nixpkgs/CONTRIBUTING.md

Contributing to Nixpkgs

This document is for people wanting to contribute to the implementation of Nixpkgs. This involves interacting with implementation changes that are proposed using GitHub pull requests to the Nixpkgs repository (which you're in right now).

As such, a GitHub account is required, which you can sign up for here. Additionally this document assumes that you already know how to use GitHub and Git. If that's not the case, we recommend learning about it first here.

Overview

This file contains general contributing information, but individual parts also have more specific information to them in their respective README.md files, linked here:

• lib: Sources and documentation of the library functions
• maintainers: Nixpkgs maintainer and team listings, maintainer scripts
• pkgs: Package and builder definitions
• doc: Sources and infrastructure for the Nixpkgs manual
• nixos: Implementation of NixOS

How to propose a change

This section describes in some detail how changes can be made and proposed with pull requests.

Note

Be aware that contributing implies licensing those contributions under the terms of COPYING, an MIT-like license.

0. Set up a local version of Nixpkgs to work with using GitHub and Git

   1. Fork the Nixpkgs repository.
   2. Clone the forked repository into a local nixpkgs directory.
   3. Configure the upstream Nixpkgs repository.

1. Create and switch to a new Git branch, ideally such that:

   • The name of the branch hints at the change you'd like to implement, e.g. update-hello.
   • The base of the branch includes the most recent changes on the master branch.

     Note

     Depending on the change you may want to use a different branch, see

   # Make sure you have the latest changes from upstream Nixpkgs
   git fetch upstream
   
   # Create and switch to a new branch based off the master branch in Nixpkgs
   git switch --create update-hello upstream/master
   

   To avoid having to download and build potentially many derivations, at the expense of using a potentially outdated version, you can base the branch off a specific Git commit instead:

   • The commit of the latest nixpkgs-unstable channel, available here.
   • The commit of a local Nixpkgs downloaded using nix-channel, available using nix-instantiate --eval --expr '(import <nixpkgs/lib>).trivial.revisionWithDefault null'
   • If you're using NixOS, the commit of your NixOS installation, available with nixos-version --revision.

   Once you have an appropriate commit you can use it instead of upstream/master in the above command:

   git switch --create update-hello <the desired base commit>
   

2. Make the desired changes in the local Nixpkgs repository using an editor of your choice. Make sure to:

   • Adhere to both the general code conventions, and the code conventions specific to the part you're making changes to. See the overview section for more specific information.
   • Test the changes. See the overview section for more specific information.
   • If necessary, document the change. See the overview section for more specific information.

3. Commit your changes using git commit. Make sure to adhere to the commit conventions.

   Repeat the steps 2 and 3 as many times as necessary. Advance to the next step if all the commits (viewable with git log) make sense together.

4. Push your commits to your fork of Nixpkgs.

   git push --set-upstream origin HEAD
   

   The above command will output a link that allows you to directly quickly do the next step:

   remote: Create a pull request for 'update-hello' on GitHub by visiting:
   remote:      https://github.com/myUser/nixpkgs/pull/new/update-hello
   

5. Create a pull request from the new branch in your Nixpkgs fork to the upstream Nixpkgs repository. Generally you should use master as the pull requests base branch. See for when a different branch should be used instead. Make sure to go through the pull request template in the pre-filled default description.

6. Respond to review comments, potential CI failures and potential merge conflicts by updating the pull request. Always keep the pull request in a mergeable state.

   To add new commits, repeat steps 2-3 and push the result using

   git push
   

   To change existing commits you will have to rewrite Git history. Useful Git commands that can help a lot with this are git commit --patch --amend and git rebase --interactive. With a rewritten history you need to force-push the commits using

   git push --force-with-lease
   

   In case of merge conflicts you will also have to rebasing the branch on top of current master. Sometimes this can be done on GitHub directly, but if not you will have to rebase locally using

   git fetch upstream
   git rebase upstream/master
   git push --force-with-lease
   

Flow of changes

After a Nixpkgs pull requests is merged, it eventually makes it to the official Hydra CI. Hydra regularly evaluates and builds Nixpkgs, updating the official channels when specific Hydra jobs succeeded. See Nix Channel Status for the current channels and their state. Here's a brief overview of the main Git branches and what channels they're used for:

• master: The main branch, used for the unstable channels such as nixpkgs-unstable, nixos-unstable and nixos-unstable-small.
• release-YY.MM (e.g. release-23.05): The NixOS release branches, used for the stable channels such as nixos-23.05, nixos-23.05-small and nixpkgs-23.05-darwin.

