[sane-devel] Passing all scanner usage through local saned

[thierryh at vivaldi.net]

Le 2024-05-09 15:39, Guillaume Girol via sane-devel a écrit :
> Hello,
> 
> I'm a downstream maintainer of SANE on NixOS, a linux distribution. I
> consider modifying NixOS to make it so all application using libsane
> would actually only use saned, and never load the backends to directly
> access scanners themselves. I send this message to the mailing list to
> know if I missed reasons it could be a bad idea.
> 
> Rationale
> =========
> 
> NixOS is quite different from typical linux distributions in that there
> is no globally installed library. /usr/lib does not exist and without
> special packaging care, dlopen() never finds anything. Each library is
> installed in a unique prefix /nix/store/unique-id/lib/foo.so so several
> versions of the same library can coexist. Exeecutable find the
> libraries they depend on via explicit RPATH. As a result I can have an
> old version of python in /nix/store/some-id/bin/python using an old
> version of glibc in /nix/store/other-id/lib/libc.so via RPATH and the
> rest of the system using a newer version of glibc in /nix/store/yet-
> another-id/lib/libc.so also via RPATH.
> 
> This does not work for SANE as it uses dlopen() to find backends
> depending on a single, global configuration file. For now we have an
> exception to the "no lib installed globally" design for SANE. However
> this is not perfect: if I use an old version of, say, simple-scan, it
> may dlopen() more recent version of some SANE backend, which depends on
> a more recent glibc than the version that old simple-scan brings with
> its RPATH. Thus dlopen() fails. In practice this means that software
> installed from the past or the future often fails to scan, whereas this
> usually works quite well for other functionnality.
> 
> Solution considered
> ===================
> 
> To solve this, I want applications using SANE to stop using dlopen() to
> access the scanner, but to use a network protocol instead. This is a
> bit like glibc can talk to nscd over a socket instead of dlopening nss
> modules.
> 
> The solution I'm considering consists in linking all normal
> applications to a stripped-down variant of libsane which only has the
> network backend and is configured to connect to a saned instance on
> localhost (instead of obeying what you would find in /etc/sane.d on
> typical distros). Each application comes with its own copy of this
> libsane, it's not global anymore. (Don't mind the disk space cost, all
> of NixOS works this way, this is part of the tradeoff). When the user
> configures their system for scanner support, a global instance of saned
> is setup which is linked against a full-fledged copy of libsane obeying
> the full configuration files you would typically find in /etc/sane.d.
> 
> When an old application attempts to scan, it uses its own, old version
> of libsane to talk to a recent saned which dlopen()s sane backends of
> matching version, so all works fine.
> 
> Possible issues?
> ================
> 
> Any issues with this design I missed? In my light testing, it appears
> to work quite fine. The problems I found:
> - all users can use the scanners instead of just users in the lp or
> scanner group previously. I suppose this can be solved with a firewall
> (firewalls can filter the uid of local sockets).
> - network scanners are not available. I propose a fix in
> https://gitlab.com/sane-project/backends/-/merge_requests/834
> - is the network protocol that saned uses stable across versions? In my
> testing, it is, but maybe it's by sheer luck.
> - maybe stuff I have not considered.
> 
Hi,

Just a question about this operation:
How are the scanners discovered?

Thierry
> 
> Cheers,
> Guillaume Girol