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Over the past weeks, we've seen one oversight with the new enforcement of `pkgs/by-name` for new packages. This documents the problem and the recommendation for resolving it.
201 lines
6.5 KiB
Markdown
201 lines
6.5 KiB
Markdown
# Name-based package directories
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The structure of this directory maps almost directly to top-level package attributes.
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Add new top-level packages to Nixpkgs using this mechanism [whenever possible](#limitations).
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Packages found in the name-based structure are automatically included, without needing to be added to `all-packages.nix`. However if the implicit attribute defaults need to be changed for a package, this [must still be declared in `all-packages.nix`](#changing-implicit-attribute-defaults).
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## Example
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The top-level package `pkgs.some-package` may be declared by setting up this file structure:
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```
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pkgs
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└── by-name
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├── so
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┊ ├── some-package
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┊ └── package.nix
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```
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Where `some-package` is the package name and `so` is the lowercased 2-letter prefix of the package name.
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The `package.nix` may look like this:
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```nix
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# A function taking an attribute set as an argument
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{
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# Get access to top-level attributes for use as dependencies
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lib,
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stdenv,
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libbar,
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# Make this derivation configurable using `.override { enableBar = true }`
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enableBar ? false,
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}:
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# The return value must be a derivation
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stdenv.mkDerivation {
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# ...
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buildInputs =
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lib.optional enableBar libbar;
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}
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```
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You can also split up the package definition into more files in the same directory if necessary.
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Once defined, the package can be built from the Nixpkgs root directory using:
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```
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nix-build -A some-package
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```
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See the [general package conventions](../README.md#conventions) for more information on package definitions.
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### Changing implicit attribute defaults
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The above expression is called using these arguments by default:
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```nix
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{
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lib = pkgs.lib;
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stdenv = pkgs.stdenv;
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libbar = pkgs.libbar;
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}
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```
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But the package might need `pkgs.libbar_2` instead.
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While the function could be changed to take `libbar_2` directly as an argument,
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this would change the `.override` interface, breaking code like `.override { libbar = ...; }`.
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So instead it is preferable to use the same generic parameter name `libbar`
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and override its value in [`pkgs/top-level/all-packages.nix`](../top-level/all-packages.nix):
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```nix
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libfoo = callPackage ../by-name/so/some-package/package.nix {
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libbar = libbar_2;
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};
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```
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## Manual migration guidelines
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Most packages are still defined in `all-packages.nix` and the [category hierarchy](../README.md#category-hierarchy).
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Please hold off migrating your maintained packages to this directory.
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1. An automated migration for the majority of packages [is being worked on](https://github.com/NixOS/nixpkgs/pull/211832).
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In order to save on contributor and reviewer time, packages should only be migrated manually afterwards if they couldn't be migrated automatically.
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1. Manual migrations should only be lightly encouraged if the relevant code is being worked on anyways.
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For example with a package update or refactoring.
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1. Manual migrations should not remove definitions from `all-packages.nix` with custom arguments.
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That is a backwards-incompatible change because it changes the `.override` interface.
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Such packages may still be moved to `pkgs/by-name` however, while keeping the definition in `all-packages.nix`.
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See also [changing implicit attribute defaults](#changing-implicit-attribute-defaults).
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## Limitations
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There's some limitations as to which packages can be defined using this structure:
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- Only packages defined using `pkgs.callPackage`.
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This excludes packages defined using `pkgs.python3Packages.callPackage ...`.
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Instead:
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- Either change the package definition to work with `pkgs.callPackage`.
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- Or use the [category hierarchy](../README.md#category-hierarchy).
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- Only top-level packages.
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This excludes packages for other package sets like `pkgs.pythonPackages.*`.
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Refer to the definition and documentation of the respective package set to figure out how such packages can be declared.
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## Validation
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CI performs [certain checks](../test/nixpkgs-check-by-name/README.md#validity-checks) on the `pkgs/by-name` structure.
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This is done using the [`nixpkgs-check-by-name` tool](../test/nixpkgs-check-by-name).
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You can locally emulate the CI check using
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```
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$ ./pkgs/test/nixpkgs-check-by-name/scripts/run-local.sh master
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```
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See [here](../../.github/workflows/check-by-name.yml) for more info.
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## Recommendation for new packages with multiple versions
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These checks of the `pkgs/by-name` structure can cause problems in combination:
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1. New top-level packages using `callPackage` must be defined via `pkgs/by-name`.
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2. Packages in `pkgs/by-name` cannot refer to files outside their own directory.
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This means that outside `pkgs/by-name`, multiple already-present top-level packages can refer to some common file.
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If you open a PR to another instance of such a package, CI will fail check 1,
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but if you try to move the package to `pkgs/by-name`, it will fail check 2.
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This is often the case for packages with multiple versions, such as
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```nix
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foo_1 = callPackage ../tools/foo/1.nix { };
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foo_2 = callPackage ../tools/foo/2.nix { };
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```
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The best way to resolve this is to not use `callPackage` directly, such that check 1 doesn't trigger.
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This can be done by using `inherit` on a local package set:
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```nix
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inherit
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({
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foo_1 = callPackage ../tools/foo/1.nix { };
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foo_2 = callPackage ../tools/foo/2.nix { };
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})
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foo_1
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foo_2
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;
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```
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While this may seem pointless, this can in fact help with future package set refactorings,
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because it establishes a clear connection between related attributes.
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### Further possible refactorings
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This is not required, but the above solution also allows refactoring the definitions into a separate file:
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```nix
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inherit (import ../tools/foo pkgs)
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foo_1 foo_2;
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```
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```nix
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# pkgs/tools/foo/default.nix
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pkgs: {
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foo_1 = callPackage ./1.nix { };
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foo_2 = callPackage ./2.nix { };
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}
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```
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Alternatively using [`callPackages`](https://nixos.org/manual/nixpkgs/unstable/#function-library-lib.customisation.callPackagesWith)
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if `callPackage` isn't used underneath and you want the same `.override` arguments for all attributes:
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```nix
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inherit (callPackages ../tools/foo { })
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foo_1 foo_2;
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```
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```nix
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# pkgs/tools/foo/default.nix
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{
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stdenv
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}: {
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foo_1 = stdenv.mkDerivation { /* ... */ };
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foo_2 = stdenv.mkDerivation { /* ... */ };
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}
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```
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### Exposing the package set
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This is not required, but the above solution also allows exposing the package set as an attribute:
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```nix
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foo-versions = import ../tools/foo pkgs;
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# Or using callPackages
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# foo-versions = callPackages ../tools/foo { };
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inherit (foo-versions) foo_1 foo_2;
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```
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