When a channel is updated, a corresponding Git branch is also updated to point to the corresponding commit. So e.g. the nixpkgs-unstable branch corresponds to the Git commit from the nixpkgs-unstable channel.

Releases

The NixOS release process is documented in the NixOS Release Wiki

Most changes added to the stable release branches are cherry-picked (“backported”) from the master and staging branches.

Backporting changes

Follow these steps to backport a change into a release branch in compliance with the commit policy.

You can add a label such as backport release-23.05 to a PR, so that merging it will automatically create a backport (via a GitHub Action). This also works for pull requests that have already been merged, and might take a couple of minutes to trigger.

You can also create the backport manually:

1. Take note of the commits in which the change was introduced into master branch.
2. Check out the target release branch, e.g. release-23.05. Do not use a channel branch like nixos-23.05 or nixpkgs-23.05-darwin.
3. Create a branch for your change, e.g. git checkout -b backport.
4. When the reason to backport is not obvious from the original commit message, use git cherry-pick -xe <original commit> and add a reason. Otherwise use git cherry-pick -x <original commit>. That's fine for minor version updates that only include security and bug fixes, commits that fixes an otherwise broken package or similar. Please also ensure the commits exists on the master branch; in the case of squashed or rebased merges, the commit hash will change and the new commits can be found in the merge message at the bottom of the master pull request.
5. Push to GitHub and open a backport pull request. Make sure to select the release branch (e.g. release-23.05) as the target branch of the pull request, and link to the pull request in which the original change was committed to master. The pull request title should be the commit title with the release version as prefix, e.g. [23.05].
6. When the backport pull request is merged and you have the necessary privileges you can also replace the label 9.needs: port to stable with 8.has: port to stable on the original pull request. This way maintainers can keep track of missing backports easier.

Automatically backporting a Pull Request

Assign label backport <branch> (e.g. backport release-21.11) to the PR and a backport PR is automatically created after the PR is merged.

Manually backporting changes

Cherry-pick changes via git cherry-pick -x <original commit> so that the original commit id is included in the commit message.

Add a reason for the backport when it is not obvious from the original commit message. You can do this by cherry picking with git cherry-pick -xe <original commit>, which allows editing the commit message. This is not needed for minor version updates that include security and bug fixes but don't add new features or when the commit fixes an otherwise broken package.

Here is an example of a cherry-picked commit message with good reason description:

zfs: Keep trying root import until it works

Works around #11003.

(cherry picked from commit 98b213a11041af39b39473906b595290e2a4e2f9)

Reason: several people cannot boot with ZFS on NVMe

Other examples of reasons are:

• Previously the build would fail due to, e.g., getaddrinfo not being defined
• The previous download links were all broken
• Crash when starting on some X11 systems

Acceptable backport criteria

The stable branch does have some changes which cannot be backported. Most notable are breaking changes. The desire is to have stable users be uninterrupted when updating packages.

However, many changes are able to be backported, including:

• New Packages / Modules
• Security / Patch updates
• Version updates which include new functionality (but no breaking changes)
• Services which require a client to be up-to-date regardless. (E.g. spotify, steam, or discord)
• Security critical applications (E.g. firefox)

Criteria for Backporting changes

Anything that does not cause user or downstream dependency regressions can be backported. This includes:

• New Packages / Modules
• Security / Patch updates
• Version updates which include new functionality (but no breaking changes)
• Services which require a client to be up-to-date regardless. (E.g. spotify, steam, or discord)
• Security critical applications (E.g. firefox)

Staging

The staging workflow exists to batch Hydra builds of many packages together.

It works by directing commits that cause mass rebuilds to a separate staging branch that isn't directly built by Hydra. Regularly, the staging branch is manually merged into a staging-next branch to be built by Hydra using the nixpkgs:staging-next jobset. The staging-next branch should then only receive direct commits in order to fix Hydra builds. Once it is verified that there are no major regressions, it is merged into master using a pull requests. This is done manually in order to ensure it's a good use of Hydra's computing resources. By keeping the staging-next branch separate from staging, this batching does not block developers from merging changes into staging.

