Using a selector to detect closed socket channel.

Question

Basically our app uses one thread to perform a select on multiple socket channels. When a socket is closed (either remotely or locally) the server app needs to unregister the client (so we need to detect closed channels). We've been using blocking IO previously (no dramas), but now we're dealing with many connections/clients we need to rework it as we're getting excessive amounts of threads. Anyway, that's just background.
Here's the problem we're getting, I have boiled it down to the following.
SocketChannel channel = SocketChannel.open();
channel.connect(socketAddress); // for the test this is localhost.
// this make sure socket is open, remote end just echos, waits 3 seconds and closes the socket.
channel.write(ByteBuffer.wrap("testLocalSocketClose\n".getBytes()));
channel.configureBlocking(false);
Selector selector = Selector.open();
LOG.fine("registering channel");
channel.register(selector, SelectionKey.OP_READ, channel);
LOG.fine("closing channel");
channel.close();
LOG.fine("waiting...");
int i = selector.select();
// this never happens (or it does very rarely)
LOG.fine("selector woke with " + i + " selected");I would have expected the selector to return the selection key of the dead channel. Given that it doesn't and this scenario is possible (channel closing just after a read operation but before another select is called - in separate threads obviously). How can we reliably detect/be informed that the channel has been closed?
(I saw somewhere someone mention adding the OP_WRITE to the key, I have tried this as well and it makes no difference).
Many Thanks.
Bob.

Answer 1

May I suggest you look at your application and reassess; either it's wrong or it's your understanding of what our issue is.
Please try the simple test below.
WSADATA ws;
WSAStartup(0x0101, &ws);
SOCKET sock = socket(PF_INET, SOCK_STREAM, 0);
struct sockaddr_in server;
server.sin_family = AF_INET;
server.sin_addr.s_addr = htonl(INADDR_ANY);
server.sin_port = htons(9000);
cout << "binding" << endl;
bind(sock, (struct sockaddr *)&server, sizeof(server));
cout << "listening" << endl;
listen(sock, SOMAXCONN);
struct sockaddr_in client;
int client_size = sizeof(client);
memset(&client, 0, client_size);
cout << "accepting" << endl;
SOCKET clientSock = accept(sock, (struct sockaddr*)&client, &client_size);
// shutdown socket.
cout << "shutting down client socket" << endl;
shutdown(clientSock, SD_BOTH);
// setup select
FD_SET fd;
TIMEVAL tv;
FD_ZERO(&fd);
tv.tv_sec  = static_cast<int>(6000);
tv.tv_usec = 0;
FD_SET(clientSock, &fd);
cout << "selecting" << endl;
int rc = select(0, &fd, NULL, NULL, &tv);     
cout << rc << ", " << FD_ISSET(clientSock, &fd) << endl;
char msg[500];
if (FD_ISSET(clientSock, &fd)) {
     cout << recv(clientSock, msg, 500, 0) << endl;
     cout << WSAGetLastError() << endl;
cout << "closing" << endl;
closesocket(clientSock);Telnet to port 9000, you get the following output immediately:
binding
listening
accepting
shutting down client socket
selecting
1, 1
-1
10058
closing
The solution I posted previously re calling shutdownInput/Output in Java isn't correct however, I left OP_WRITE on by mistake, which always returns ready, my fault.  Apologies.
Whatever the behaviour, it will be the same for Selector.select() as it is for select().
Clearly not.
Edited by: Bawb on 29-Jul-2011 07:01, I had left OP_WRITE on which was returning ready each time, when I realised and took it out I removed the shutdown code which made me think I hadn't sorted this. Apologies again.
Still reckon an RFE for OP_SHUTDOWN should be added to Java.
Thanks.