There's a race condition in OpenSSH (all versions - haven't checked -current)
such that if SIGCHLD arrives after the check for child_terminated, but before
entering select() (with no timeout) AND if the session's pty/pipes have
received EOF before SIGCHLD is handled, then sshd will block in select()
forever, while the client hangs waiting for the session-exit message that
will never arrive (because sshd will never send it, because it's blocking
in select()).

Markus Friedl posted a fix for it (see URL associated with this bug report).
It works. It should be applied.

Thanks.