# Contributing to Nixpkgs packages

This document is for people wanting to contribute specifically to the package collection in Nixpkgs.
See the [CONTRIBUTING.md](../CONTRIBUTING.md) document for more general information.

## Overview

- [`top-level`](./top-level): Entrypoints, package set aggregations
  - [`impure.nix`](./top-level/impure.nix), [`default.nix`](./top-level/default.nix), [`config.nix`](./top-level/config.nix): Definitions for the evaluation entry point of `import <nixpkgs>`
  - [`stage.nix`](./top-level/stage.nix), [`all-packages.nix`](./top-level/all-packages.nix), [`by-name-overlay.nix`](./top-level/by-name-overlay.nix), [`splice.nix`](./top-level/splice.nix): Definitions for the top-level attribute set made available through `import <nixpkgs> {…}`
  - `*-packages.nix`, [`linux-kernels.nix`](./top-level/linux-kernels.nix), [`unixtools.nix`](./top-level/unixtools.nix): Aggregations of nested package sets defined in `development`
  - [`aliases.nix`](./top-level/aliases.nix), [`python-aliases.nix`](./top-level/python-aliases.nix): Aliases for package definitions that have been renamed or removed
  - `release*.nix`, [`make-tarball.nix`](./top-level/make-tarball.nix), [`packages-config.nix`](./top-level/packages-config.nix), [`metrics.nix`](./top-level/metrics.nix), [`nixpkgs-basic-release-checks.nix`](./top-level/nixpkgs-basic-release-checks.nix): Entry-points and utilities used by Hydra for continuous integration
- [`development`](./development)
  - `*-modules`, `*-packages`, `*-pkgs`: Package definitions for nested package sets
  - All other directories loosely categorise top-level package definitions, see [category hierarchy][categories]
- [`build-support`](./build-support): [Builders](https://nixos.org/manual/nixpkgs/stable/#part-builders)
  - `fetch*`: [Fetchers](https://nixos.org/manual/nixpkgs/stable/#chap-pkgs-fetchers)
- [`stdenv`](./stdenv): [Standard environment](https://nixos.org/manual/nixpkgs/stable/#part-stdenv)
- [`pkgs-lib`](./pkgs-lib): Definitions for utilities that need packages but are not needed for packages
- [`test`](./test): Tests not directly associated with any specific packages
- [`by-name`](./by-name): Top-level packages organised by name ([docs](./by-name/README.md))
- All other directories loosely categorise top-level package definitions, see [category hierarchy][categories]

## Quick Start to Adding a Package

We welcome new contributors of new packages to Nixpkgs, arguably the greatest software database known.
However, each new package comes with a cost for the maintainers, Continuous Integration, caching servers and users downloading Nixpkgs.

Before adding a new package, please consider the following questions:

* Is the package ready for general use?
  We don't want to include projects that are too immature or are going to be abandoned immediately.
  In case of doubt, check with upstream.
* Does the project have a clear license statement?
  Remember that software is unfree by default (all rights reserved), and merely providing access to the source code does not imply its redistribution.
  In case of doubt, ask upstream.
* How realistic is it that it will be used by other people?
  It's good that nixpkgs caters to various niches, but if it's a niche of 5 people it's probably too small.
  A good estimate is checking upstream issues and pull requests, or other software repositories.
    * Library packages should have at least one dependent.
      If possible, that dependent should be packaged in the same PR the library is added in, as a sanity check.
      If it is not possible to package the dependent, a minimal test program should be added to `passthru.tests`.
* Is the software actively maintained upstream?
  Especially packages that are security-critical, rely on fast-moving dependencies, or affect data integrity should see regular maintenance.
* Are you willing to maintain the package?
  You should care enough about the package to be willing to keep it up and running for at least one complete Nixpkgs' release life-cycle.
  * In case you are not able to maintain the package you wrote, you can seek someone to fill that role, effectively adopting the package.

If any of these questions' answer is no, then you should probably not add the package.

Special care has to be taken with security-critical software components.
Because entries in the Nix store are inert and do nothing by themselves, packages should be considered by their intended use, e.g. when used together with a NixOS module.

* Any package that immediately would need to be tagged with `meta.knownVulnerabilities` is unlikely to be fit for nixpkgs.
* Any package depending on a known-vulnerable library should be considered carefully.
* Packages typically used with untrusted data should have a maintained and responsible upstream.
  For example:
  * Any package which does not follow upstream security policies should be considered vulnerable.
    In particular, packages that vendor or fork web engines like Blink, Gecko or Webkit need to keep up with the frequent updates of those projects.
  * Any security-critical fast-moving package such as Chrome or Firefox (or their forks) must have at least one committer among the maintainers, who actively reviews, merges and backports updates.
    This ensures no critical fixes are delayed unnecessarily, endangering unsuspecting users.
  * Services which typically work on web traffic are working on untrusted input.
  * Data (such as archives or rich documents) commonly shared over untrusted channels (e.g. email) is untrusted.
* Applications in the Unix authentication stack such as PAM/D-Bus modules or SUID binaries should be considered carefully, and should have a maintained and responsible upstream.
* Encryption libraries should have a maintained and responsible upstream.
* Security-critical components that are part of larger packages should be unvendored (=use the nixpkgs package as dependency, instead of vendored and pinned sources).
* A "responsible upstream" includes various aspects, such as:
  * channels to disclose security concerns
  * being responsive to security concerns, providing fixes or workarounds
  * transparent public disclosure of security issues when they are found or fixed
  * These aspects are sometimes hard to verify, in which case an upstream that is not known to be irresponsible should be considered as responsible.
* Source-available software should be built from source where possible.
  Binary blobs risk supply chain attacks and vendored outdated libraries.

This section describes a general framework of understanding and exceptions might apply.

Luckily it's pretty easy to maintain your own package set with Nix, which can then be added to the [Nix User Repository](https://github.com/nix-community/nur) project or included in [search.nixos.org's list of indexed 3rd-party flakes](https://github.com/NixOS/nixos-search/blob/main/flakes/manual.toml)

---

Now that this is out of the way.
To add a package to Nixpkgs:

1. Checkout the Nixpkgs source tree:

   ```ShellSession
   $ git clone https://github.com/NixOS/nixpkgs
   $ cd nixpkgs
   ```

2. Create a package directory `pkgs/by-name/so/some-package` where `some-package` is the package name and `so` is the lowercased 2-letter prefix of the package name:

   ```ShellSession
   $ mkdir -p pkgs/by-name/so/some-package
   ```

   For more detailed information, see [here](./by-name/README.md).

3. Create a `package.nix` file in the package directory, containing a Nix expression — a piece of code that describes how to build the package.
   In this case, it should be a _function_ that is called with the package dependencies as arguments, and returns a build of the package in the Nix store.

