Re: docs: Update vhost-user spec regarding backend program conventions

["Marc-André Lureau" <marcandre.lureau@redhat.com>]

Hi

On Tue, Feb 11, 2020 at 4:24 PM Boeuf, Sebastien
<sebastien.boeuf@intel.com> wrote:
>
> From c073d528b8cd7082832fd1825dc33dd65b305aa2 Mon Sep 17 00:00:00 2001
> From: Sebastien Boeuf <sebastien.boeuf@intel.com>
> Date: Tue, 11 Feb 2020 16:01:22 +0100
> Subject: [PATCH] docs: Update vhost-user spec regarding backend program
>  conventions
>
> The vhost-user specification is not clearly stating the expected
> behavior from a backend program whenever the client disconnects.
>
> This patch addresses the issue by defining the default behavior and
> proposing an alternative through a command line option.
>
> By default, a backend program will have to keep listening even if the
> client disconnects, unless told otherwise through the newly introduced
> option --exit-on-disconnect.
>
> Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  docs/interop/vhost-user.rst | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
> index 5f8b3a456b..da9a1ebc4c 100644
> --- a/docs/interop/vhost-user.rst
> +++ b/docs/interop/vhost-user.rst
> @@ -1323,6 +1323,10 @@ The backend program must end (as quickly and cleanly as possible) when
>  the SIGTERM signal is received. Eventually, it may receive SIGKILL by
>  the management layer after a few seconds.
>
> +By default, the backend program continues running after the client
> +disconnects. It accepts only 1 connection at a time on each UNIX domain
> +socket.

I don't think that's the most common behaviour. libvhost-user will
panic() on disconnect in general, unless the error/exit is handled
gracefully by the backend.

The most common case is to have 1-1 relation between device/qemu
instance and backend.

Why not restart the backend for another instance?

> +
>  The following command line options have an expected behaviour. They
>  are mandatory, unless explicitly said differently:
>
> @@ -1337,6 +1341,12 @@ are mandatory, unless explicitly said differently:
>    vhost-user socket as file descriptor FDNUM. It is incompatible with
>    --socket-path.
>
> +--exit-on-disconnect
> +
> +  When this option is provided, the backend program must terminate when
> +  the client disconnects. This can be used to keep the backend program's
> +  lifetime synchronized with its client process.

This section list options that are mandatory. It's probably a bit late
to add more mandatory options (I regret already some of them)

Do we need to specify the behaviour on client disconnect? Can't we
leave that to the backend and management layer to decide?


> +
>  --print-capabilities
>
>    Output to stdout the backend capabilities in JSON format, and then
> --
> 2.20.1
>
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