guix/doc/contributing.texi

@node Contributing
@chapter Contributing

This project is a cooperative effort, and we need your help to make it
grow!  Please get in touch with us on @email{guix-devel@@gnu.org} and
@code{#guix} on the Freenode IRC network.  We welcome ideas, bug
reports, patches, and anything that may be helpful to the project.  We
particularly welcome help on packaging (@pxref{Packaging Guidelines}).

@cindex code of conduct, of contributors
@cindex contributor covenant
We want to provide a warm, friendly, and harassment-free environment, so
that anyone can contribute to the best of their abilities.  To this end
our project uses a ``Contributor Covenant'', which was adapted from
@url{https://contributor-covenant.org/}.  You can find a local version in
the @file{CODE-OF-CONDUCT} file in the source tree.

Contributors are not required to use their legal name in patches and
on-line communication; they can use any name or pseudonym of their
choice.

@menu
* Building from Git::           The latest and greatest.
* Running Guix Before It Is Installed::  Hacker tricks.
* The Perfect Setup::           The right tools.
* Packaging Guidelines::        Growing the distribution.
* Coding Style::                Hygiene of the contributor.
* Submitting Patches::          Share your work.
* Tracking Bugs and Patches::   Using Debbugs.
* Commit Access::               Pushing to the official repository.
* Updating the Guix Package::   Updating the Guix package definition.
@end menu

@node Building from Git
@section Building from Git

If you want to hack Guix itself, it is recommended to use the latest
version from the Git repository:

@example
git clone https://git.savannah.gnu.org/git/guix.git
@end example

@cindex authentication, of a Guix checkout
How do you ensure that you obtained a genuine copy of the repository?
To do that, run @command{guix git authenticate}, passing it the commit
and OpenPGP fingerprint of the @dfn{channel introduction}
(@pxref{Invoking guix git authenticate}):

@c The commit and fingerprint below must match those of the channel
@c introduction in '%default-channels'.
@example
git fetch origin keyring:keyring
guix git authenticate 9edb3f66fd807b096b48283debdcddccfea34bad \
  "BBB0 2DDF 2CEA F6A8 0D1D  E643 A2A0 6DF2 A33A 54FA"
@end example

@noindent
This command completes with exit code zero on success; it prints an
error message and exits with a non-zero code otherwise.

As you can see, there is a chicken-and-egg problem: you first need to
have Guix installed.  Typically you would install Guix System
(@pxref{System Installation}) or Guix on top of another distro
(@pxref{Binary Installation}); in either case, you would verify the
OpenPGP signature on the installation medium.  This ``bootstraps'' the
trust chain.

The easiest way to set up a development environment for Guix is, of
course, by using Guix!  The following command starts a new shell where
all the dependencies and appropriate environment variables are set up to
hack on Guix:

@example
guix environment guix --pure
@end example

@xref{Invoking guix environment}, for more information on that command.

If you are unable to use Guix when building Guix from a checkout, the
following are the required packages in addition to those mentioned in the
installation instructions (@pxref{Requirements}).

@itemize
@item @url{https://gnu.org/software/autoconf/, GNU Autoconf};
@item @url{https://gnu.org/software/automake/, GNU Automake};
@item @url{https://gnu.org/software/gettext/, GNU Gettext};
@item @url{https://gnu.org/software/texinfo/, GNU Texinfo};
@item @url{https://www.graphviz.org/, Graphviz};
@item @url{https://www.gnu.org/software/help2man/, GNU Help2man (optional)}.
@end itemize

On Guix, extra dependencies can be added by instead running @command{guix
environment} with @option{--ad-hoc}:

@example
guix environment guix --pure --ad-hoc help2man git strace
@end example

Run @command{./bootstrap} to generate the build system infrastructure
using Autoconf and Automake.  If you get an error like this one:

@example
configure.ac:46: error: possibly undefined macro: PKG_CHECK_MODULES
@end example

@noindent
it probably means that Autoconf couldn’t find @file{pkg.m4}, which is
provided by pkg-config.  Make sure that @file{pkg.m4} is available.  The
same holds for the @file{guile.m4} set of macros provided by Guile.  For
instance, if you installed Automake in @file{/usr/local}, it wouldn’t
look for @file{.m4} files in @file{/usr/share}.  In that case, you have
to invoke the following command:

@example
export ACLOCAL_PATH=/usr/share/aclocal
@end example

@xref{Macro Search Path,,, automake, The GNU Automake Manual}, for
more information.

