path: root/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](https://github.com/) [pull requests](https://docs.github.com/pull-requests) to the [Nixpkgs](https://github.com/nixos/nixpkgs/) repository (which you're in right now).

As such, a GitHub account is required, which you can sign up for [here](https://github.com/signup).
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](https://docs.github.com/en/get-started/quickstart/hello-world).

## 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`](./lib): Sources and documentation of the [library functions](https://nixos.org/manual/nixpkgs/stable/#chap-functions)
- [`maintainers`](./maintainers): Nixpkgs maintainer and team listings, maintainer scripts
- [`pkgs`](./pkgs): Package and [builder](https://nixos.org/manual/nixpkgs/stable/#part-builders) definitions
- [`doc`](./doc): Sources and infrastructure for the [Nixpkgs manual](https://nixos.org/manual/nixpkgs/stable/)
- [`nixos`](./nixos): Implementation of [NixOS](https://nixos.org/manual/nixos/stable/)

## 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](./COPYING), an MIT-like license.

0. Set up a local version of Nixpkgs to work with using GitHub and Git
   1. [Fork](https://docs.github.com/en/get-started/quickstart/fork-a-repo#forking-a-repository) the [Nixpkgs repository](https://github.com/nixos/nixpkgs/).
   1. [Clone the forked repository](https://docs.github.com/en/get-started/quickstart/fork-a-repo#cloning-your-forked-repository) into a local `nixpkgs` directory.
   1. [Configure the upstream Nixpkgs repository](https://docs.github.com/en/get-started/quickstart/fork-a-repo#configuring-git-to-sync-your-fork-with-the-upstream-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 <!-- TODO link to branch section -->

   ```bash
   # 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](https://www.git-scm.com/docs/gitglossary#def_commit) instead:
   - The commit of the latest `nixpkgs-unstable` channel, available [here](https://channels.nixos.org/nixpkgs-unstable/git-revision).
   - The commit of a local Nixpkgs downloaded using [nix-channel](https://nixos.org/manual/nix/stable/command-ref/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:
   ```bash
   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](#code-conventions), and the code conventions specific to the part you're making changes to.
     See the [overview section](#overview) for more specific information.
   - Test the changes.
     See the [overview section](#overview) for more specific information.
   - If necessary, document the change.
     See the [overview section](#overview) for more specific information.

3. Commit your changes using `git commit`.
   Make sure to adhere to the [commit conventions](#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](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/creating-a-pull-request#creating-the-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 <!-- TODO branch section link --> for when a different branch should be used instead.
   Make sure to go through the [pull request template](#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](https://git-scm.com/book/en/v2/Git-Tools-Rewriting-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](https://git-scm.com/book/en/v2/Git-Branching-Rebasing) on top of current `master`.
   Sometimes this can be done [on GitHub directly](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/keeping-your-pull-request-in-sync-with-the-base-branch#updating-your-pull-request-branch), but if not you will have to rebase locally using
   ```
   git fetch upstream
   git rebase upstream/master
   git push --force-with-lease
   ```

#### 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.


```console
# 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.

```console
# 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:

```markdown
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.
```

### Backporting changes

Follow these steps to backport a change into a release branch in compliance with the [commit policy](https://nixos.org/nixpkgs/manual/#submitting-changes-stable-release-branches).

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](.github/workflows/backport.yml)).
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.

#### 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`)

### Hotfixing pull requests {#submitting-changes-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 {#chap-reviewing-contributions}

::: {.warning}
The following section is a draft, and the policy for reviewing is still being discussed in issues such as [#11166](https://github.com/NixOS/nixpkgs/issues/11166) and [#20836](https://github.com/NixOS/nixpkgs/issues/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](https://github.com/NixOS/nixpkgs/pulls?q=is%3Apr+is%3Aopen+sort%3Aupdated-desc) and the [least recently](https://github.com/NixOS/nixpkgs/pulls?q=is%3Apr+is%3Aopen+sort%3Aupdated-asc) updated pull requests. We highly encourage looking at [this list of ready to merge, unreviewed pull requests](https://github.com/NixOS/nixpkgs/pulls?q=is%3Apr+is%3Aopen+review%3Anone+status%3Asuccess+-label%3A%222.status%3A+work-in-progress%22+no%3Aproject+no%3Aassignee+no%3Amilestone).

