WIP: Rust bindings generation & cmake integration

[PhilippGackstatter]

This PR adds the elektra-sys crate for the Rust bindings. (See also #2730)
This crate only contains the raw bindings. A separate elektra crate will be added shortly in a future PR.

My goals for this PR are

• Generate bindings with rust-bindgen during the build phase
• Integrate cargo (rusts build tool) into the existing CMake build system
  • Building
  • Testing
• Add some basic tests for the bindings
• Change docker scripts such that cargo projects can be built

I will look into publishing the crate to crates.io later, once I am closer to the completion of both crates.

Basics

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Checklist

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For docu fixes, spell checking, and similar none of these points below
need to be checked.

• I added unit tests
• I ran all tests locally and everything went fine
• affected documentation is fixed
• I added code comments, logging, and assertions (see Coding Guidelines)
• meta data is updated (e.g. README.md of plugins and METADATA.ini)

Review

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• Code is conforming to our Coding Guidelines
• APIs are conforming to our Design Guidelines
• Code is consistent to our Design Decisions
• Documentation is concise and good to read
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Labels

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[PhilippGackstatter]

Some open questions

1. Do I only need to generate bindings for the kdb.h header, or are there other ones that I need for the low-level api, without plugin support?
2. Do I have to change all the docker scripts in order to install rustup (which is used to install cargo and rustc)? I suppose there is no automated way to do that.
3. Rust-bindgen offers two ways to generate bindings. One is via commandline, which is therefore a manual process and needs to be repeated if anything in the C-API changes. The other is via a build script, which is run every time cargo build runs. That means bindings are regenerated on each build. This is what is currently implemented. However, it requires everyone who uses the bindings to have the necessary headers that elekra needs. I imagine, if someone just installs elektra but did not compile it, he may not meet all the necessary requirements. Maybe it make more sense to regenerate the headers every once in a while, manually, since the C-API doesn't change very much any more?

[PhilippGackstatter]

Everything passes except for 6 - testmod_dbus (Failed) and 87 - testmod_dbusrecv (Failed). I've already rebased to a working master build. Why is this still failing?

[sanssecours]

Everything passes except for 6 - testmod_dbus (Failed) and 87 - testmod_dbusrecv (Failed). I've already rebased to a working master build. Why is this still failing?

Those two tests are unreliable on systems with high load (see also issue #2439). I just restarted the failing build job.

[markus2330]

Thank you!

[markus2330]

@PhilippGackstatter I hope it is okay that I merged this?

Can you rebase the other PR on top of this?

[PhilippGackstatter]

@PhilippGackstatter I hope it is okay that I merged this?

Yes, totally.

Can you rebase the other PR on top of this?

I'll try!