   ```ShellSession
   $ emacs pkgs/by-name/so/some-package/package.nix
   $ git add pkgs/by-name/so/some-package/package.nix
   ```

   If the package is written in a language other than C, you should use [the corresponding language framework](https://nixos.org/manual/nixpkgs/stable/#chap-language-support).

   You can have a look at the existing Nix expressions under `pkgs/` to see how it’s done, some of which are also using the [category hierarchy](#category-hierarchy).
   Here are some good ones:

   - GNU Hello: [`pkgs/by-name/he/hello/package.nix`](./by-name/he/hello/package.nix).
     Trivial package, which specifies some `meta` attributes which is good practice.

   - GNU cpio: [`pkgs/by-name/cp/cpio/package.nix`](./by-name/cp/cpio/package.nix).
     Also a simple package.
     The generic builder in `stdenv` does everything for you.
     It has no dependencies beyond `stdenv`.

   - GNU Multiple Precision arithmetic library (GMP): [`pkgs/development/libraries/gmp`](development/libraries/gmp).
     Also done by the generic builder, but has a dependency on `m4`.

   - Pan, a GTK-based newsreader: [`pkgs/by-name/pa/pan/package.nix`](./by-name/pa/pan/package.nix).
     Has an optional dependency on `gspell`, which is only built if `spellCheck` is `true`.

   - Apache HTTPD: [`pkgs/servers/http/apache-httpd/2.4.nix`](servers/http/apache-httpd/2.4.nix).
     A bunch of optional features, variable substitutions in the configure flags, a post-install hook, and miscellaneous hackery.

   - buildMozillaMach: [`pkgs/build-support/build-mozilla-mach/default.nix`](./build-support/build-mozilla-mach/default.nix).
     A reusable build function for Firefox, Thunderbird and Librewolf.

   - JDiskReport, a Java utility: [`pkgs/by-name/jd/jdiskreport/package.nix`](./by-name/jd/jdiskreport/package.nix).
     Nixpkgs doesn’t have a decent `stdenv` for Java yet so this is pretty ad-hoc.

   - XML::Simple, a Perl module: [`pkgs/top-level/perl-packages.nix`](top-level/perl-packages.nix) (search for the `XMLSimple` attribute).
     Most Perl modules are so simple to build that they are defined directly in `perl-packages.nix`; no need to make a separate file for them.

   - Discord Game SDK: [`pkgs/by-name/di/discord-gamesdk/package.nix`](./by-name/di/discord-gamesdk/package.nix).
     Shows how binary-only packages can be supported.
     In particular, the `autoPatchelfHook` is used to set the RUNPATH and ELF interpreter of the executables so that the right libraries are found at runtime.

   Some notes:

   - Add yourself as the maintainer of the package.

     - If this is your first time contributing (welcome!), [add yourself to the maintainers list](../maintainers/README.md#how-to-become-a-maintainer) in a separate commit.

   - All other [`meta`](https://nixos.org/manual/nixpkgs/stable/#chap-meta) attributes are optional, but it’s still a good idea to provide at least the `description`, `homepage` and [`license`](https://nixos.org/manual/nixpkgs/stable/#sec-meta-license).

   - The exact syntax and semantics of the Nix expression language, including the built-in functions, can be found in the [Nix language reference](https://nixos.org/manual/nix/stable/language/).

5. To test whether the package builds, run the following command from the root of the nixpkgs source tree:

   ```ShellSession
   $ nix-build -A some-package
   ```

   where `some-package` should be the package name.
   You may want to add the flag `-K` to keep the temporary build directory in case something fails.
   If the build succeeds, a symlink `./result` to the package in the Nix store is created.

6. If you want to install the package into your profile (optional), do

   ```ShellSession
   $ nix-env -f . -iA libfoo
   ```

7. Optionally commit the new package and open a pull request [to nixpkgs](https://github.com/NixOS/nixpkgs/pulls), or use [the Patches category](https://discourse.nixos.org/t/about-the-patches-category/477) on Discourse for sending a patch without a GitHub account.

## Commit conventions

- Make sure you read about the [commit conventions](../CONTRIBUTING.md#commit-conventions) common to Nixpkgs as a whole.

- Format the commit messages in the following way:

  ```
  (pkg-name): (from -> to | init at version | refactor | etc)

  (Motivation for change. Link to release notes. Additional information.)
  ```

  Examples:

  * nginx: init at 2.0.1
  * qt6Packages.qtdeclarative: fix build
  * firefox: 54.0.1 -> 55.0

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

(using "→" instead of "->" is also accepted)

For package sets with multiple versions, such as `perlPackages` (aliased to
`perl5Packages`) and `python3Packages` (aliased to `python313Packages` at the
time of writing), please use the unversioned attribute in your commit message
unless the change is specific to one version.

Using the `(pkg-name):` prefix is important beyond just being a convention: it queues automatic builds by CI.
More sophisticated prefixes are also possible:

| Message                                                                  | Automatic Builds                                           |
|--------------------------------------------------------------------------|------------------------------------------------------------|
| `vim: 1.0.0 -> 2.0.0`                                                    | `vim`                                                      |
| `vagrant: fix dependencies for version 2.0.2`                            | `vagrant`                                                  |
| `python3Packages.requests: 1.0.0 -> 2.0.0`                               | `python3Packages.requests`                                 |
| `python3Packagess.{numpy,scipy}: fix build`                              | `python3Packages.numpy` , `python3Packages.scipy`          |

When opening a PR with multiple commits, CI creates a single build job for all detected packages.
If `passthru.tests` attributes are available, these will be built as well.

If the title of the _PR_ begins with `WIP:` or contains `[WIP]` anywhere, its packages are not built automatically.
Other than that, PR titles have meaning only for humans.
It is recommended to keep the PR title in sync with the commit title, to make it easier to find.
For PRs with multiple commits, the PR title should be a general summary of these commits.

> [!NOTE]
> Marking a PR as a draft does not prevent automatic builds.

## Category Hierarchy
[categories]: #category-hierarchy

Most top-level packages are organised in a loosely-categorised directory hierarchy in this directory.
See the [overview](#overview) for which directories are part of this.

This category hierarchy is partially deprecated and will be migrated away over time.
The new `pkgs/by-name` directory ([docs](./by-name/README.md)) should be preferred instead.
The category hierarchy may still be used for packages that should be imported using an alternate `callPackage`, such as `python3Packages.callPackage` or `libsForQt5.callPackage`.

If that is the case for a new package, here are some rules for picking the right category.
Many packages fall under several categories; what matters is the _primary_ purpose of a package.
For example, the `libxml2` package builds both a library and some tools; but it’s a library foremost, so it goes under `pkgs/development/libraries`.

