nixos/doc: Add Developing the Test Driver

nixos/doc/manual/development/developing-the-test-driver.chapter.md

@@ -0,0 +1,43 @@
+
+# Developing the NixOS Test Driver {#chap-developing-the-test-driver}
+
+The NixOS test framework is a project of its own.
+
+It consists of roughly the following components:
+
+ - `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.
+
+These components are exposed publicly through:
+
+ - `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.
+
+Beyond the test driver itself, its integration into NixOS and Nixpkgs is important.
+
+ - `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.
+
+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`.
+
+## Testing changes to the test framework {#sec-test-the-test-framework}
+
+When making significant changes to the test framework, we run the tests on Hydra, to avoid disrupting the larger NixOS project.
+
+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.
+
+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.
+
+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.
+
+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.

nixos/doc/manual/development/development.md

@@ -10,5 +10,6 @@ bootspec.chapter.md
 what-happens-during-a-system-switch.chapter.md
 writing-documentation.chapter.md
 nixos-tests.chapter.md
+developing-the-test-driver.chapter.md
 testing-installer.chapter.md
 ```