nix-filter - a small self-contained source filtering lib

STATUS: beta

When using nix within a project, developers often use src = ./.; for a project like this:

{ stdenv }:
stdenv.mkDerivation {
  name = "my-project";
  src = ./.;
}

This works but has an issue; on each build, nix will copy the whole project source code into the /nix/store. Including the .git folder and any temporary files left over by the editor.

The main workaround is to use either builtins.fetchGit ./. or one of the many gitignore filter projects but this is not precise enough. If the project README changes, it should not rebuild the project. If the nix code changes, it should not rebuild the project.

This project is a small library that makes it easy to filter in and out what files should go into a nix derivation.

Example usage

nix-filter works best for projects where the files needed for a build are easy to match. Using it with only an exclude argument will likely not reduce the reasons for rebuilds by a lot. Here's an Example:

{ stdenv, nix-filter }:
stdenv.mkDerivation {
  name = "my-project";
  src = nix-filter {
    root = ./.;
    # If no include is passed, it will include all the paths.
    include = [
      # Include the "src" path relative to the root.
      "src"
      # Include this specific path. The path must be under the root.
      ./package.json
      # Include all files with the .js extension
      (nix-filter.matchExt "js")
    ];

    # Works like include, but the reverse.
    exclude = [
      ./main.js
    ];
  };
}

Usage

Import this folder. Eg:

let
  nix-filter = import ./path/to/nix-filter;
in
 # ...

The top-level is a functor that takes:

• root of type path: pointing to the root of the source to add to the /nix/store.
• name of type string (optional): the name of the derivation (defaults to "source")
• include of type list(string|path|matcher) (optional): a list of patterns to include (defaults to all).
• exclude of type list(string|path|matcher) (optional): a list of patterns to exclude (defaults to none).

The include and exclude take a matcher, and automatically convert the string and path types to a matcher.

The matcher is a function that takes a path and type and returns true if the pattern matches.

The flake usage is like this:

{
  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixos-22.11";
    nix-filter.url = "github:numtide/nix-filter";
  };
  outputs = { self, nixpkgs, nix-filter }:
    let
      pkgs = nixpkgs.legacyPackages.x86_64-linux;
      # Avoid calling it nix-filter as it may result in an infinite recursion
      filter = nix-filter.lib;
     # ....
    in
    {
      # ...
    };
}

Builtin matchers

The functor also contains a number of matchers:

• nix-filter.matchExt: ext -> returns a function that matches the given file extension.
• nix-filter.inDirectory: directory -> returns a function that matches a directory and any path inside of it.
• nix-filter.isDirectory: matches all paths that are directories

Combining matchers

• and: a -> b -> c combines the result of two matchers into a new matcher.
• or_: a -> b -> c combines the result of two matchers into a new matcher.

NOTE: or is a keyword in nix, which is why we use a variation here.

REMINDER: both, include & exlude already XOR elements, so or_ is not useful at the top level.

Design notes

This solution uses builtins.path { path, name, filter ? path: type: true } under the hood, which ships with Nix.

While traversing the filesystem, starting from path, it will call filter on each file and folder recursively. If the filter returns false then the file or folder is ignored. If a folder is ignored, it won't recurse into it anymore.

Because of that, it makes it difficult to implement recursive glob matchers. Something like **/*.js would necessarily have to add every folder, to be able to traverse them. And those empty folders will end-up in the output.

If we want to control rebuild, it's important to have a fixed set of folders.

One possibility is to use a two-pass system, where first all the folders are being added, and then the empty ones are being filtered out. But all of this happens at Nix evaluation time. Nix evaluation is already slow enough like that.

That's why nix-filter is asking the users to explicitly list all the folders that they want to include, and using only an exclude is not recommended.

Future development

Add more matchers.

Related projects

• globset - a superior implementation that is compatible with nixpkgs filesets. See https://github.com/pdtpartners/globset
• nixpkgs' lib.cleanSourceWith.
• All the git-based solutions. See https://github.com/hercules-ci/gitignore.nix#comparison

License

Copyright (c) 2021 Numtide under the MIT.