<details>
<summary>Categories</summary>

**If it’s used to support _software development_:**

- **If it’s a _library_ used by other packages:**

  - `development/libraries` (e.g. `libxml2`)

- **If it’s a _compiler_:**

  - `development/compilers` (e.g. `gcc`)

- **If it’s an _interpreter_:**

  - `development/interpreters` (e.g. `guile`)

- **If it’s a (set of) development _tool(s)_:**

  - **If it’s a _parser generator_ (including lexers):**

    - `development/tools/parsing` (e.g. `bison`, `flex`)

  - **If it’s a _build manager_:**

    - `development/tools/build-managers` (e.g. `gnumake`)

  - **If it’s a _language server_:**

    - `development/tools/language-servers` (e.g. `ccls` or `nil`)

  - **Else:**

    - `development/tools/misc` (e.g. `binutils`)

- **Else:**

  - `development/misc`

**If it’s a (set of) _tool(s)_:**

(A tool is a relatively small program, especially one intended to be used non-interactively.)

- **If it’s for _networking_:**

  - `tools/networking` (e.g. `wget`)

- **If it’s for _text processing_:**

  - `tools/text` (e.g. `diffutils`)

- **If it’s a _system utility_, i.e., something related or essential to the operation of a system:**

  - `tools/system` (e.g. `cron`)

- **If it’s an _archiver_ (which may include a compression function):**

  - `tools/archivers` (e.g. `zip`, `tar`)

- **If it’s a _compression_ program:**

  - `tools/compression` (e.g. `gzip`, `bzip2`)

- **If it’s a _security_-related program:**

  - `tools/security` (e.g. `nmap`, `gnupg`)

- **Else:**

  - `tools/misc`

**If it’s a _shell_:**

- `shells` (e.g. `bash`)

**If it’s a _server_:**

- **If it’s a web server:**

  - `servers/http` (e.g. `apache-httpd`)

- **Else:**

  - `servers/misc`

**If it’s a _desktop environment_:**

- `desktops` (e.g. `kde`, `gnome`, `enlightenment`)

**If it’s a _window manager_:**

- `applications/window-managers` (e.g. `awesome`, `stumpwm`)

**If it’s an _application_:**

A (typically large) program with a distinct user interface, primarily used interactively.

- **If it’s a _version management system_:**

  - `applications/version-management` (e.g. `subversion`)

- **If it’s a _terminal emulator_:**

  - `applications/terminal-emulators` (e.g. `alacritty` or `rxvt`)

- **If it’s a _file manager_:**

  - `applications/file-managers` (e.g. `mc` or `ranger` or `pcmanfm`)

- **If it’s for _video playback / editing_:**

  - `applications/video` (e.g. `vlc`)

- **If it’s for _graphics viewing / editing_:**

  - `applications/graphics` (e.g. `gimp`)

- **If it’s for _networking_:**

  - **If it’s a _mailreader_:**

    - `applications/networking/mailreaders` (e.g. `thunderbird`)

  - **If it’s a _newsreader_:**

    - `applications/networking/newsreaders` (e.g. `pan`)

  - **If it’s a _web browser_:**

    - `applications/networking/browsers` (e.g. `firefox`)

  - **Else:**

    - `applications/networking/misc`

- **Else:**

  - `applications/misc`

**If it’s _data_ (i.e., does not have a straight-forward executable semantics):**

- **If it’s a _font_:**

  - `data/fonts`

- **If it’s an _icon theme_:**

  - `data/icons`

- **If it’s related to _SGML/XML processing_:**

  - **If it’s an _XML DTD_:**

    - `data/sgml+xml/schemas/xml-dtd` (e.g. `docbook`)

  - **If it’s an _XSLT stylesheet_:**

    (Okay, these are executable...)

    - `data/sgml+xml/stylesheets/xslt` (e.g. `docbook-xsl`)

- **If it’s a _theme_ for a _desktop environment_, a _window manager_ or a _display manager_:**

  - `data/themes`

**If it’s a _game_:**

- `games`

**Else:**

- `misc`

</details>

# Conventions

The key words _must_, _must not_, _required_, _shall_, _shall not_, _should_, _should not_, _recommended_, _may_, and _optional_ in this section are to be interpreted as described in [RFC 2119](https://tools.ietf.org/html/rfc2119).
Only _emphasized_ words are to be interpreted in this way.

## Package naming

In Nixpkgs, there are generally three different names associated with a package:

- The `pname` attribute of the derivation.
  This is what most users see, in particular when using `nix-env`.

- The attribute name used for the package in the [`pkgs/by-name` structure](./by-name/README.md) or in [`all-packages.nix`](./top-level/all-packages.nix), and when passing it as a dependency in recipes.

- The filename for (the directory containing) the Nix expression.

Most of the time, these are the same.
For instance, the package `e2fsprogs` has a `pname` attribute `"e2fsprogs"`, is bound to the attribute name `e2fsprogs` in `all-packages.nix`, and the Nix expression is in `pkgs/os-specific/linux/e2fsprogs/default.nix`.

Follow these guidelines:

- For the `pname` attribute:

  - It _should_ be identical to the upstream package name.

  - It _must not_ contain uppercase letters.

    Example: Use `"mplayer"` instead of `"MPlayer"`

- For the package attribute name:

  - It _must_ be a valid identifier in Nix.

  - If the `pname` starts with a digit, the attribute name _should_ be prefixed with an underscore.
    Otherwise the attribute name _should not_ be prefixed with an underscore.

    Example: The corresponding attribute name for `0ad` should be `_0ad`.

  - New attribute names _should_ be the same as the value in `pname`.

    Hyphenated names _should not_ be converted to [snake case](https://en.wikipedia.org/wiki/Snake_case) or [camel case](https://en.wikipedia.org/wiki/Camel_case).
    This was done historically, but is not necessary any more.
    [The Nix language allows dashes in identifiers since 2012](https://github.com/NixOS/nix/commit/95c74eae269b2b9e4bc514581b5caa1d80b54acc).

  - If there are multiple versions of a package, this _should_ be reflected in the attribute names in `all-packages.nix`.

    Example: `json-c_0_9` and `json-c_0_11`

    If there is an obvious “default” version, make an extra attribute.

    Example: `json-c = json-c_0_9;`

    See also [versioning][versioning].

## Versioning
[versioning]: #versioning

These are the guidelines for the `version` attribute of a package:

- It _must_ start with a digit.
  This is required for backwards-compatibility with [how `nix-env` parses derivation names](https://nix.dev/manual/nix/latest/command-ref/nix-env#selectors).

  Example: `"0.3.1rc2"` or `"0-unstable-1970-01-01"`

- If a package is a commit from a repository without a version assigned, then the `version` attribute _should_ be the latest upstream version preceding that commit, followed by `-unstable-` and the date of the (fetched) commit.
  The date _must_ be in `"YYYY-MM-DD"` format.

