Multiple socket servers can listen on the same socket address (using
`SO_REUSEPORT` or shared file descriptors). Accordingly, when a new
connection is incoming, multiple event-loops could report the socket to
be readable and try to a `accept()` an incoming connection from the same
socket.
This wouldn't be an issue with the underlying `accept()` system call.
The first call would succeed and subsequent calls would report `EAGAIN`
or `EWOULDBLOCK` for the other servers given the socket resource is in
non-blocking mode.
However, PHP's `stream_socket_accept()` implementation first runs a
`poll()` on the socket resource before performing an `accept()`. This
means multiple instances listening on the same address could end up in a
race condition where some may be "stuck" in the pending `poll()`.
We work around this by specifiying a `0` timeout to ensure the `poll()`
doesn't block in this case. This allows all servers to either complete
successfully or report an error and continue processing right away.