agama-project / agama / 15303935258

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Split and refactor the Rust CI tests (#2417)

## Problem

- Running the Rust CI takes quite long time, usually around 9 minutes.
In most cases you have to wait to get the green check mark for merging
your changes.

## Solution

- Split the single huge job into several smaller independent parts which
can be executed in parallel
- Cache the compiled crates for faster rebuilds (it uses a different
cache for each job, the clippy cache is much smaller than the test
cache)
- Run directly in the Ubuntu host, actually we do not need the openSUSE
container for running the tests
- Faster start, avoids downloading the container (in a corner case just
downloading the image might [take 17
minutes](https://github.com/agama-project/agama/actions/runs/15294771338/job/43021160058#step:2:107))
  - The Rust tooling is already preinstalled in the default image
  - It uses `rustup` as well so there is basically no difference

## Other changes

- Install tarpaulin from sources.
 
I'd avoid downloading random binaries from the internet and running them
in the system. In the GitHub Actions which run in an isolated single-use
system it is acceptable, but if somebody wants to replicate the same
steps locally it would be nice to avoid this. The compiled tarpaulin is
cached with all other crates for the next run, so it is basically built
only once.

## Testing

- See [example
run](https://github.com/lslezak/agama/actions/runs/15249520255), with
hot cache it takes about 3 minutes for the tests to finish, the other
jobs running in parallel are faster.

### Hot cache


![image](https://github.com/user-attachments/assets/39a95203-60b2-43bd-aed3-810f4370e584)

### Cold cache

Note: This includes compiling the tarpaulin tool from sources in the
test job.


![image](https://github.com/user-attachments/assets/9b3335b3-fdaf-4438-ab77-1702f23f568e)


## Review note

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easy. It is better to review the [new file
content](h... (continued)

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