In order for the staging and staging-next branches to be up-to-date with the latest commits on master, there are regular automated merges from master into staging-next and staging. This is implemented using GitHub workflows here and here.

Note

Changes must be sufficiently tested before being merged into any branch. Hydra builds should not be used as testing platform.

Here is a Git history diagram showing the flow of commits between the three branches:

%%{init: {
    'theme': 'base',
    'themeVariables': {
        'gitInv0': '#ff0000',
        'gitInv1': '#ff0000',
        'git2': '#ff4444',
        'commitLabelFontSize': '15px'
    },
    'gitGraph': {
        'showCommitLabel':true,
        'mainBranchName': 'master',
        'rotateCommitLabel': true
    }
} }%%
gitGraph
    commit id:" "
    branch staging-next
    branch staging

    checkout master
    checkout staging
    checkout master
    commit id:"    "
    checkout staging-next
    merge master id:"automatic"
    checkout staging
    merge staging-next id:"automatic "

    checkout staging-next
    merge staging type:HIGHLIGHT id:"manual"
    commit id:"fixup"

    checkout master
    checkout staging
    checkout master
    commit id:"       "
    checkout staging-next
    merge master id:"automatic  "
    checkout staging
    merge staging-next id:"automatic   "

    checkout staging-next
    commit id:"fixup "
    checkout master
    merge staging-next type:HIGHLIGHT id:"manual (PR)"

Here's an overview of the different branches:

branch	master	staging	staging-next
Used for development	✔️	✔️	❌
Built by Hydra	✔️	❌	✔️
Mass rebuilds	❌	✔️	⚠️ Only to fix Hydra builds
Critical security fixes	✔️ for non-mass-rebuilds	❌	✔️ for mass-rebuilds
Automatically merged into	staging-next	-	staging
Manually merged into	-	staging-next	master

The staging workflow is used for all main branches, master and release-YY.MM, with corresponding names:

• master/release-YY.MM
• staging/staging-YY.MM
• staging-next/staging-next-YY.MM

Mass rebuilds

Which changes cause mass rebuilds is not formally defined. In order to help the decision, CI automatically assigns rebuild labels to pull requests based on the number of packages they cause rebuilds for. As a rule of thumb, if the number of rebuilds is over 500, it can be considered a mass rebuild. To get a sense for what changes are considered mass rebuilds, see previously merged pull requests to the staging branches.

Rebasing between branches (i.e. from master to staging)

From time to time, changes between branches must be rebased, for example, if the number of new rebuilds they would cause is too large for the target branch. When rebasing, care must be taken to include only the intended changes, otherwise many CODEOWNERS will be inadvertently requested for review. To achieve this, rebasing should not be performed directly on the target branch, but on the merge base between the current and target branch. As an additional precautionary measure, you should temporarily mark the PR as draft for the duration of the operation. This reduces the probability of mass-pinging people. (OfBorg might still request a couple of persons for reviews though.)

In the following example, we assume that the current branch, called feature, is based on master, and we rebase it onto the merge base between master and staging so that the PR can eventually be retargeted to staging without causing a mess. The example uses upstream as the remote for NixOS/nixpkgs.git while origin is the remote you are pushing to.

# Rebase your commits onto the common merge base
git rebase --onto upstream/staging... upstream/master
# Force push your changes
git push origin feature --force-with-lease

The syntax upstream/staging... is equivalent to upstream/staging...HEAD and stands for the merge base between upstream/staging and HEAD (hence between upstream/staging and upstream/master).

Then change the base branch in the GitHub PR using the Edit button in the upper right corner, and switch from master to staging. After the PR has been retargeted it might be necessary to do a final rebase onto the target branch, to resolve any outstanding merge conflicts.

# Rebase onto target branch
git rebase upstream/staging
# Review and fixup possible conflicts
git status
# Force push your changes
git push origin feature --force-with-lease

Something went wrong and a lot of people were pinged

It happens. Remember to be kind, especially to new contributors. There is no way back, so the pull request should be closed and locked (if possible). The changes should be re-submitted in a new PR, in which the people originally involved in the conversation need to manually be pinged again. No further discussion should happen on the original PR, as a lot of people are now subscribed to it.

