internal/http3: add Server support for handling HEAD requests

When handling HEAD requests, Server will now always send an HTTP
response with an empty body, even if the HTTP handler would normally
write to the body.

For golang/go#70914

Change-Id: Id656a8f9901b97357a343e204761f6a47f87f9fe
Reviewed-on: https://go-review.googlesource.com/c/net/+/740160
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Nicholas Husin <husin@google.com>

internal/http3/server.go

@@ -207,11 +207,21 @@ func (sc *serverConn) handleRequestStream(st *stream) error {
 	}
 	defer req.Body.Close()
 
-	responseWriter := sc.newResponseWriter(st)
-	defer responseWriter.close()
+	rw := &responseWriter{
+		st:         st,
+		headers:    make(http.Header),
+		isHeadResp: req.Method == "HEAD",
+		bw: &bodyWriter{
+			st:     st,
+			remain: -1,
+			flush:  false,
+			name:   "response",
+		},
+	}
+	defer rw.close()
 
 	// TODO: handle panic coming from the HTTP handler.
-	sc.handler.ServeHTTP(responseWriter, req)
+	sc.handler.ServeHTTP(rw, req)
 	return nil
 }
 
@@ -234,20 +244,7 @@ type responseWriter struct {
 	headers http.Header
 	// TODO: support 1xx status
 	wroteHeader bool // Non-1xx header has been (logically) written.
-}
-
-func (sc *serverConn) newResponseWriter(st *stream) *responseWriter {
-	rw := &responseWriter{
-		st:      st,
-		headers: make(http.Header),
-		bw: &bodyWriter{
-			st:     st,
-			remain: -1,
-			flush:  false,
-			name:   "response",
-		},
-	}
-	return rw
+	isHeadResp  bool // response is for a HEAD request.
 }
 
 func (rw *responseWriter) Header() http.Header {
@@ -295,6 +292,9 @@ func (rw *responseWriter) Write(b []byte) (int, error) {
 	if !rw.wroteHeader {
 		rw.writeHeaderLocked(http.StatusOK)
 	}
+	if rw.isHeadResp {
+		return 0, nil
+	}
 	return rw.bw.Write(b)
 }
 

internal/http3/server_test.go

@@ -150,6 +150,30 @@ func TestServerBody(t *testing.T) {
 	})
 }
 
+func TestServerHeadResponseNoBody(t *testing.T) {
+	bodyContent := []byte("response body that will not be sent for HEAD requests")
+	synctest.Test(t, func(t *testing.T) {
+		ts := newTestServer(t, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			w.Write(bodyContent)
+		}))
+		tc := ts.connect()
+		tc.greet()
+
+		reqStream := tc.newStream(streamTypeRequest)
+		reqStream.writeHeaders(http.Header{":method": {http.MethodGet}})
+		synctest.Wait()
+		reqStream.wantHeaders(http.Header{":status": {"200"}})
+		reqStream.wantData(bodyContent)
+		reqStream.wantClosed("request is complete")
+
+		reqStream = tc.newStream(streamTypeRequest)
+		reqStream.writeHeaders(http.Header{":method": {http.MethodHead}})
+		synctest.Wait()
+		reqStream.wantHeaders(http.Header{":status": {"200"}})
+		reqStream.wantClosed("request is complete")
+	})
+}
+
 type testServer struct {
 	t  testing.TB
 	s  *Server