fix deadlock when concurrently closing server and transport (#4332)

* server: fix deadlock when closing concurrently with transport

* add test for checking no deadlock

server.go

@@ -98,7 +98,7 @@ type baseServer struct {
 		protocol.Version,
 	) quicConn
 
-	closeOnce sync.Once
+	closeMx   sync.Mutex
 	errorChan chan struct{} // is closed when the server is closed
 	closeErr  error
 	running   chan struct{} // closed as soon as run() returns
@@ -338,15 +338,19 @@ func (s *baseServer) Close() error {
 }
 
 func (s *baseServer) close(e error, notifyOnClose bool) {
-	s.closeOnce.Do(func() {
+	s.closeMx.Lock()
+	if s.closeErr != nil {
+		s.closeMx.Unlock()
+		return
+	}
 	s.closeErr = e
 	close(s.errorChan)
-
 	<-s.running
+	s.closeMx.Unlock()
+
 	if notifyOnClose {
 		s.onClose()
 	}
-	})
 }
 
 // Addr returns the server's network address

transport_test.go

@@ -122,6 +122,27 @@ var _ = Describe("Transport", func() {
 		tr.Close()
 	})
 
+	It("closes transport concurrently with listener", func() {
+		// try 10 times to trigger race conditions
+		for i := 0; i < 10; i++ {
+			packetChan := make(chan packetToRead)
+			tr := &Transport{Conn: newMockPacketConn(packetChan)}
+			ln, err := tr.Listen(&tls.Config{}, nil)
+			Expect(err).ToNot(HaveOccurred())
+			ch := make(chan bool)
+			// Close transport and listener concurrently.
+			go func() {
+				ch <- true
+				Expect(ln.Close()).To(Succeed())
+				ch <- true
+			}()
+			<-ch
+			close(packetChan)
+			Expect(tr.Close()).To(Succeed())
+			<-ch
+		}
+	})
+
 	It("drops unparseable QUIC packets", func() {
 		addr := &net.UDPAddr{IP: net.IPv4(9, 8, 7, 6), Port: 1234}
 		packetChan := make(chan packetToRead)