Do not write SETTINGS in response to ACKs.

Reported-by talgendler in Issue #42

cmux.go

@@ -116,8 +116,9 @@ type cMux struct {
 func matchersToMatchWriters(matchers []Matcher) []MatchWriter {
 	mws := make([]MatchWriter, 0, len(matchers))
 	for _, m := range matchers {
+		cm := m
 		mws = append(mws, func(w io.Writer, r io.Reader) bool {
-			return m(r)
+			return cm(r)
 		})
 	}
 	return mws

cmux_test.go

@@ -622,6 +622,35 @@ func TestErrorHandler(t *testing.T) {
 	}
 }
 
+func TestMultipleMatchers(t *testing.T) {
+	defer leakCheck(t)()
+	errCh := make(chan error)
+	defer func() {
+		select {
+		case err := <-errCh:
+			t.Fatal(err)
+		default:
+		}
+	}()
+	l, cleanup := testListener(t)
+	defer cleanup()
+
+	matcher := func(r io.Reader) bool {
+		return true
+	}
+	unmatcher := func(r io.Reader) bool {
+		return false
+	}
+
+	muxl := New(l)
+	lis := muxl.Match(unmatcher, matcher, unmatcher)
+
+	go runTestHTTPServer(errCh, lis)
+	go safeServe(errCh, muxl)
+
+	runTestHTTP1Client(t, l.Addr())
+}
+
 func TestClose(t *testing.T) {
 	defer leakCheck(t)()
 	errCh := make(chan error)

matchers.go

@@ -240,6 +240,11 @@ func matchHTTP2Field(w io.Writer, r io.Reader, name string, matches func(string)
 
 		switch f := f.(type) {
 		case *http2.SettingsFrame:
+			// Sender acknoweldged the SETTINGS frame. No need to write
+			// SETTINGS again.
+			if f.IsAck() {
+				break
+			}
 			if err := framer.WriteSettings(); err != nil {
 				return false
 			}