Close the connections buffered for the listeners

This commit closes the connections that has been buffered
on the connection channel of the cmux listeners, when the
root listener is closed. With this change it is guaranteed
that the connections are either closed or handed of to the
child listeners (returned via Accept()).

There are a couple of changes to the tests to cover corner
cases and the new behavior.

cmux.go

@@ -98,6 +98,10 @@ func (m *cMux) Serve() error {
 
 		for _, sl := range m.sls {
 			close(sl.l.connc)
+			// Drain the connections enqueued for the listener.
+			for c := range sl.l.connc {
+				_ = c.Close()
+			}
 		}
 	}()
 
@@ -126,12 +130,8 @@ func (m *cMux) serve(c net.Conn, donec <-chan struct{}, wg *sync.WaitGroup) {
 			if matched {
 				select {
 				case sl.l.connc <- muc:
-				default:
-					select {
-					case <-donec:
-						_ = c.Close()
-					default:
-					}
+				case <-donec:
+					_ = c.Close()
 				}
 				return
 			}

cmux_test.go

@@ -262,17 +262,17 @@ func TestHTTP2(t *testing.T) {
 	})
 	h2l := muxl.Match(HTTP2())
 	go safeServe(errCh, muxl)
+	muxedConn, err := h2l.Accept()
 	close(l.connCh)
-	if muxedConn, err := h2l.Accept(); err != nil {
+	if err != nil {
 		t.Fatal(err)
-	} else {
-		var b [len(http2.ClientPreface)]byte
-		if _, err := muxedConn.Read(b[:]); err != io.EOF {
-			t.Fatal(err)
-		}
-		if string(b[:]) != http2.ClientPreface {
-			t.Errorf("got unexpected read %s, expected %s", b, http2.ClientPreface)
-		}
+	}
+	var b [len(http2.ClientPreface)]byte
+	if _, err := muxedConn.Read(b[:]); err != io.EOF {
+		t.Fatal(err)
+	}
+	if string(b[:]) != http2.ClientPreface {
+		t.Errorf("got unexpected read %s, expected %s", b, http2.ClientPreface)
 	}
 }
 
@@ -374,9 +374,14 @@ func TestClose(t *testing.T) {
 	// Listener is closed.
 	close(l.connCh)
 
-	// Second connection goes through.
+	// Second connection either goes through or it is closed.
 	if _, err := anyl.Accept(); err != nil {
-		t.Fatal(err)
+		if err != ErrListenerClosed {
+			t.Fatal(err)
+		}
+		if _, err := c2.Read([]byte{}); err != io.ErrClosedPipe {
+			t.Fatalf("connection is not closed and is leaked: %v", err)
+		}
 	}
 }