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 {#reviewing-contributions-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 {#reviewing-contributions--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.

<!--
The following paragraphs about how to deal with unactive contributors is just a proposition and should be modified to what the community agrees to be the right policy.

Please note that contributors with commit rights unactive for more than three months will have their commit rights revoked.
-->

Please see the discussion in [GitHub nixpkgs issue #50105](https://github.com/NixOS/nixpkgs/issues/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](https://discourse.nixos.org) with references of packages and modules they maintain so the maintainership can be taken over by other contributors.

## (Flow of changes) | Commit policy {#submitting-changes-commit-policy}

Most contributions are based on and merged into these branches:

* `master` is the main branch where all small contributions go
* `staging` is branched from master, changes that have a big impact on
  Hydra builds go to this branch
* `staging-next` is branched from staging and only fixes to stabilize
  and security fixes with a big impact on Hydra builds should be
  contributed to this branch. This branch is merged into master when
  deemed of sufficiently high quality

- Commits must be sufficiently tested before being merged, both for the master and staging branches.
- Hydra builds for master and staging should not be used as testing platform, it’s a build farm for changes that have been already tested.
- 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.

### Branches {#submitting-changes-branches}

The `nixpkgs` repository has three major branches:
- `master`
- `staging`
- `staging-next`

The most important distinction between them is that `staging`
(colored red in the diagram below) can receive commits which cause
a mass-rebuild (for example, anything that changes the `drvPath` of
`stdenv`).  The other two branches `staging-next` and `master`
(colored green in the diagram below) can *not* receive commits which
cause a mass-rebuild.

Arcs between the branches show possible merges into these branches,
either from other branches or from independently submitted PRs.  The
colors of these edges likewise show whether or not they could
trigger a mass rebuild (red) or must not trigger a mass rebuild
(green).

Hydra runs automatic builds for the green branches.

Notice that the automatic merges are all green arrows.  This is by
design.  Any merge which might cause a mass rebuild on a branch
which has automatic builds (`staging-next`, `master`) will be a
manual merge to make sure it is good use of compute power.

Nixpkgs has two branches so that there is one branch (`staging`)
which accepts mass-rebuilding commits, and one fast-rebuilding
branch which accepts independent PRs (`master`).  The `staging-next`
branch allows the Hydra operators to batch groups of commits to
`staging` to be built.  By keeping the `staging-next` branch
separate from `staging`, this batching does not block
developers from merging changes into `staging`.

```{.graphviz caption="Staging workflow"}
digraph {
    master [color="green" fontcolor=green]
    "staging-next" [color="green" fontcolor=green]
    staging [color="red" fontcolor=red]

    "small changes" [fontcolor=green shape=none]
    "small changes" -> master [color=green]

    "mass-rebuilds and other large changes" [fontcolor=red shape=none]
    "mass-rebuilds and other large changes" -> staging [color=red]

    "critical security fixes" [fontcolor=green shape=none]
    "critical security fixes" -> master [color=green]

    "staging fixes which do not cause staging to mass-rebuild" [fontcolor=green shape=none]
    "staging fixes which do not cause staging to mass-rebuild" -> "staging-next" [color=green]

    "staging-next" -> master [color="red"] [label="manual merge"] [fontcolor="red"]
    "staging" -> "staging-next" [color="red"] [label="manual merge"] [fontcolor="red"]

    master -> "staging-next" [color="green"] [label="automatic merge (GitHub Action)"] [fontcolor="green"]
    "staging-next" -> staging [color="green"] [label="automatic merge (GitHub Action)"] [fontcolor="green"]
}
```

[This GitHub Action](https://github.com/NixOS/nixpkgs/blob/master/.github/workflows/periodic-merge-6h.yml) brings changes from `master` to `staging-next` and from `staging-next` to `staging` every 6 hours; these are the green arrows in the diagram above.  The red arrows in the diagram above are done manually and much less frequently.  You can get an idea of how often these merges occur by looking at the git history.


#### Master branch {#submitting-changes-master-branch}

The `master` branch is the main development branch.