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

[thierryh at vivaldi.net]

Le 2024-05-12 09:55, ThierryFR via sane-devel a écrit :
> 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?
My bad.
I've just tested it and it works pretty well.
> 
> Thierry
>> 
>> Cheers,
>> Guillaume Girol