  Example: Given a project had its latest releases `2.2` in November 2021 and `3.0` in January 2022, a commit authored on March 15, 2022 for an upcoming bugfix release `2.2.1` would have `version = "2.2-unstable-2022-03-15"`.

- If a project has no suitable preceding releases - e.g., no versions at all, or an incompatible versioning or tagging scheme - then the latest upstream version in the above schema should be `0`.

  Example: Given a project that has no tags or released versions at all, or applies versionless tags like `latest` or `YYYY-MM-DD-Build`, a commit authored on March 15, 2022 would have `version = "0-unstable-2022-03-15"`.

Because every version of a package in Nixpkgs creates a potential maintenance burden, old versions of a package should not be kept unless there is a good reason to do so.
For instance, Nixpkgs contains several versions of GCC because other packages don’t build with the latest version of GCC.
Other examples are having both the latest stable and latest pre-release version of a package, or to keep several major releases of an application that differ significantly in functionality.

If there is only one version of a package, its Nix expression should be named (e.g) `pkgs/by-name/xy/xyz/package.nix`.
If there are multiple versions, this should be reflected in the attribute name.
If you wish to share code between the Nix expressions of each version, you cannot rely upon `pkgs/by-name`'s automatic attribute creation, and must create the attributes yourself in `all-packages.nix`.
See also [`pkgs/by-name/README.md`'s section on this topic](https://github.com/NixOS/nixpkgs/blob/master/pkgs/by-name/README.md#recommendation-for-new-packages-with-multiple-versions).

## Meta attributes

The `meta` attribute set should always be placed last in the derivation and any other "meta"-like attribute sets like `passthru` should be written before it.

* `meta.description` must:
  * Be short, just one sentence.
  * Be capitalized.
  * Not start with the definite or an indefinite article.
  * Not start with the package name.
    * More generally, it should not refer to the package name.
  * Not end with a period (or any punctuation for that matter).
  * Provide factual information.
    * Avoid subjective language.
* `meta.license` must be set and match the upstream license.
  * If there is no upstream license, `meta.license` should default to `lib.licenses.unfree`.
  * If in doubt, try to contact the upstream developers for clarification.
* `meta.sourceProvenance` must be set if the package is not built from source.
  * If you are repackaging a `.deb`, `.rpm`, `.whl`, or any other format provided by your upstream, this should almost always be set to `lib.sourceTypes.binaryNativeCode`.
* `meta.mainProgram` must be set to the name of the executable which facilitates the primary function or purpose of the package, if there is such an executable in `$bin/bin/` (or `$out/bin/`, if there is no `"bin"` output).
  * Packages that only have a single executable in the applicable directory above should set `meta.mainProgram`.
    For example, the package `ripgrep` only has a single executable `rg` under `$out/bin/`, so `ripgrep.meta.mainProgram` is set to `"rg"`.
  * Packages like `polkit_gnome` that have no executables in the applicable directory should not set `meta.mainProgram`.
  * Packages like `e2fsprogs` that have multiple executables, none of which can be considered the main program, should not set `meta.mainProgram`.
  * Packages which are not primarily used for a single executable do not need to set `meta.mainProgram`.
  * Always prefer using a hardcoded string (don't use `pname`, for example).
  * When in doubt, ask for reviewer input.
* `meta.maintainers` must be set for new packages.

See the Nixpkgs manual for more details on [standard meta-attributes](https://nixos.org/nixpkgs/manual/#sec-standard-meta-attributes).

## Import From Derivation

[Import From Derivation](https://nixos.org/manual/nix/unstable/language/import-from-derivation) (IFD) is disallowed in Nixpkgs for performance reasons:
[Hydra](https://github.com/NixOS/hydra) evaluates the entire package set, and sequential builds during evaluation would increase evaluation times to become impractical.

Import From Derivation can be worked around in some cases by committing generated intermediate files to version control and reading those instead.

## `overrideAttrs` and `overridePythonAttrs`

Please do not introduce new uses of `overrideAttrs` or `overridePythonAttrs` in Nixpkgs.
These functions are useful for out-of-tree code because they allow easy overriding a package without changing its source in Nixpkgs, but when contributing to Nixpkgs you *can* change the source of other packages. So instead of using the escape hatch that is overriding, you should try to provide proper support for the functionality you need, in ways that are visible and can be understood and accounted for by the maintainers of the patched package.
Using `overrideAttrs` and `overridePythonAttrs` in Nixpkgs causes maintainability problems:

* It's easy for multiple packages to end up duplicating basically the same override without noticing.
* It's not clear when working on an overridden package that it's being overridden elsewhere in Nixpkgs, so `overrideAttrs` and `overridePythonAttrs` are fragile and can break accidentally when the overridden package is changed.
* Package maintainers will not be requested for review of overrides, even though they are likely to have important knowledge about the package.
* It is easy for overridden packages to be forgotten and remain around long after they are no longer necessary.
* Dependency closures end up being bigger than necessary due to unnecessarily including multiple versions of the same package.

Instead, keep all instances of the same package next to each other, and try to minimize how many different instances of a package are in Nixpkgs.
If you need a patch applied to a dependency, discuss with the maintainer of that dependency whether it would be acceptable to apply to the main version of the package.
If you need a different version of a dependency, first try modifying your package to work with the version in Nixpkgs — it's often not very hard! — and if that's not possible, try to factor out a function that can build multiple versions of the package, including the main version.
If you need to enable or disable optional functionality of a dependency, add an explicit flag to the package and use `override` instead.

## Sources

Always fetch source files using [Nixpkgs fetchers](https://nixos.org/manual/nixpkgs/unstable/#chap-pkgs-fetchers).
Use reproducible sources with a high degree of availability.
Prefer protocols that support proxies.

A list of schemes for `mirror://` URLs can be found in [`pkgs/build-support/fetchurl/mirrors.nix`](build-support/fetchurl/mirrors.nix), and is supported by [`fetchurl`](https://nixos.org/manual/nixpkgs/unstable/#fetchurl).
Other fetchers which end up relying on `fetchurl` may also support mirroring.

The preferred source hash type is `sha256`.

