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

[Daniel P . Berrangé]

On Mon, Jan 30, 2023 at 02:58:01PM +0000, Daniel P. Berrangé wrote:
> > 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.
> 
> I don't think we should change SocketAddress at all, just use the
> @fd member that already exists, by fixing its semantics to always
> take an alphanumeric FD name and deprecate the use of pure numeric
> FD numbers.
> 
> Currently the socket code has an annoying structural connection
> to the monitor code, because the monitor is tracking the passed
> in FDs.
> 
> It would be nice to separate this by introducing some dedicated
> APIs for handling named FDs. The monitor can feed named FDs into
> this from getfd, and the CLI can feed named FDS into this from
> activation.   The socket code can then use these named FD handling
> APIs and thus isolate/detach itself from the monitor.

Some worked examples of what I would like to see being possible.

First the bare minimum, allowing a zero-conf setup (ok not possible
today, but lets assume QEMU can configure machine hardware entirely
via QMP):

  $ cat /etc/system/system/myvm.service
  [Unit]
  Description=My VM
  [Service]
  Exec=/usr/bin/qemu-system-x86_64 

  $ cat /etc/system/system/myvm-qmp.socket
  [Unit]
  Description=My VM QMP
  [Socket]
  ListenStream=/var/run/myvm-qmp.socket
  FileDescriptorName=qmp

So in this example, /usr/bin/qemu-system-x86_64 would be started
with no args, and LISTEN_FDS=1 and LISTEN_FDNAMES=qmp. QEMU will
have declared that 'qmp' is a reserved activation name that results
in creation of a QMP chardev.

This would be functionally equivalent to the following more explicit
syntax, which avoids the reserved name:

  $ cat /etc/system/system/myvm.service
  [Unit]
  Description=My VM
  [Service]
  Exec=/usr/bin/qemu-system-x86_64 -chardev socket,fd=control,id=qmp0 -qmp 
chardev=qmp0

  $ cat /etc/system/system/myvm-qmp.socket
  [Unit]
  Description=My VM QMP
  [Socket]
  ListenStream=/var/run/myvm-qmp.socket
  FileDescriptorName=control

which is LISTEN_FDS=1, LISTEN_FDNAMES=control

A more advanced usage would be to pass many FDs for different
tasks:

  $ cat /etc/system/system/myvm.service
  [Unit]
  Description=My VM
  [Service]
  Exec=/usr/bin/qemu-system-x86_64 \
     -chardev socket,fd=control,id=qmp0 -qmp chardev=qmp0 \
     -vnc fd=vnc-listen

  $ cat /etc/system/system/myvm-qmp.socket
  [Unit]
  Description=My VM QMP
  [Socket]
  ListenStream=/var/run/myvm-qmp.socket
  FileDescriptorName=control

  $ cat /etc/system/system/myvm-vnc.socket
  [Unit]
  Description=My VM VNC
  [Socket]
  ListenStream=0.0.0.0:5901
  FileDescriptorName=vnc

which is LISTEN_FDS=2, LISTEN_FDNAMES=control,vnc


This can apply to q-s-d too with the -export flag taking a FD name.
eg here we use the implicit QMP socket, and the explicit NBD exports:

  $ cat /etc/system/system/mydisks.service
  [Unit]
  Description=My DISKS
  [Service]
  Exec=/usr/bin/qemu-storage-daemon \
     -blockdev id=pubdisk,.... \
     -blockdev id=privdisk,.... \
     -export 
type=nbd,id=nbdpub,node-name=pubdisk,addr.type=fd,addr.fd=nbd-public
     -export 
type=nbd,id=nbdpub,node-name=privdisk,addr.type=fd,addr.fd=nbd-private

  $ cat /etc/system/system/mydisks-qmp.socket
  [Unit]
  Description=My disks QMP
  [Socket]
  ListenStream=/var/run/mydisks-qmp.socket
  FileDescriptorName=qmp

  $ cat /etc/system/system/mydisks-nbd-public.socket
  [Unit]
  Description=My disks Public
  [Socket]
  ListenStream=0.0.0.0:9000
  FileDescriptorName=nbd-public

  $ cat /etc/system/system/mydisks-nbd-private.socket
  [Unit]
  Description=My disks private
  [Socket]
  ListenStream=/var/run/mydisks-nbd-private.sock
  FileDescriptorName=nbd-private


which is LISTEN_FDS=3, LISTEN_FDNAMES=qmp,nbd-public,nbd-private, giving
you a QMP server on a UNIX socket, a private disk on a NBD UNIX socket
and a public disk on a NBD TCP socket.

The above system units are overly simplified for illustration, real
world ones would have more such as SocketMode for unit sockets and
dependencies, etc

With regards,
Daniel
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