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nixos/doc: Add Developing the Test Driver #216660
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roberth
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nixos/doc/manual/development/developing-the-test-driver.chapter.md
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| # Developing the NixOS Test Driver {#chap-developing-the-test-driver} | ||
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| The NixOS test framework is a project of its own. | ||
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| It consists of roughly the following components: | ||
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| - `nixos/lib/test-driver`: The Python framework that sets up the test and runs the [`testScript`](#test-opt-testScript) | ||
| - `nixos/lib/testing`: The Nix code responsible for the wiring, written using the (NixOS) Module System. | ||
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| These components are exposed publicly through: | ||
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| - `nixos/lib/default.nix`: The public interface that exposes the `nixos/lib/testing` entrypoint. | ||
| - `flake.nix`: Exposes the `lib.nixos`, including the public test interface. | ||
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| Beyond the test driver itself, its integration into NixOS and Nixpkgs is important. | ||
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| - `pkgs/top-level/all-packages.nix`: Defines the `nixosTests` attribute, used | ||
| by the package `tests` attributes and OfBorg. | ||
| - `nixos/release.nix`: Defines the `tests` attribute built by Hydra, independently, but analogous to `nixosTests` | ||
| - `nixos/release-combined.nix`: Defines which tests are channel blockers. | ||
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| Finally, we have legacy entrypoints that users should move away from, but are cared for on a best effort basis. | ||
| These include `pkgs.nixosTest`, `testing-python.nix` and `make-test-python.nix`. | ||
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| ## Testing changes to the test framework {#sec-test-the-test-framework} | ||
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| When making significant changes to the test framework, we run the tests on Hydra, to avoid disrupting the larger NixOS project. | ||
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| For this, we use the `python-test-refactoring` branch in the `NixOS/nixpkgs` repository, and its [corresponding Hydra jobset](https://hydra.nixos.org/jobset/nixos/python-test-refactoring). | ||
| This branch is used as a pointer, and not as a feature branch. | ||
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| 1. Rebase the PR onto a recent, good evaluation of `nixos-unstable` | ||
| 2. Create a baseline evaluation by force-pushing this revision of `nixos-unstable` to `python-test-refactoring`. | ||
| 3. Note the evaluation number (we'll call it `<previous>`) | ||
| 4. Push the PR to `python-test-refactoring` and evaluate the PR on Hydra | ||
| 5. Create a comparison URL by navigating to the latest build of the PR and adding to the URL `?compare=<previous>`. This is not necessary for the evaluation that comes right after the baseline. | ||
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| Review the removed tests and newly failed tests using the constructed URL; otherwise you will accidentally compare iterations of the PR instead of changes to the PR base. | ||
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| As we currently have some flaky tests, newly failing tests are expected, but should be reviewed to make sure that | ||
| - The number of failures did not increase significantly. | ||
| - All failures that do occur can reasonably be assumed to fail for a different reason than the changes. | ||
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The document itself is good.
At the same time, what do you think about rigorously dropping everything what's flaky? Flaky tests are a nuisance to everyone who invests good work to get changes through and have the side-effect of wearing down motivation to painstaking.
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As long as we don't mistake flaky Hydra for flaky tests, this would absolutely be a win.
We could perhaps move them to a
flakyattrset, so that they can be fixed and moved back when they have a record of being reliable again.Reasons to prefer dropping over moving:
Reasons to prefer moving:
Hermit emulates the scheduler deterministically. This should let us find debuggable counterexamples, I think. Might be too new/experimental, and might not support KVM. We'll want to run it only on the flaky tests, because it is slower by an order of magnitude, but NixOS isn't poor so that's not a reason to avoid it.
So yeah, I think moving those tests would be great!
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Having different sets of which only one is really mandatory for contributors to satisfy sounds like a great improvement, yes.
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Opened #216828 for this.
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FWIW, @JulienMalka wanted to explore Hermit usage in NixOS tests, I don't know if he has something to share, but just mentioning him as it can be interesting.
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Indeed I think it'd be very interesting to deep a little further into Hermit, see what improvements in test reproducibility we can get and at what cost. I'll certainly work on that subject soon, available to chat more about it.
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What parts of the nixos test driver stack would be touched by using hermit? My gut feel says that this would be "just" configuring the guest vm's nixos config with kernel patch + config properly, is that about right?
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Ideally Hermit supports KVM, so that we only have to wrap the top level invocation in
nixos/lib/testing/run.nix. Otherwise, we have a qemu invocation innixos/modules/virtualisation/qemu-vm.nix, which is included (a bit indirectly) fromnixos/lib/testing/nodes.nix. Configuring the kernel works the way it normally does in NixOS, I think.