Examples going from bad to best practices:

- Bad: Uses `git://` which won't be proxied.

  ```nix
  {
    src = fetchgit {
      url = "git://github.com/NixOS/nix.git";
      rev = "1f795f9f44607cc5bec70d1300150bfefcef2aae";
      hash = "sha256-7D4m+saJjbSFP5hOwpQq2FGR2rr+psQMTcyb1ZvtXsQ=";
    };
  }
  ```

- Better: This is ok, but an archive fetch will still be faster.

  ```nix
  {
    src = fetchgit {
      url = "https://github.com/NixOS/nix.git";
      rev = "1f795f9f44607cc5bec70d1300150bfefcef2aae";
      hash = "sha256-7D4m+saJjbSFP5hOwpQq2FGR2rr+psQMTcyb1ZvtXsQ=";
    };
  }
  ```

- Best: Fetches a snapshot archive for the given revision.

  ```nix
  {
    src = fetchFromGitHub {
      owner = "NixOS";
      repo = "nix";
      rev = "1f795f9f44607cc5bec70d1300150bfefcef2aae";
      hash = "sha256-7D4m+saJjbSFP5hOwpQq2FGR2rr+psQMTcyb1ZvtXsQ=";
    };
  }
  ```

> [!Note]
> When fetching from GitHub, always reference revisions by their full commit hash.
> GitHub shares commit hashes among all forks and returns `404 Not Found` when a short commit hash is ambiguous.
> It already happened in Nixpkgs for short, 6-character commit hashes.
>
> Pushing large amounts of auto generated commits into forks is a practical vector for a denial-of-service attack, and was already [demonstrated against GitHub Actions Beta](https://blog.teddykatz.com/2019/11/12/github-actions-dos.html).

## Patches

Sometimes, changes are needed to the source to allow building a derivation in nixpkgs, or to get earlier access to an upstream fix or improvement.
When using the `patches` parameter to `mkDerivation`, make sure the patch name clearly describes the reason for the patch, or add a comment.

> [!Note]
> The version of the package does not need to be changed just because a patch is applied.
> Declarative package installations don't depend on the version, while imperative `nix-env` installations can use [`upgrade --eq/leq/--always`](https://nix.dev/manual/nix/2.25/command-ref/nix-env/upgrade#flags).
>
> See [Versioning](#versioning) for details on package versioning.

The following describes two ways to include the patch.
Regardless of how the patch is included, you _must_ ensure its purpose is clear and obvious.
This enables other maintainers to more easily determine when old patches are no longer required.
Typically, you can improve clarity with carefully considered filenames, attribute names, and/or comments; these should explain the patch's _intention_.
Additionally, it may sometimes be helpful to clarify _how_ it resolves the issue.
For example: _"fix gcc14 build by adding missing include"_.

### Fetching patches

In the interest of keeping our maintenance burden and the size of Nixpkgs to a minimum, patches already merged upstream or published elsewhere _should_ be retrieved using `fetchpatch2`:

```nix
{
  patches = [
    (fetchpatch2 {
      name = "make-no-atomics-a-soft-failure.patch";
      url = "https://github.com/boostorg/math/commit/7d482f6ebc356e6ec455ccb5f51a23971bf6ce5b.patch?full_index=1";
      hash = "sha256-9Goa0NTUdSOs1Vm+FnkoSFhw0o8ZLNOw6cLUqCVnF5Y=";
    })
  ];
}
```

> [!Warning]
> If the patch file contains short commit hashes, use `fetchpatch` instead of `fetchpatch2` ([tracking issue](https://github.com/NixOS/nixpkgs/issues/257446)).
> This is the case if the patch contains a line similar to `index 0c97fcc35..f533e464a 100644`.
> Depending on the patch source it is possible to expand the commit hash, in which case using `fetchpatch2` is acceptable (e.g. GitHub supports appending `?full_index=1` to the URL, as seen above).

If a patch is available online but does not cleanly apply, it can be modified in some fixed ways by using additional optional arguments for `fetchpatch2`.
Check [the `fetchpatch` reference](https://nixos.org/manual/nixpkgs/unstable/#fetchpatch) for details.

When adding patches in this manner you should be reasonably sure that the used URL is stable.
Patches referencing open pull requests will change when the PR is updated and code forges (such as GitHub) usually garbage collect commits that are no longer reachable due to rebases/amends.

### Vendoring patches

In the following cases, a `.patch` file _should_ be added to Nixpkgs repository, instead of retrieved:

- solves problems unique to packaging in Nixpkgs
- cannot be fetched easily
- has a high chance to disappear in the future due to unstable or unreliable URLs

The latter avoids link rot when the upstream abandons, squashes or rebases their change, in which case the commit may get garbage-collected.

```nix
{ patches = [ ./0001-add-missing-include.patch ]; }
```

If you do need to create this sort of patch file, one way to do so is with git:

1. Move to the root directory of the source code you're patching.

    ```ShellSession
    $ cd the/program/source
    ```

2. If a git repository is not already present, create one and stage all of the source files.

    ```ShellSession
    $ git init
    $ git add -A
    ```

3. Edit some files to make whatever changes need to be included in the patch.

4. Use git to create a diff, and pipe the output to a patch file:

    ```ShellSession
    $ git diff -a > nixpkgs/pkgs/the/package/0001-changes.patch
    ```

## Deprecating/removing packages

There is currently no policy when to remove a package.

Before removing a package, one should try to find a new maintainer or fix smaller issues first.

### Steps to remove a package from Nixpkgs

We use jbidwatcher as an example for a discontinued project here.

1. Have Nixpkgs checked out locally and up to date.
1. Create a new branch for your change, e.g. `git checkout -b jbidwatcher`
1. Remove the actual package including its directory, e.g. `git rm -rf pkgs/applications/misc/jbidwatcher`
1. Remove the package from the list of all packages (`pkgs/top-level/all-packages.nix`).
1. Add an alias for the package name in `pkgs/top-level/aliases.nix` (There is also `pkgs/applications/editors/vim/plugins/aliases.nix`.
   Package sets typically do not have aliases, so we can't add them there.)

    For example in this case:

    ```nix
    {
      jbidwatcher = throw "jbidwatcher was discontinued in march 2021"; # added 2021-03-15
    }
    ```

    The throw message should explain in short why the package was removed for users that still have it installed.

1. Test if the changes introduced any issues by running `nix-env -qaP -f . --show-trace`.
   It should show the list of packages without errors.
1. Commit the changes.
   Explain again why the package was removed.
   If it was declared discontinued upstream, add a link to the source.

    ```ShellSession
    $ git add pkgs/applications/misc/jbidwatcher/default.nix pkgs/top-level/all-packages.nix pkgs/top-level/aliases.nix
    $ git commit
    ```

    Example commit message:

    ```
    jbidwatcher: remove

    project was discontinued in march 2021. the program does not work anymore because ebay changed the login.

    https://web.archive.org/web/20210315205723/http://www.jbidwatcher.com/
    ```

1. Push changes to your GitHub fork with `git push`
1. Create a pull request against Nixpkgs.
   Mention the package maintainer.