The following message (or a version thereof) might be left when closing to describe the situation, since closing and locking without any explanation is kind of rude:

It looks like you accidentally mass-pinged a bunch of people, which are now subscribed
and getting notifications for everything in this pull request. Unfortunately, they
cannot be automatically unsubscribed from the issue (removing review request does not
unsubscribe), therefore development cannot continue in this pull request anymore.

Please open a new pull request with your changes, link back to this one and ping the
people actually involved in here over there.

In order to avoid this in the future, there are instructions for how to properly
rebase between branches in our [contribution guidelines](https://github.com/NixOS/nixpkgs/blob/master/CONTRIBUTING.md#rebasing-between-branches-ie-from-master-to-staging).
Setting your pull request to draft prior to rebasing is strongly recommended.
In draft status, you can preview the list of people that are about to be requested
for review, which allows you to sidestep this issue.
This is not a bulletproof method though, as OfBorg still does review requests even on draft PRs.

Hotfixing pull requests

• Make the appropriate changes in you branch.
• Don’t create additional commits, do
  • git rebase -i
  • git push --force to your branch.

Reviewing contributions

::: {.warning} The following section is a draft, and the policy for reviewing is still being discussed in issues such as #11166 and #20836. :::

The Nixpkgs project receives a fairly high number of contributions via GitHub pull requests. Reviewing and approving these is an important task and a way to contribute to the project.

The high change rate of Nixpkgs makes any pull request that remains open for too long subject to conflicts that will require extra work from the submitter or the merger. Reviewing pull requests in a timely manner and being responsive to the comments is the key to avoid this issue. GitHub provides sort filters that can be used to see the most recently and the least recently updated pull requests. We highly encourage looking at this list of ready to merge, unreviewed pull requests.

When reviewing a pull request, please always be nice and polite. Controversial changes can lead to controversial opinions, but it is important to respect every community member and their work.

GitHub provides reactions as a simple and quick way to provide feedback to pull requests or any comments. The thumb-down reaction should be used with care and if possible accompanied with some explanation so the submitter has directions to improve their contribution.

Pull request reviews should include a list of what has been reviewed in a comment, so other reviewers and mergers can know the state of the review.

All the review template samples provided in this section are generic and meant as examples. Their usage is optional and the reviewer is free to adapt them to their liking.

Other submissions

Other type of submissions requires different reviewing steps.

If you consider having enough knowledge and experience in a topic and would like to be a long-term reviewer for related submissions, please contact the current reviewers for that topic. They will give you information about the reviewing process. The main reviewers for a topic can be hard to find as there is no list, but checking past pull requests to see who reviewed or git-blaming the code to see who committed to that topic can give some hints.

Container system, boot system and library changes are some examples of the pull requests fitting this category.

(Merging a pull request) | Merging pull requests

The Nixpkgs committers are people who have been given permission to merge.

It is possible for community members that have enough knowledge and experience on a special topic to contribute by merging pull requests.

In case the PR is stuck waiting for the original author to apply a trivial change (a typo, capitalisation change, etc.) and the author allowed the members to modify the PR, consider applying it yourself. (or commit the existing review suggestion) You should pay extra attention to make sure the addition doesn't go against the idea of the original PR and would not be opposed by the author.

Please see the discussion in GitHub nixpkgs issue #50105 for information on how to proceed to be granted this level of access.

In a case a contributor definitively leaves the Nix community, they should create an issue or post on Discourse with references of packages and modules they maintain so the maintainership can be taken over by other contributors.

Code conventions

Release notes

If you removed pkgs or made some major NixOS changes, write about it in the release notes for the next stable release. For example nixos/doc/manual/release-notes/rl-2003.xml.

File naming and organisation

Names of files and directories should be in lowercase, with dashes between words — not in camel case. For instance, it should be all-packages.nix, not allPackages.nix or AllPackages.nix.

Syntax

• Use 2 spaces of indentation per indentation level in Nix expressions, 4 spaces in shell scripts.

