check-by-name: Update contributor docs

Now that the tooling is pinned instead of being fetched indirectly from
the channel, the docs need some updating

.github/workflows/check-by-name.yml

@@ -4,8 +4,8 @@
 # When you make changes to this workflow, also update pkgs/test/nixpkgs-check-by-name/scripts/run-local.sh adequately
 name: Check pkgs/by-name
 
-# The pre-built tool is fetched from a channel,
+# The tool is pinned to a pre-built version on Hydra,
-# making it work predictable on all PRs.
+# see pkgs/test/nixpkgs-check-by-name/scripts/README.md
 on:
   # Using pull_request_target instead of pull_request avoids having to approve first time contributors
   pull_request_target
@@ -92,6 +92,7 @@ jobs:
           echo "base=$base" >> "$GITHUB_ENV"
       - uses: cachix/install-nix-action@7ac1ec25491415c381d9b62f0657c7a028df52a7 # v24
       - name: Fetching the pinned tool
+        # Update the pinned version using pkgs/test/nixpkgs-check-by-name/scripts/update-pinned-tool.sh
         run: |
           # Get the direct /nix/store path from the pin to avoid having to evaluate Nixpkgs
           toolPath=$(jq -r '."ci-path"' pkgs/test/nixpkgs-check-by-name/scripts/pinned-tool.json)

nixos/release-combined.nix

@@ -171,8 +171,8 @@ in rec {
         (onSystems ["x86_64-linux"] "nixpkgs.mesa_i686") # i686 sanity check + useful
         ["nixpkgs.tarball"]
 
-        # Ensure that nixpkgs-check-by-name is available in all release channels and nixos-unstable,
+        # Ensure that nixpkgs-check-by-name is available in nixos-unstable,
-        # so that a pre-built version can be used in CI for PR's on the corresponding development branches.
+        # so that a pre-built version can be used in CI for PR's
         # See ../pkgs/test/nixpkgs-check-by-name/README.md
         (onSystems ["x86_64-linux"] "nixpkgs.tests.nixpkgs-check-by-name")
       ];

pkgs/test/nixpkgs-check-by-name/README.md

@@ -1,9 +1,13 @@
 # Nixpkgs pkgs/by-name checker
 
 This directory implements a program to check the [validity](#validity-checks) of the `pkgs/by-name` Nixpkgs directory.
-It is being used by [this GitHub Actions workflow](../../../.github/workflows/check-by-name.yml).
 This is part of the implementation of [RFC 140](https://github.com/NixOS/rfcs/pull/140).
 
+A [pinned version](./scripts/pinned-tool.json) of this tool is used by [this GitHub Actions workflow](../../../.github/workflows/check-by-name.yml).
+See [./scripts](./scripts/README.md#update-pinned-toolsh) for how to update the pinned version.
+
+The source of the tool being right inside Nixpkgs allows any Nixpkgs committer to make updates to it.
+
 ## Interface
 
 The interface of the tool is shown with `--help`:
@@ -96,18 +100,3 @@ Tests are declared in [`./tests`](./tests) as subdirectories imitating Nixpkgs w
 - `expected` (optional):
   A file containing the expected standard output.
   The default is expecting an empty standard output.
-
-## Hydra builds
-
-This program will always be available pre-built for `x86_64-linux` on the `nixos-unstable` channel and `nixos-XX.YY` channels.
-This is ensured by including it in the `tested` jobset description in [`nixos/release-combined.nix`](../../../nixos/release-combined.nix).
-
-This allows CI for PRs to development branches `master` and `release-XX.YY` to fetch the pre-built program from the corresponding channel and use that to check the PR. This has the following benefits:
-- It allows CI to check all PRs, even if they would break the CI tooling.
-- It makes the CI check very fast, since no Nix builds need to be done, even for mass rebuilds.
-- It improves security, since we don't have to build potentially untrusted code from PRs.
-  The tool only needs a very minimal Nix evaluation at runtime, which can work with [readonly-mode](https://nixos.org/manual/nix/stable/command-ref/opt-common.html#opt-readonly-mode) and [restrict-eval](https://nixos.org/manual/nix/stable/command-ref/conf-file.html#conf-restrict-eval).
-- It allows anybody to make updates to the tooling and for those updates to be automatically used by CI without needing a separate release mechanism.
-
-The tradeoff is that there's a delay between updates to the tool and those updates being used by CI.
-This needs to be considered when updating the [API](#api).

pkgs/test/nixpkgs-check-by-name/scripts/README.md

@@ -19,6 +19,20 @@ Arguments:
 ## `./update-pinned-tool.sh`
 
 Updates the pinned CI tool in [`./pinned-tool.json`](./pinned-tool.json) to the
-[latest version from the `nixos-unstable` channel](https://hydra.nixos.org/job/nixos/trunk-combined/nixpkgs.tests.nixpkgs-check-by-name.x86_64-linux)
+[latest version from the `nixos-unstable` channel](https://hydra.nixos.org/job/nixos/trunk-combined/nixpkgs.tests.nixpkgs-check-by-name.x86_64-linux).
+
+This script needs to be called manually when the CI tooling needs to be updated.
+
+The `pinned-tool.json` file gets populated with both:
+- The `/nix/store` path for `x86_64-linux`, such that CI doesn't have to evaluate Nixpkgs and can directly fetch it from the cache instead.
+- The Nixpkgs revision, such that the `./run-local.sh` script can be used to run the checks locally on any system.
+
+To ensure that the tool is always pre-built for `x86_64-linux` in the `nixos-unstable` channel,
+it's included in the `tested` jobset description in [`nixos/release-combined.nix`](../../../nixos/release-combined.nix).
+
+Why not just build the tooling right from the PRs Nixpkgs version?
+- Because it allows CI to check all PRs, even if they would break the CI tooling.
+- Because it makes the CI check very fast, since no Nix builds need to be done, even for mass rebuilds.
+- Because it improves security, since we don't have to build potentially untrusted code from PRs.
+  The tool only needs a very minimal Nix evaluation at runtime, which can work with [readonly-mode](https://nixos.org/manual/nix/stable/command-ref/opt-common.html#opt-readonly-mode) and [restrict-eval](https://nixos.org/manual/nix/stable/command-ref/conf-file.html#conf-restrict-eval).
 
-This script is called manually once the CI tooling needs to be updated.