 	<-done
+
+	if res := conn.writeBuf.String(); !strings.Contains(res, "Connection: close") {
+		t.Errorf("Expected a Connection: close header; got response: %s", res)
+	}
 }
 
 func TestTimeoutHandler(t *testing.T) {
@@ -1144,17 +1148,13 @@ func TestClientWriteShutdown(t *testing.T) {
 // Tests that chunked server responses that write 1 byte at a time are
 // buffered before chunk headers are added, not after chunk headers.
 func TestServerBufferedChunking(t *testing.T) {
-	if true {
-		t.Logf("Skipping known broken test; see Issue 2357")
-		return
-	}
 	conn := new(testConn)
 	conn.readBuf.Write([]byte("GET / HTTP/1.1\r\n\r\n"))
 	done := make(chan bool)
 	ls := &oneConnListener{conn}
 	go Serve(ls, HandlerFunc(func(rw ResponseWriter, req *Request) {
 		defer close(done)
-		rw.Header().Set("Content-Type", "text/plain") // prevent sniffing, which buffers
+		rw.(Flusher).Flush() // force the Header to be sent, in chunking mode, not counting the length
 		rw.Write([]byte{'x'})
 		rw.Write([]byte{'y'})
 		rw.Write([]byte{'z'})