Threads are not notified when the stream they are reading is closed

If a thread is blocked on a read system call on a file descriptor
and that file descriptor is closed by a different thread, the first
thread (the reader) is not notified that its file descriptor is
closed.

This creates a serious race condition becuase if a new file is
opened and receives the same file descriptor, then the first reader
thread may accidentally read some data from the new file before
it discovers that the old file descriptor was closed.