tcp: handle listen after shutdown properly

Right now, sentry panics in this case:
panic: close of nil channel

goroutine 67 [running]:
pkg/tcpip/transport/tcp/tcp.(*endpoint).listen(0xc0000ce000, 0x9, 0x0)
        pkg/tcpip/transport/tcp/endpoint.go:2208 +0x170
pkg/tcpip/transport/tcp/tcp.(*endpoint).Listen(0xc0000ce000, 0x9, 0xc0003a1ad0)
        pkg/tcpip/transport/tcp/endpoint.go:2179 +0x50

Fixes #2468

PiperOrigin-RevId: 307896725

pkg/tcpip/transport/tcp/endpoint.go

@@ -2158,8 +2158,6 @@ func (e *endpoint) shutdownLocked(flags tcpip.ShutdownFlags) *tcpip.Error {
 			//
 			// By not removing this endpoint from the demuxer mapping, we
 			// ensure that any other bind to the same port fails, as on Linux.
-			// TODO(gvisor.dev/issue/2468): We need to enable applications to
-			// start listening on this endpoint again similar to Linux.
 			e.rcvListMu.Lock()
 			e.rcvClosed = true
 			e.rcvListMu.Unlock()
@@ -2188,26 +2186,31 @@ func (e *endpoint) listen(backlog int) *tcpip.Error {
 	e.LockUser()
 	defer e.UnlockUser()
 
-	// Allow the backlog to be adjusted if the endpoint is not shutting down.
-	// When the endpoint shuts down, it sets workerCleanup to true, and from
-	// that point onward, acceptedChan is the responsibility of the cleanup()
-	// method (and should not be touched anywhere else, including here).
-	if e.EndpointState() == StateListen && !e.workerCleanup {
-		// Adjust the size of the channel iff we can fix existing
-		// pending connections into the new one.
+	if e.EndpointState() == StateListen && !e.closed {
 		e.acceptMu.Lock()
 		defer e.acceptMu.Unlock()
-		if len(e.acceptedChan) > backlog {
-			return tcpip.ErrInvalidEndpointState
-		}
-		if cap(e.acceptedChan) == backlog {
-			return nil
-		}
-		origChan := e.acceptedChan
-		e.acceptedChan = make(chan *endpoint, backlog)
-		close(origChan)
-		for ep := range origChan {
-			e.acceptedChan <- ep
+		if e.acceptedChan == nil {
+			// listen is called after shutdown.
+			e.acceptedChan = make(chan *endpoint, backlog)
+			e.shutdownFlags = 0
+			e.rcvListMu.Lock()
+			e.rcvClosed = false
+			e.rcvListMu.Unlock()
+		} else {
+			// Adjust the size of the channel iff we can fix
+			// existing pending connections into the new one.
+			if len(e.acceptedChan) > backlog {
+				return tcpip.ErrInvalidEndpointState
+			}
+			if cap(e.acceptedChan) == backlog {
+				return nil
+			}
+			origChan := e.acceptedChan
+			e.acceptedChan = make(chan *endpoint, backlog)
+			close(origChan)
+			for ep := range origChan {
+				e.acceptedChan <- ep
+			}
 		}
 
 		// Notify any blocked goroutines that they can attempt to

pkg/tcpip/transport/tcp/endpoint_state.go

@@ -247,6 +247,11 @@ func (e *endpoint) Resume(s *stack.Stack) {
 			if err := e.Listen(backlog); err != nil {
 				panic("endpoint listening failed: " + err.String())
 			}
+			e.LockUser()
+			if e.shutdownFlags != 0 {
+				e.shutdownLocked(e.shutdownFlags)
+			}
+			e.UnlockUser()
 			listenLoading.Done()
 			tcpip.AsyncLoading.Done()
 		}()

test/syscalls/linux/socket_inet_loopback.cc

@@ -319,6 +319,49 @@ TEST_P(SocketInetLoopbackTest, TCPListenUnbound) {
   tcpSimpleConnectTest(listener, connector, false);
 }
 
+TEST_P(SocketInetLoopbackTest, TCPListenShutdownListen) {
+  const auto& param = GetParam();
+
+  const TestAddress& listener = param.listener;
+  const TestAddress& connector = param.connector;
+
+  constexpr int kBacklog = 5;
+
+  // Create the listening socket.
+  FileDescriptor listen_fd = ASSERT_NO_ERRNO_AND_VALUE(
+      Socket(listener.family(), SOCK_STREAM, IPPROTO_TCP));
+  sockaddr_storage listen_addr = listener.addr;
+  ASSERT_THAT(bind(listen_fd.get(), reinterpret_cast<sockaddr*>(&listen_addr),
+                   listener.addr_len),
+              SyscallSucceeds());
+
+  ASSERT_THAT(listen(listen_fd.get(), kBacklog), SyscallSucceeds());
+  ASSERT_THAT(shutdown(listen_fd.get(), SHUT_RD), SyscallSucceeds());
+  ASSERT_THAT(listen(listen_fd.get(), kBacklog), SyscallSucceeds());
+
+  // Get the port bound by the listening socket.
+  socklen_t addrlen = listener.addr_len;
+  ASSERT_THAT(getsockname(listen_fd.get(),
+                          reinterpret_cast<sockaddr*>(&listen_addr), &addrlen),
+              SyscallSucceeds());
+  const uint16_t port =
+      ASSERT_NO_ERRNO_AND_VALUE(AddrPort(listener.family(), listen_addr));
+
+  sockaddr_storage conn_addr = connector.addr;
+  ASSERT_NO_ERRNO(SetAddrPort(connector.family(), &conn_addr, port));
+
+  for (int i = 0; i < kBacklog; i++) {
+    auto client = ASSERT_NO_ERRNO_AND_VALUE(
+        Socket(connector.family(), SOCK_STREAM, IPPROTO_TCP));
+    ASSERT_THAT(connect(client.get(), reinterpret_cast<sockaddr*>(&conn_addr),
+                        connector.addr_len),
+                SyscallSucceeds());
+  }
+  for (int i = 0; i < kBacklog; i++) {
+    ASSERT_THAT(accept(listen_fd.get(), nullptr, nullptr), SyscallSucceeds());
+  }
+}
+
 TEST_P(SocketInetLoopbackTest, TCPListenShutdown) {
   auto const& param = GetParam();