This is what the pull request looks like in this case: [https://github.com/NixOS/nixpkgs/pull/116470](https://github.com/NixOS/nixpkgs/pull/116470)

## Package tests

To run the main types of tests locally:

- Run package-internal tests with `nix-build --attr pkgs.PACKAGE.passthru.tests`
- Run [NixOS tests](https://nixos.org/manual/nixos/unstable/#sec-nixos-tests) with `nix-build --attr nixosTests.NAME`, where `NAME` is the name of the test listed in `nixos/tests/all-tests.nix`
- Run [global package tests](https://nixos.org/manual/nixpkgs/unstable/#sec-package-tests) with `nix-build --attr tests.PACKAGE`, where `PACKAGE` is the name of the test listed in `pkgs/test/default.nix`
- See `lib/tests/NAME.nix` for instructions on running specific library tests

Tests are important to ensure quality and make reviews and automatic updates easy.

The following types of tests exist:

* [NixOS **module tests**](https://nixos.org/manual/nixos/stable/#sec-nixos-tests), which spawn one or more NixOS VMs.
  They exercise both NixOS modules and the packaged programs used within them.
  For example, a NixOS module test can start a web server VM running the `nginx` module, and a client VM running `curl` or a graphical `firefox`, and test that they can talk to each other and display the correct content.
* Nix **package tests** are a lightweight alternative to NixOS module tests.
  They should be used to create simple integration tests for packages, but cannot test NixOS services, and some programs with graphical user interfaces may also be difficult to test with them.
* The **`checkPhase` of a package**, which should execute the unit tests that are included in the source code of a package.

Here in the nixpkgs manual we describe mostly _package tests_; for _module tests_ head over to the corresponding [section in the NixOS manual](https://nixos.org/manual/nixos/stable/#sec-nixos-tests).

### Writing inline package tests

For very simple tests, they can be written inline:

```nix
# ... ,
{ yq-go }:

buildGoModule rec {
  # …

  passthru.tests = {
    simple = runCommand "${pname}-test" { } ''
      echo "test: 1" | ${yq-go}/bin/yq eval -j > $out
      [ "$(cat $out | tr -d $'\n ')" = '{"test":1}' ]
    '';
  };
}
```

Any derivation can be specified as a test, even if it's in a different file.
Such a derivation that implements a test can depend on the package under test, even in the presence of `overrideAttrs`.

In the following example, `(my-package.overrideAttrs f).passthru.tests` will work as expected, as long as the definition of `tests` does not rely on the original `my-package` or overrides all occurrences of `my-package`:

```nix
# my-package/default.nix
{ stdenv, callPackage }:
stdenv.mkDerivation (finalAttrs: {
  # ...
  passthru.tests.example = callPackage ./example.nix { my-package = finalAttrs.finalPackage; };
})
```

```nix
# my-package/example.nix
{
  runCommand,
  lib,
  my-package,
  ...
}:
runCommand "my-package-test"
  {
    nativeBuildInputs = [ my-package ];
    src = lib.sources.sourcesByRegex ./. [
      ".*.in"
      ".*.expected"
    ];
  }
  ''
    my-package --help
    my-package <example.in >example.actual
    diff -U3 --color=auto example.expected example.actual
    mkdir $out
  ''
```

### Writing larger package tests
[larger-package-tests]: #writing-larger-package-tests

This is an example using the `phoronix-test-suite` package with the current best practices.

Add the tests in `passthru.tests` to the package definition like this:

```nix
{
  stdenv,
  lib,
  fetchurl,
  callPackage,
}:

stdenv.mkDerivation {
  # …

  passthru.tests = {
    simple-execution = callPackage ./tests.nix { };
  };

  meta = {
    # …
  };
}
```

Create `tests.nix` in the package directory:

```nix
{ runCommand, phoronix-test-suite }:

let
  inherit (phoronix-test-suite) pname version;

in
runCommand "${pname}-tests" { meta.timeout = 60; } ''
  # automatic initial setup to prevent interactive questions
  ${phoronix-test-suite}/bin/phoronix-test-suite enterprise-setup >/dev/null
  # get version of installed program and compare with package version
  if [[ `${phoronix-test-suite}/bin/phoronix-test-suite version` != *"${version}"*  ]]; then
    echo "Error: program version does not match package version"
    exit 1
  fi
  # run dummy command
  ${phoronix-test-suite}/bin/phoronix-test-suite dummy_module.dummy-command >/dev/null
  # needed for Nix to register the command as successful
  touch $out
''
```

### Running package tests

You can run these tests with:

```ShellSession
$ cd path/to/nixpkgs
$ nix-build -A phoronix-test-suite.tests
```

### Examples of package tests

Here are examples of package tests:

- [Jasmin compile test](by-name/ja/jasmin/test-assemble-hello-world/default.nix)
- [Lobster compile test](by-name/lo/lobster/test-can-run-hello-world.nix)
- [Spacy annotation test](development/python-modules/spacy/annotation-test/default.nix)
- [Libtorch test](development/libraries/science/math/libtorch/test/default.nix)
- [Multiple tests for nanopb](./by-name/na/nanopb/package.nix)

### Linking NixOS module tests to a package

Like [package tests][larger-package-tests] as shown above, [NixOS module tests](https://nixos.org/manual/nixos/stable/#sec-nixos-tests) can also be linked to a package, so that the tests can be easily run when changing the related package.

For example, assuming we're packaging `nginx`, we can link its module test via `passthru.tests`:

```nix
{
  stdenv,
  lib,
  nixosTests,
}:

stdenv.mkDerivation {
  # ...

  passthru.tests = {
    nginx = nixosTests.nginx;
  };

  # ...
}
```

## Automatic package updates
[automatic-package-updates]: #automatic-package-updates

The [community bot `r-ryantm`](https://nix-community.org/update-bot/), periodically tries to update all packages in Nixpkgs.
`r-ryantm` runs the program [`nixpkgs-update`](https://nix-community.github.io/nixpkgs-update/) to find new versions of packages.
In most cases, `nixpkgs-update` will be capable of finding new versions and perform the update with out any special instructions.
Putting a `passthru.updateScript` attribute sets an explicit update procedure for `nixpkgs-update`, but this is not required for most cases.
To learn more about the default update procedures, read their [FAQ for Nixpkgs maintainers](https://nix-community.github.io/nixpkgs-update/nixpkgs-maintainer-faq/).

> [!Note]
> A common pattern is to use the [`nix-update-script`](../pkgs/by-name/ni/nix-update/nix-update-script.nix) function provided in Nixpkgs, which makes automatic updates use [`nix-update`](https://github.com/Mic92/nix-update):
>
> ```nix
> { stdenv, nix-update-script }:
> stdenv.mkDerivation {
>   # ...
>   passthru.updateScript = nix-update-script { };
> }
> ```
>
> `nix-update` is a little bit more flexible than `nixpkgs-update` in performing updates, so it can be useful for cases such as:
>
>  - A `nix-update` CLI flag like `--version branch` or `--version-regex` are needed to make the update work.
>  - You don't want to rely upon new versions to be listed in [Repology](https://repology.org/), and `nix-update` finds new versions easily (e.g GitLab projects).

