[Qemu-devel] [PATCH v2] qemu-nbd: Implement socket activation.

["Richard W.M. Jones" <rjones@redhat.com>]

v2:

 - A few small fixed identified by Dan Berrange.

The original cover letter is below.

Rich.


Socket activation (sometimes known as systemd socket activation)
allows an Internet superserver to pass a pre-opened listening socket
to the process, instead of having qemu-nbd open a socket itself.  This
is done via the LISTEN_FDS and LISTEN_PID environment variables, and a
standard file descriptor range.

This patch partially implements socket activation.

The limitation of this implementation is that qemu-nbd can only listen
on a single file descriptor, and so if LISTEN_FDS > 1 (eg. for
listening on multiple interfaces or ports) socket activation will
fail.  However for the simple case of listening on a single port, and
either all interfaces with IPv4+IPv6, or just a loopback interface,
the current implementation works fine.  Fixing this properly would
require considerable changes throughout qemu, since qemu's currently
handling of getaddrinfo is plainly wrong.

To use qemu-nbd from systemd, you create
/etc/systemd/system/nbd.socket:

  [Unit]
  Description=QEMU Network Block Device server
  [Socket]
  ListenStream=10809
  [Install]
  WantedBy=sockets.target

and /etc/systemd/system/nbd.service:

  [Service]
  ExecStart=/usr/sbin/qemu-nbd -v -t /path/to/file

and enable the socket service (only):

  systemctl enable nbd.socket
  systemctl start nbd.socket

and then connecting to port 10809 will start qemu-nbd and service the
file, with systemd opening the listening socket.

In the ExecStart line, the qemu-nbd -v option is only needed if you
want enhanced debugging.  The -t option is required unless you want to
fiddle with systemd settings for rate-limiting.

If you try to use the -p and similar options with socket activation
then qemu-nbd will give an error.

(I wasn't sure where to document this -- there is no obvious
documentation for qemu-nbd beyond the simple list of command line
arguments)

This is based on the implementations in
libvirt (src/util/virutil.c:virGetListenFDs) and
nbdkit (src/main.c:get_socket_activation), and also on Denis Plotnikov's
implementation of --server-sock-fd
(https://lists.gnu.org/archive/html/qemu-devel/2016-09/msg07781.html).