• Do not use tab characters, i.e. configure your editor to use soft tabs. For instance, use (setq-default indent-tabs-mode nil) in Emacs. Everybody has different tab settings so it’s asking for trouble.

• Use lowerCamelCase for variable names, not UpperCamelCase. Note, this rule does not apply to package attribute names, which instead follow the rules in .

• Function calls with attribute set arguments are written as

  foo {
    arg = ...;
  }
  

  not

  foo
  {
    arg = ...;
  }
  

  Also fine is

  foo { arg = ...; }
  

  if it's a short call.

• In attribute sets or lists that span multiple lines, the attribute names or list elements should be aligned:

  # A long list.
  list = [
    elem1
    elem2
    elem3
  ];
  
  # A long attribute set.
  attrs = {
    attr1 = short_expr;
    attr2 =
      if true then big_expr else big_expr;
  };
  
  # Combined
  listOfAttrs = [
    {
      attr1 = 3;
      attr2 = "fff";
    }
    {
      attr1 = 5;
      attr2 = "ggg";
    }
  ];
  

• Short lists or attribute sets can be written on one line:

  # A short list.
  list = [ elem1 elem2 elem3 ];
  
  # A short set.
  attrs = { x = 1280; y = 1024; };
  

• Breaking in the middle of a function argument can give hard-to-read code, like

  someFunction { x = 1280;
    y = 1024; } otherArg
    yetAnotherArg
  

  (especially if the argument is very large, spanning multiple lines).

  Better:

  someFunction
    { x = 1280; y = 1024; }
    otherArg
    yetAnotherArg
  

  or

  let res = { x = 1280; y = 1024; };
  in someFunction res otherArg yetAnotherArg
  

• The bodies of functions, asserts, and withs are not indented to prevent a lot of superfluous indentation levels, i.e.

  { arg1, arg2 }:
  assert system == "i686-linux";
  stdenv.mkDerivation { ...
  

  not

  { arg1, arg2 }:
    assert system == "i686-linux";
      stdenv.mkDerivation { ...
  

• Function formal arguments are written as:

  { arg1, arg2, arg3 }:
  

  but if they don't fit on one line they're written as:

  { arg1, arg2, arg3
  , arg4, ...
  , # Some comment...
    argN
  }:
  

• Functions should list their expected arguments as precisely as possible. That is, write

  { stdenv, fetchurl, perl }: ...
  

  instead of

  args: with args; ...
  

  or

  { stdenv, fetchurl, perl, ... }: ...
  

  For functions that are truly generic in the number of arguments (such as wrappers around mkDerivation) that have some required arguments, you should write them using an @-pattern:

  { stdenv, doCoverageAnalysis ? false, ... } @ args:
  
  stdenv.mkDerivation (args // {
    ... if doCoverageAnalysis then "bla" else "" ...
  })
  

  instead of

  args:
  
  args.stdenv.mkDerivation (args // {
    ... if args ? doCoverageAnalysis && args.doCoverageAnalysis then "bla" else "" ...
  })
  

• Unnecessary string conversions should be avoided. Do

  rev = version;
  

  instead of

  rev = "${version}";
  

• Building lists conditionally should be done with lib.optional(s) instead of using if cond then [ ... ] else null or if cond then [ ... ] else [ ].

  buildInputs = lib.optional stdenv.isDarwin iconv;
  

  instead of

  buildInputs = if stdenv.isDarwin then [ iconv ] else null;
  

  As an exception, an explicit conditional expression with null can be used when fixing a important bug without triggering a mass rebuild. If this is done a follow up pull request should be created to change the code to lib.optional(s).

• Arguments should be listed in the order they are used, with the exception of lib, which always goes first.

Commit conventions

• When changing the bootloader installation process, extra care must be taken. Grub installations cannot be rolled back, hence changes may break people’s installations forever. For any non-trivial change to the bootloader please file a PR asking for review, especially from @edolstra.

• Create a commit for each logical unit.

• Check for unnecessary whitespace with git diff --check before committing.

• If you have commits pkg-name: oh, forgot to insert whitespace: squash commits in this case. Use git rebase -i.

• Format the commit messages in the following way:

  (pkg-name | nixos/<module>): (from -> to | init at version | refactor | etc)
  
  (Motivation for change. Link to release notes. Additional information.)
  