The `passthru.updateScript` attribute can contain one of the following:

- an executable file, either on the file system:

  ```nix
  { stdenv }:
  stdenv.mkDerivation {
    # ...
    passthru.updateScript = ./update.sh;
  }
  ```

  or inside the expression itself:

  ```nix
  { stdenv, writeScript }:
  stdenv.mkDerivation {
    # ...
    passthru.updateScript = writeScript "update-zoom-us" ''
      #!/usr/bin/env nix-shell
      #!nix-shell -i bash -p curl pcre2 common-updater-scripts

      set -eu -o pipefail

      version="$(curl -sI https://zoom.us/client/latest/zoom_x86_64.tar.xz | grep -Fi 'Location:' | pcre2grep -o1 '/(([0-9]\.?)+)/')"
      update-source-version zoom-us "$version"
    '';
  }
  ```

- a list, a script file followed by arguments to be passed to it:

  ```nix
  { stdenv }:
  stdenv.mkDerivation {
    # ...
    passthru.updateScript = [
      ../../update.sh
      pname
      "--requested-release=unstable"
    ];
  }
  ```

- an attribute set containing:
  - `command`

    A string or list in the [format expected by `passthru.updateScript`][automatic-package-updates]

  - `attrPath` (optional)

    A string containing the canonical attribute path for the package.

    If present, it will be passed to the update script instead of the attribute path on which the package was discovered during Nixpkgs traversal.

  - `supportedFeatures` (optional)

    A list of the [extra features the script supports][supported-features].

    ```nix
    { stdenv }:
    stdenv.mkDerivation rec {
      pname = "my-package";
      # ...
      passthru.updateScript = {
        command = [
          ../../update.sh
          pname
        ];
        attrPath = pname;
        supportedFeatures = [
          # ...
        ];
      };
    }
    ```

### How are update scripts executed?

Update scripts are to be invoked by the [automatic package update script](../maintainers/scripts/update.nix).
You can run `nix-shell maintainers/scripts/update.nix` in the root of Nixpkgs repository for information on how to use it.
`update.nix` offers several modes for selecting packages to update, and it will execute update scripts for all matched packages that have an `updateScript` attribute.

Update scripts will be run inside the [Nixpkgs development shell](../shell.nix), providing access to some useful tools for CI.
Furthermore each update script will be passed the following environment variables:

- [`UPDATE_NIX_NAME`] – content of the `name` attribute of the updated package
- [`UPDATE_NIX_PNAME`] – content of the `pname` attribute of the updated package
- [`UPDATE_NIX_OLD_VERSION`] – content of the `version` attribute of the updated package
- [`UPDATE_NIX_ATTR_PATH`] – attribute path the `update.nix` discovered the package on (or the package's specified `attrPath` when available).
  Example: `pantheon.elementary-terminal`

> [!Note]
> An update script will be usually run from the root of the Nixpkgs repository, but you should not rely on that.
> Also note that `update.nix` executes update scripts in parallel by default, so you should avoid running `git commit` or any other commands that cannot handle that.

While update scripts should not create commits themselves, `update.nix` supports automatically creating commits when running it with `--arg commit true`.
If you need to customize commit message, you can have the update script implement the `commit` feature.

### Supported features
[update-script-supported-features]: #supported-features

- `commit`

  This feature allows update scripts to *ask* `update.nix` to create Git commits.

  When support of this feature is declared, whenever the update script exits with `0` return status, it is expected to print a JSON list containing an object described below for each updated attribute to standard output.
  Example:

  ```json
  [
    {
      "attrPath": "volume_key",
      "oldVersion": "0.3.11",
      "newVersion": "0.3.12",
      "files": [
        "/path/to/nixpkgs/pkgs/development/libraries/volume-key/default.nix"
      ]
    }
  ]
  ```
  :::

  When `update.nix` is run with `--arg commit true`, it will create a separate commit for each of the objects.
  An empty list can be returned when the script did not update any files; for example, when the package is already at the latest version.

  The commit object contains the following values:

  - `attrPath` – a string containing the attribute path
  - `oldVersion` – a string containing the old version
  - `newVersion` – a string containing the new version
  - `files` – a non-empty list of file paths (as strings) to add to the commit
  - `commitBody` (optional) – a string with extra content to be appended to the default commit message (useful for adding changelog links)
  - `commitMessage` (optional) – a string to use instead of the default commit message

  If the returned list contains exactly one object (e.g. `[{}]`), all values are optional and will be determined automatically.

## Reviewing contributions

### Package updates

A package update is the most trivial and common type of pull request.
These pull requests mainly consist of updating the version part of the package name and the source hash.

It can happen that non-trivial updates include patches or more complex changes.

Reviewing process:

- Ensure that the package versioning [fits the guidelines](#versioning).
- Ensure that the commit text [fits the guidelines](../CONTRIBUTING.md#commit-conventions).
- Ensure that the package maintainers are notified.
  - The continuous integration system will make GitHub notify users based on the submitted changes, but it can happen that it misses some of the package maintainers.
- Ensure that the meta field information [fits the guidelines](#meta-attributes) and is correct:
  - License can change with version updates, so it should be checked to match the upstream license.
  - If the package has no maintainer, a maintainer must be set.
    This can be the update submitter or a community member that accepts to take maintainership of the package.
- Verify any change of upstream.
  - If switching from e.g. PyPi to GitHub, verify that the repo is the official one.
  - If switching to a fork, check with external sources like other package repositories for community consensus.
- Ensure any special packaging choices and required context are documented in i.e. the name of a patch or in a comment.
  - If a special version of a package is pinned, document why, so others know if/when it can be unpinned.
  - If any (especially opinionated) patch or `substituteInPlace` is applied, document why.
  - If any non-default build flags are set, document why.
  - If checks are partially or fully disabled, document why.
- Ensure that the code contains no typos.
- Build the package locally.
  - Pull requests are often targeted to the master or staging branch, and building the pull request locally when it is submitted can trigger many source builds.
  - It is possible to rebase the changes on nixos-unstable or nixpkgs-unstable for easier review by running the following commands from a nixpkgs clone.

