Make unblockPipeW nonblocking (pun not intended) to fix bug 2633. The PTYTask thread would get stuck in readTask waiting for the mainThread to complete -[PTYSession readTask:] while the main thread was stuck in -unblock waiting to write to unblockPipeW, which is only read by the PTYTask thread. Since unblockPipeW was blocking, it must have had a lot of pending writes that hadn't been read off, and dropping one on the floor won't matter because the thread will definitely get unblocked.