Then, run @command{./configure} as usual.  Make sure to pass
@code{--localstatedir=@var{directory}} where @var{directory} is the
@code{localstatedir} value used by your current installation (@pxref{The
Store}, for information about this), usually @file{/var}.  Note that you
will probably not run @command{make install} at the end (you don't have
to) but it's still important to pass the right @code{localstatedir}.

Finally, you have to invoke @code{make check} to run tests
(@pxref{Running the Test Suite}).  If anything
fails, take a look at installation instructions (@pxref{Installation})
or send a message to the @email{guix-devel@@gnu.org, mailing list}.

From there on, you can authenticate all the commits included in your
checkout by running:

@example
make authenticate
@end example

The first run takes a couple of minutes, but subsequent runs are faster.

Or, when your configuration for your local Git repository doesn't match
the default one, you can provide the reference for the @code{keyring}
branch through the variable @code{GUIX_GIT_KEYRING}.  The following
example assumes that you have a Git remote called @samp{myremote}
pointing to the official repository:

@example
make authenticate GUIX_GIT_KEYRING=myremote/keyring
@end example

@quotation Note
You are advised to run @command{make authenticate} after every
@command{git pull} invocation.  This ensures you keep receiving valid
changes to the repository.
@end quotation


@node Running Guix Before It Is Installed
@section Running Guix Before It Is Installed

In order to keep a sane working environment, you will find it useful to
test the changes made in your local source tree checkout without
actually installing them.  So that you can distinguish between your
``end-user'' hat and your ``motley'' costume.

To that end, all the command-line tools can be used even if you have not
run @code{make install}.  To do that, you first need to have an
environment with all the dependencies available (@pxref{Building from
Git}), and then simply prefix each command with @command{./pre-inst-env}
(the @file{pre-inst-env} script lives in the top build tree of Guix; it
is generated by running @command{./bootstrap} followed by
@command{./configure}).  As an example, here is how you would build the
@code{hello} package as defined in your working tree (this assumes
@command{guix-daemon} is already running on your system; it's OK if it's
a different version):

@example
$ ./pre-inst-env guix build hello
@end example

@noindent
Similarly, an example for a Guile session using the Guix modules:

@example
$ ./pre-inst-env guile -c '(use-modules (guix utils)) (pk (%current-system))'

;;; ("x86_64-linux")
@end example

@noindent
@cindex REPL
@cindex read-eval-print loop
@dots{} and for a REPL (@pxref{Using Guile Interactively,,, guile, Guile
Reference Manual}):

@example
$ ./pre-inst-env guile
scheme@@(guile-user)> ,use(guix)
scheme@@(guile-user)> ,use(gnu)
scheme@@(guile-user)> (define snakes
                       (fold-packages
                         (lambda (package lst)
                           (if (string-prefix? "python"
                                               (package-name package))
                               (cons package lst)
                               lst))
                         '()))
scheme@@(guile-user)> (length snakes)
$1 = 361
@end example

If you are hacking on the daemon and its supporting code or if
@command{guix-daemon} is not already running on your system, you can
launch it straight from the build tree@footnote{The @option{-E} flag to
@command{sudo} guarantees that @code{GUILE_LOAD_PATH} is correctly set
such that @command{guix-daemon} and the tools it uses can find the Guile
modules they need.}:

@example
$ sudo -E ./pre-inst-env guix-daemon --build-users-group=guixbuild
@end example

The @command{pre-inst-env} script sets up all the environment variables
necessary to support this, including @env{PATH} and @env{GUILE_LOAD_PATH}.

Note that @command{./pre-inst-env guix pull} does @emph{not} upgrade the
local source tree; it simply updates the @file{~/.config/guix/current}
symlink (@pxref{Invoking guix pull}).  Run @command{git pull} instead if
you want to upgrade your local source tree.


