older versions of libsystemd lack all functions use for our `bus` module

[codyps]

In particular, ubuntu-18.04's libsystemd lacks 2 functions which result in link errors (when the bus feature is not disabled):

/home/runner/work/rust-systemd/rust-systemd/src/bus/mod.rs:1269: undefined reference to `sd_bus_slot_set_destroy_callback'
/home/runner/work/rust-systemd/rust-systemd/src/bus/mod.rs:1270: undefined reference to `sd_bus_slot_set_floating'

We should consider looking into whether these can be avoided an provide a usable API in systemd (high level crate). Possible that bus (the feature) could be subsetted (so some of bus is usable with old libsystemd. Minimally, detecting the presence of these functions at configuration time and avoiding emitting the rust definitions would help us get better error
messages (ie: at compile instead of at link).

openssl has to deal with similar issues and probably has some approach we can adapt.

[siegfriedweber]

We have a similar problem with sd_journal_open_namespace which was added in systemd v245 (see the systemd changelog). I solved it by creating a feature flag systemd_v245 (see https://github.com/siegfriedweber/rust-systemd/commit/29efbfe356e31a24ed67eb8ae1abd28dd0b8904f).

In my opinion, a feature flag is not necessary for systemd versions which are already in the stable branches of the major Linux distributions. On Debian stable it is currently version 241 (https://packages.debian.org/buster/libsystemd-dev). But there should be flags for newer versions.

I am not familiar with the systemd API, so I cannot provide a pull request for all API changes. But I could create a pull request with the systemd_v245 flag if desired. Alternatively you can just cherry-pick the mentioned commit if you like the approach.

[codyps]

I think starting with a systemd_v245 feature flag makes sense here. We need some solution for this, and that seems like a minimal one.

I'm fine with the feature not being 100% accurate (missing some API items) initially, it'll be better than the current situation.

[lucab]

Similar situation here, hitting a linking issue due to missing sd_journal_open_namespace on RHEL 8, where systemd is at v239.