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

[Daniel P . Berrangé]

On Mon, Jan 30, 2023 at 03:44:48PM +0000, Richard W.M. Jones wrote:
> On Mon, Jan 30, 2023 at 02:58:01PM +0000, Daniel P. Berrangé wrote:
> > Obviously at startup QEMU can trivially inherit the FDs from whatever
> > spawned it. The only task is to identify the FDs that are passed into,
> > and systemd defined a mechanism for this using LISTEN_FDNAMES. IOW the
> > socket activation can fully replace 'getfd' for purpose of initial
> > startup. This will get rid of the annoying difference that SocketAddress
> > only allows numeric FDs at startup and named FDs at runtime, by making
> > named FDs the consistent standard. We could thus deprecate the use of
> > non-named numeric FDs in SocketAddress to improve our sanity.
> > 
> > The question is how to define semantics for the LISTEN_FDNAMES while
> > also still remaining back compat with the existing QEMU utilities
> > that allow socket activation. Some kind of naming scheme would need
> > to be decided upon, as well as handling the use of activation without
> > LISTEN_FDNAMES being set. 
> 
> If I understand LISTEN_FDNAMES correctly, it's the names of the
> protocols to be used (rather clumsily expressed through IANA
> registered names from /etc/services).  It would be valid to use
> LISTEN_FDNAMES=http:http for example, for a service that must use HTTP
> on both sockets.

That's not accurate, it is not related to IANA service names.

> In other words it's not just names of file descriptors that you can
> make up.

By default the FD name matches the name of the systemd .socket unit
file, but it can be set to an arbitrary string using FileDescriptorName=.
It is upto the application to decide if it wants to require specific
file naming schemes, or is happy to receive the default system .socket
based names.

With regards,
Daniel
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