    ```ShellSession
    $ git fetch origin nixos-unstable
    $ git fetch origin pull/PRNUMBER/head
    $ git rebase --onto nixos-unstable BASEBRANCH FETCH_HEAD
    ```

    - The first command fetches the nixos-unstable branch.
    - The second command fetches the pull request changes, `PRNUMBER` is the number at the end of the pull request title and `BASEBRANCH` the base branch of the pull request.
    - The third command rebases the pull request changes to the nixos-unstable branch.
  - The [nixpkgs-review](https://github.com/Mic92/nixpkgs-review) tool can be used to review a pull request content in a single command.
    `PRNUMBER` should be replaced by the number at the end of the pull request title.
    You can also provide the full github pull request url.

    ```ShellSession
    $ nix-shell -p nixpkgs-review --run "nixpkgs-review pr PRNUMBER"
    ```
- Run every binary.

Sample template for a package update review is provided below.

```markdown
##### Reviewed points

- [ ] package name fits guidelines
- [ ] package version fits guidelines
- [ ] package builds on ARCHITECTURE
- [ ] executables tested on ARCHITECTURE
- [ ] any change of upstream are verified
- [ ] the motives for any special packaging choices are documented
- [ ] all depending packages build
- [ ] patches have a comment describing either the upstream URL or a reason why the patch wasn't upstreamed
- [ ] patches that are remotely available are fetched rather than vendored

##### Possible improvements

##### Comments
```

### New packages

New packages are a common type of pull requests.
These pull requests consist of adding a new nix-expression for a package.

Review process:

- Ensure that all file paths [fit the guidelines](../CONTRIBUTING.md#file-naming-and-organisation).
- Ensure that the package name and version [fits the guidelines](#package-naming).
- Ensure that the package versioning [fits the guidelines](#versioning).
- Ensure that the commit text [fits the guidelines](../CONTRIBUTING.md#commit-conventions).
- Ensure that the source is fetched from an official location, one of our [trusted mirrors](./build-support/fetchurl/mirrors.nix), or a mirror trusted by the authors.
- Ensure that the meta fields [fit the guidelines](#meta-attributes) and contain the correct information:
  - License must match the upstream license.
  - Platforms should be set (or the package will not get binary substitutes).
  - Maintainers must be set.
    This can be the package submitter or a community member that accepts taking up maintainership of the package.
  - The `meta.mainProgram` must be set if a main executable exists.
- Ensure any special packaging choices and required context are documented in, i.e., the name of a patch or in a comment.
  - If a special version of a package is pinned, document why, so others know if/when it can be unpinned.
  - If any (especially opinionated) patch or `substituteInPlace` is applied, document why.
  - If any non-default build flags are set, document why.
  - If checks are partially or fully disabled, document why.
- Report detected typos.
- Ensure the package source:
  - Uses `mirror://` URLs when available.
  - Uses the most appropriate functions (e.g. packages from GitHub should use `fetchFromGitHub`).
- Build the package locally.
- Run every binary.

Sample template for a new package review is provided below.

```markdown
##### Reviewed points

- [ ] package path fits guidelines
- [ ] package name fits guidelines
- [ ] package version fits guidelines
- [ ] package builds on ARCHITECTURE
- [ ] executables tested on ARCHITECTURE
- [ ] `meta.description` is set and fits guidelines
- [ ] `meta.license` fits upstream license
- [ ] `meta.platforms` is set
- [ ] `meta.maintainers` is set
- [ ] `meta.mainProgram` is set, if applicable.
- [ ] build time only dependencies are declared in `nativeBuildInputs`
- [ ] source is fetched from an official or trusted location
- [ ] source is fetched using the appropriate function
- [ ] the motives for any special packaging choices are documented
- [ ] the list of `phases` is not overridden
- [ ] when a phase (like `installPhase`) is overridden it starts with `runHook preInstall` and ends with `runHook postInstall`.
- [ ] patches have a comment describing either the upstream URL or a reason why the patch wasn't upstreamed
- [ ] patches that are remotely available are fetched rather than vendored

##### Possible improvements

##### Comments
```

## Security

### Submitting security fixes
[security-fixes]: #submitting-security-fixes

Security fixes are submitted in the same way as other changes and thus the same guidelines apply.

- If a new version fixing the vulnerability has been released, update the package;
- If the security fix comes in the form of a patch and a CVE is available, then add the patch to the Nixpkgs tree, and apply it to the package.
  The name of the patch should be the CVE identifier, so e.g. `CVE-2019-13636.patch`; If a patch is fetched the name needs to be set as well, e.g.:

  ```nix
  (fetchpatch {
    name = "CVE-2019-11068.patch";
    url = "https://gitlab.gnome.org/GNOME/libxslt/commit/e03553605b45c88f0b4b2980adfbbb8f6fca2fd6.patch";
    hash = "sha256-SEKe/8HcW0UBHCfPTTOnpRlzmV2nQPPeL6HOMxBZd14=";
  })
  ```

If a security fix applies to both master and a stable release then, similar to regular changes, they are preferably delivered via master first and cherry-picked to the release branch.

Critical security fixes may bypass the staging branches and be delivered directly to release branches such as `master` and `release-*`.

### Vulnerability Roundup

#### Issues

Vulnerable packages in Nixpkgs are managed using issues.
Currently opened ones can be found using the following:

[github.com/NixOS/nixpkgs/issues?q=is:issue+is:open+"Vulnerability+roundup"](https://github.com/NixOS/nixpkgs/issues?q=is%3Aissue+is%3Aopen+%22Vulnerability+roundup%22)

Each issue corresponds to a vulnerable version of a package; as a consequence:

- One issue can contain several CVEs;
- One CVE can be shared across several issues;
- A single package can be concerned by several issues.


A "Vulnerability roundup" issue usually respects the following format:

```txt
<link to relevant package search on search.nix.gsc.io>, <link to relevant files in Nixpkgs on GitHub>

<list of related CVEs, their CVSS score, and the impacted NixOS version>

<list of the scanned Nixpkgs versions>

<list of relevant contributors>
```

Note that there can be an extra comment containing links to previously reported (and still open) issues for the same package.


#### Triaging and Fixing

**Note**: An issue can be a "false positive" (i.e. automatically opened, but without the package it refers to being actually vulnerable).
If you find such a "false positive", comment on the issue an explanation of why it falls into this category, linking as much information as necessary to help maintainers double check.

If you are investigating a "true positive":

- Find the earliest patched version or a code patch in the CVE details;
- Is the issue already patched (version up-to-date or patch applied manually) in Nixpkgs's `master` branch?
  - **No**:
    - [Submit a security fix][security-fixes];
    - Once the fix is merged into `master`, [submit the change to the vulnerable release branch(es)](../CONTRIBUTING.md#how-to-backport-pull-requests);
  - **Yes**: [Backport the change to the vulnerable release branch(es)](../CONTRIBUTING.md#how-to-backport-pull-requests).
- When the patch has made it into all the relevant branches (`master`, and the vulnerable releases), close the relevant issue(s).