@node The Perfect Setup
@section The Perfect Setup

The Perfect Setup to hack on Guix is basically the perfect setup used
for Guile hacking (@pxref{Using Guile in Emacs,,, guile, Guile Reference
Manual}).  First, you need more than an editor, you need
@url{https://www.gnu.org/software/emacs, Emacs}, empowered by the
wonderful @url{https://nongnu.org/geiser/, Geiser}.  To set that up, run:

@example
guix package -i emacs guile emacs-geiser
@end example

Geiser allows for interactive and incremental development from within
Emacs: code compilation and evaluation from within buffers, access to
on-line documentation (docstrings), context-sensitive completion,
@kbd{M-.} to jump to an object definition, a REPL to try out your code,
and more (@pxref{Introduction,,, geiser, Geiser User Manual}).  For
convenient Guix development, make sure to augment Guile’s load path so
that it finds source files from your checkout:

@lisp
;; @r{Assuming the Guix checkout is in ~/src/guix.}
(with-eval-after-load 'geiser-guile
  (add-to-list 'geiser-guile-load-path "~/src/guix"))
@end lisp

To actually edit the code, Emacs already has a neat Scheme mode.  But in
addition to that, you must not miss
@url{https://www.emacswiki.org/emacs/ParEdit, Paredit}.  It provides
facilities to directly operate on the syntax tree, such as raising an
s-expression or wrapping it, swallowing or rejecting the following
s-expression, etc.

@cindex code snippets
@cindex templates
@cindex reducing boilerplate
We also provide templates for common git commit messages and package
definitions in the @file{etc/snippets} directory.  These templates can
be used with @url{https://joaotavora.github.io/yasnippet/, YASnippet} to
expand short trigger strings to interactive text snippets.  You may want
to add the snippets directory to the @var{yas-snippet-dirs} variable in
Emacs.

@lisp
;; @r{Assuming the Guix checkout is in ~/src/guix.}
(with-eval-after-load 'yasnippet
  (add-to-list 'yas-snippet-dirs "~/src/guix/etc/snippets"))
@end lisp

The commit message snippets depend on @url{https://magit.vc/, Magit} to
display staged files.  When editing a commit message type @code{add}
followed by @kbd{TAB} to insert a commit message template for adding a
package; type @code{update} followed by @kbd{TAB} to insert a template
for updating a package; type @code{https} followed by @kbd{TAB} to
insert a template for changing the home page URI of a package to HTTPS.

The main snippet for @code{scheme-mode} is triggered by typing
@code{package...} followed by @kbd{TAB}.  This snippet also inserts the
trigger string @code{origin...}, which can be expanded further.  The
@code{origin} snippet in turn may insert other trigger strings ending on
@code{...}, which also can be expanded further.

@cindex insert or update copyright
@cindex @code{M-x guix-copyright}
@cindex @code{M-x copyright-update}
We additionally provide insertion and automatic update of a copyright in
@file{etc/copyright.el}.  You may want to set your full name, mail, and
load a file.

@lisp
(setq user-full-name "Alice Doe")
(setq user-mail-address "alice@@mail.org")
;; @r{Assuming the Guix checkout is in ~/src/guix.}
(load-file "~/src/guix/etc/copyright.el")
@end lisp

To insert a copyright at the current line invoke @code{M-x guix-copyright}.

To update a copyright you need to specify a @code{copyright-names-regexp}.

@lisp
(setq copyright-names-regexp
      (format "%s <%s>" user-full-name user-mail-address))
@end lisp

You can check if your copyright is up to date by evaluating @code{M-x
copyright-update}.  If you want to do it automatically after each buffer
save then add @code{(add-hook 'after-save-hook 'copyright-update)} in
Emacs.

@node Packaging Guidelines
@section Packaging Guidelines

@cindex packages, creating
The GNU distribution is nascent and may well lack some of your favorite
packages.  This section describes how you can help make the distribution
grow.