  For consistency, there should not be a period at the end of the commit message's summary line (the first line of the commit message).

  Examples:

  • nginx: init at 2.0.1

  • firefox: 54.0.1 -> 55.0

    https://www.mozilla.org/en-US/firefox/55.0/releasenotes/

  • nixos/hydra: add bazBaz option

    Dual baz behavior is needed to do foo.

  • nixos/nginx: refactor config generation

    The old config generation system used impure shell scripts and could break in specific circumstances (see #1234).

Writing good commit messages

In addition to writing properly formatted commit messages, it's important to include relevant information so other developers can later understand why a change was made. While this information usually can be found by digging code, mailing list/Discourse archives, pull request discussions or upstream changes, it may require a lot of work.

Package version upgrades usually allow for simpler commit messages, including attribute name, old and new version, as well as a reference to the relevant release notes/changelog. Every once in a while a package upgrade requires more extensive changes, and that subsequently warrants a more verbose message.

Pull requests should not be squash merged in order to keep complete commit messages and GPG signatures intact and must not be when the change doesn't make sense as a single commit.

Pull Request Template

The pull request template helps determine what steps have been made for a contribution so far, and will help guide maintainers on the status of a change. The motivation section of the PR should include any extra details the title does not address and link any existing issues related to the pull request.

When a PR is created, it will be pre-populated with some checkboxes detailed below:

Tested using sandboxing

When sandbox builds are enabled, Nix will setup an isolated environment for each build process. It is used to remove further hidden dependencies set by the build environment to improve reproducibility. This includes access to the network during the build outside of fetch* functions and files outside the Nix store. Depending on the operating system access to other resources are blocked as well (ex. inter process communication is isolated on Linux); see sandbox in Nix manual for details.

Sandboxing is not enabled by default in Nix due to a small performance hit on each build. In pull requests for nixpkgs people are asked to test builds with sandboxing enabled (see Tested using sandboxing in the pull request template) because inhttps://nixos.org/hydra/ sandboxing is also used.

Depending if you use NixOS or other platforms you can use one of the following methods to enable sandboxing before building the package:

• Globally enable sandboxing on NixOS: add the following to configuration.nix

  nix.useSandbox = true;
  

• Globally enable sandboxing on non-NixOS platforms: add the following to: /etc/nix/nix.conf

  sandbox = true
  

Built on platform(s)

Many Nix packages are designed to run on multiple platforms. As such, it’s important to let the maintainer know which platforms your changes have been tested on. It’s not always practical to test a change on all platforms, and is not required for a pull request to be merged. Only check the systems you tested the build on in this section.

Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)

Packages with automated tests are much more likely to be merged in a timely fashion because it doesn’t require as much manual testing by the maintainer to verify the functionality of the package. If there are existing tests for the package, they should be run to verify your changes do not break the tests. Tests can only be run on Linux. For more details on writing and running tests, see the section in the NixOS manual.

Tested compilation of all pkgs that depend on this change using nixpkgs-review

If you are updating a package’s version, you can use nixpkgs-review to make sure all packages that depend on the updated package still compile correctly. The nixpkgs-review utility can look for and build all dependencies either based on uncommitted changes with the wip option or specifying a GitHub pull request number.

Review changes from pull request number 12345:

nix-shell -p nixpkgs-review --run "nixpkgs-review pr 12345"

Alternatively, with flakes (and analogously for the other commands below):

nix run nixpkgs#nixpkgs-review -- pr 12345

Review uncommitted changes:

nix-shell -p nixpkgs-review --run "nixpkgs-review wip"

Review changes from last commit:

nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD"

Tested execution of all binary files (usually in ./result/bin/)

It’s important to test any executables generated by a build when you change or create a package in nixpkgs. This can be done by looking in ./result/bin and running any files in there, or at a minimum, the main executable for the package. For example, if you make a change to texlive, you probably would only check the binaries associated with the change you made rather than testing all of them.

Meets Nixpkgs contribution standards

The last checkbox is fits CONTRIBUTING.md. The contributing document has detailed information on standards the Nix community has for commit messages, reviews, licensing of contributions you make to the project, etc... Everyone should read and understand the standards the community has for contributing before submitting a pull request.