Bug 1062754: Support peer shutdown and EOF in |SocketIOBase|, r=qdot, btian

Calling read on a socket that has been closed for reading by the
peer, read returns 0. The socket is still readable however, so
polling and reading will return constant results of 0 received
bytes.

With this patch, if a socket's peer shuts down reading or if
we reached the EOF, we stop watching the file descriptor for
readability. |SocketIOBase| will detect this case exactly once
and initiate the socket's shutdown.

ipc/unixsocket/UnixSocket.cpp

@@ -389,10 +389,13 @@ UnixSocketConsumerIO::OnSocketCanReceiveWithoutBlocking()
   MOZ_ASSERT(MessageLoopForIO::current() == GetIOLoop());
   MOZ_ASSERT(GetConnectionStatus() == SOCKET_IS_CONNECTED); // see bug 990984
 
-  nsresult rv = ReceiveData(GetFd(), this);
-  if (NS_FAILED(rv)) {
+  ssize_t res = ReceiveData(GetFd(), this);
+  if (res < 0) {
+    /* I/O error */
     RemoveWatchers(READ_WATCHER|WRITE_WATCHER);
-    return;
+  } else if (!res) {
+    /* EOF or peer shutdown */
+    RemoveWatchers(READ_WATCHER);
   }
 }