Free software packages are usually distributed in the form of
@dfn{source code tarballs}---typically @file{tar.gz} files that contain
all the source files.  Adding a package to the distribution means
essentially two things: adding a @dfn{recipe} that describes how to
build the package, including a list of other packages required to build
it, and adding @dfn{package metadata} along with that recipe, such as a
description and licensing information.

In Guix all this information is embodied in @dfn{package definitions}.
Package definitions provide a high-level view of the package.  They are
written using the syntax of the Scheme programming language; in fact,
for each package we define a variable bound to the package definition,
and export that variable from a module (@pxref{Package Modules}).
However, in-depth Scheme knowledge is @emph{not} a prerequisite for
creating packages.  For more information on package definitions,
@pxref{Defining Packages}.

Once a package definition is in place, stored in a file in the Guix
source tree, it can be tested using the @command{guix build} command
(@pxref{Invoking guix build}).  For example, assuming the new package is
called @code{gnew}, you may run this command from the Guix build tree
(@pxref{Running Guix Before It Is Installed}):

@example
./pre-inst-env guix build gnew --keep-failed
@end example

Using @code{--keep-failed} makes it easier to debug build failures since
it provides access to the failed build tree.  Another useful
command-line option when debugging is @code{--log-file}, to access the
build log.

If the package is unknown to the @command{guix} command, it may be that
the source file contains a syntax error, or lacks a @code{define-public}
clause to export the package variable.  To figure it out, you may load
the module from Guile to get more information about the actual error:

@example
./pre-inst-env guile -c '(use-modules (gnu packages gnew))'
@end example

Once your package builds correctly, please send us a patch
(@pxref{Submitting Patches}).  Well, if you need help, we will be happy to
help you too.  Once the patch is committed in the Guix repository, the
new package automatically gets built on the supported platforms by
@url{@value{SUBSTITUTE-URL}, our continuous integration system}.

@cindex substituter
Users can obtain the new package definition simply by running
@command{guix pull} (@pxref{Invoking guix pull}).  When
@code{@value{SUBSTITUTE-SERVER}} is done building the package, installing the
package automatically downloads binaries from there
(@pxref{Substitutes}).  The only place where human intervention is
needed is to review and apply the patch.


@menu
* Software Freedom::            What may go into the distribution.
* Package Naming::              What's in a name?
* Version Numbers::             When the name is not enough.
* Synopses and Descriptions::   Helping users find the right package.
* Snippets versus Phases::      Whether to use a snippet, or a build phase.
* Emacs Packages::              Your Elisp fix.
* Python Modules::              A touch of British comedy.
* Perl Modules::                Little pearls.
* Java Packages::               Coffee break.
* Rust Crates::                 Beware of oxidation.
* Fonts::                       Fond of fonts.
@end menu

@node Software Freedom
@subsection Software Freedom

@c Adapted from http://www.gnu.org/philosophy/philosophy.html.
@cindex free software
The GNU operating system has been developed so that users can have
freedom in their computing.  GNU is @dfn{free software}, meaning that
users have the @url{https://www.gnu.org/philosophy/free-sw.html,four
essential freedoms}: to run the program, to study and change the program
in source code form, to redistribute exact copies, and to distribute
modified versions.  Packages found in the GNU distribution provide only
software that conveys these four freedoms.

In addition, the GNU distribution follow the
@url{https://www.gnu.org/distros/free-system-distribution-guidelines.html,free
software distribution guidelines}.  Among other things, these guidelines
reject non-free firmware, recommendations of non-free software, and
discuss ways to deal with trademarks and patents.

Some otherwise free upstream package sources contain a small and optional
subset that violates the above guidelines, for instance because this subset
is itself non-free code.  When that happens, the offending items are removed
with appropriate patches or code snippets in the @code{origin} form of the
package (@pxref{Defining Packages}).  This way, @code{guix
build --source} returns the ``freed'' source rather than the unmodified
upstream source.


@node Package Naming
@subsection Package Naming

@cindex package name
A package actually has two names associated with it.
First, there is the name of the @emph{Scheme variable}, the one following
@code{define-public}.  By this name, the package can be made known in the
Scheme code, for instance as input to another package.  Second, there is
the string in the @code{name} field of a package definition.  This name
is used by package management commands such as
@command{guix package} and @command{guix build}.

