Re: RFC: towards systemd socket activation in q-s-d

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

On Fri, Jan 27, 2023 at 03:26:15PM -0600, Eric Blake wrote:
> In https://bugzilla.redhat.com/show_bug.cgi?id=2055229, the question
> was raised on how to make qemu-storage-daemon sufficiently powerful to
> be a full-blown replacement to qemu-nbd.  One of the features still
> lacking is the ability to do systemd socket activation (qemu-nbd does
> this, qemu-storage-daemon needs a way to do it).
> 
> But that bug further noted that systemd supports LISTEN_FDNAMES to
> supply names to a passed-in fd (right now, qemu-nbd does not use
> names, but merely expects one fd in LISTEN_FDS).  Dan had the idea
> that it would be nice to write a systemd file that passes in a socket
> name for a QMP socket, as in:
> 
>  [Socket]
>  ListenStream=/var/run/myapp/qsd.qmp
>  FileDescriptorName=qmp
>  Service=myapp-qsd.service
> 
> and further notes that QAPI SocketAddressType supports @fd which is a
> name in QMP (a previously-added fd passed through the older 'getfd'
> command, rather than the newer 'add-fd' command), but an integer on
> the command line.  With LISTEN_FDNAMES, we could mix systemd socket
> activation with named fds for any command line usage that already
> supports SocketAddressType (not limited to just q-s-d usage).

I couldn't find a good description of LISTEN_FDNAMES anywhere, and we
don't use it in libnbd / nbdkit / qemu-nbd right now.  So here's my
interpretation of what this environment variable means ...

Currently systemd socket activation in qemu-nbd or nbdkit uses only
LISTEN_PID and LISTEN_FDS, usually with a single fd being passed.
(I'll ignore LISTEN_PID.)  So:

  LISTEN_FDS=1
    fd 3 --> NBD socket

This works because there's only one NBD protocol, it doesn't need to
be named.

However if we imagine that a superserver wants to run a webserver, it
might need to pass two sockets carrying distinct protocols (HTTP and
HTTPS).  In this case it would use:

  LISTEN_FDS=2
    fd 3 --> HTTP socket
    fd 4 --> HTTPS socket
  LISTEN_FDNAMES=http:https

LISTEN_FDNAMES is telling the webserver that the first socket is HTTP
and the second is HTTPS, so it knows which protocol to negotiate on
each socket.

I believe what you're saying above is that you'd like to have the
ability to pass multiple sockets to q-s-d carrying distinct protocols,
with the obvious ones being NBD + QMP (although other combinations
could be possible, eg. NBD + vhost + QMP?):

  LISTEN_FDS=2
    fd 3 --> NBD socket
    fd 4 --> QMP socket
  LISTEN_FDNAMES=nbd:qmp

If my understanding is correct, then this makes sense.  We might also
modify libnbd to pass LISTEN_FDNAMES=nbd in:

  https://gitlab.com/nbdkit/libnbd/-/blob/master/generator/states-connect-socket-activation.c

(Current servers would ignore the new environment variable.)

> I'm at a point where I can take a shot at implementing this, but want
> some feedback on whether it is better to try to shoehorn a generic
> solution into the existing @fd member of the SocketAddressType union,
> or whether it would be better to add yet another union member
> @systemd-fd or some similar name to make it explicit when a command
> line parameter wants to refer to an fd being passed through systemd
> socket activation LISTEN_FDS and friends.

It sounds like the q-s-d command lines will be even more convoluted
and inelegant than before.

That's fine, but please keep qemu-nbd around as a sane alternative.
It might in the end just be a wrapper that translates the command line
to q-s-d.  I don't believe it's ever going to be possible or desirable
to deprecate or remove qemu-nbd.

Rich.

-- 
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