Both are usually the same and correspond to the lowercase conversion of
the project name chosen upstream, with underscores replaced with
hyphens.  For instance, GNUnet is available as @code{gnunet}, and
SDL_net as @code{sdl-net}.

We do not add @code{lib} prefixes for library packages, unless these are
already part of the official project name.  But @pxref{Python
Modules} and @ref{Perl Modules} for special rules concerning modules for
the Python and Perl languages.

Font package names are handled differently, @pxref{Fonts}.


@node Version Numbers
@subsection Version Numbers

@cindex package version
We usually package only the latest version of a given free software
project.  But sometimes, for instance for incompatible library versions,
two (or more) versions of the same package are needed.  These require
different Scheme variable names.  We use the name as defined
in @ref{Package Naming}
for the most recent version; previous versions use the same name, suffixed
by @code{-} and the smallest prefix of the version number that may
distinguish the two versions.

The name inside the package definition is the same for all versions of a
package and does not contain any version number.

For instance, the versions 2.24.20 and 3.9.12 of GTK+ may be packaged as follows:

@lisp
(define-public gtk+
  (package
    (name "gtk+")
    (version "3.9.12")
    ...))
(define-public gtk+-2
  (package
    (name "gtk+")
    (version "2.24.20")
    ...))
@end lisp
If we also wanted GTK+ 3.8.2, this would be packaged as
@lisp
(define-public gtk+-3.8
  (package
    (name "gtk+")
    (version "3.8.2")
    ...))
@end lisp

@c See <https://lists.gnu.org/archive/html/guix-devel/2016-01/msg00425.html>,
@c for a discussion of what follows.
@cindex version number, for VCS snapshots
Occasionally, we package snapshots of upstream's version control system
(VCS) instead of formal releases.  This should remain exceptional,
because it is up to upstream developers to clarify what the stable
release is.  Yet, it is sometimes necessary.  So, what should we put in
the @code{version} field?

Clearly, we need to make the commit identifier of the VCS snapshot
visible in the version string, but we also need to make sure that the
version string is monotonically increasing so that @command{guix package
--upgrade} can determine which version is newer.  Since commit
identifiers, notably with Git, are not monotonically increasing, we add
a revision number that we increase each time we upgrade to a newer
snapshot.  The resulting version string looks like this:

@example
2.0.11-3.cabba9e
  ^    ^    ^
  |    |    `-- upstream commit ID
  |    |
  |    `--- Guix package revision
  |
latest upstream version
@end example

It is a good idea to strip commit identifiers in the @code{version}
field to, say, 7 digits.  It avoids an aesthetic annoyance (assuming
aesthetics have a role to play here) as well as problems related to OS
limits such as the maximum shebang length (127 bytes for the Linux
kernel).  It is best to use the full commit identifiers in
@code{origin}s, though, to avoid ambiguities.  A typical package
definition may look like this:

@lisp
(define my-package
  (let ((commit "c3f29bc928d5900971f65965feaae59e1272a3f7")
        (revision "1"))          ;Guix package revision
    (package
      (version (git-version "0.9" revision commit))
      (source (origin
                (method git-fetch)
                (uri (git-reference
                      (url "git://example.org/my-package.git")
                      (commit commit)))
                (sha256 (base32 "1mbikn@dots{}"))
                (file-name (git-file-name name version))))
      ;; @dots{}
      )))
@end lisp

@node Synopses and Descriptions
@subsection Synopses and Descriptions

@cindex package description
@cindex package synopsis
As we have seen before, each package in GNU@tie{}Guix includes a
synopsis and a description (@pxref{Defining Packages}).  Synopses and
descriptions are important: They are what @command{guix package
--search} searches, and a crucial piece of information to help users
determine whether a given package suits their needs.  Consequently,
packagers should pay attention to what goes into them.

Synopses must start with a capital letter and must not end with a
period.  They must not start with ``a'' or ``the'', which usually does
not bring anything; for instance, prefer ``File-frobbing tool'' over ``A
tool that frobs files''.  The synopsis should say what the package
is---e.g., ``Core GNU utilities (file, text, shell)''---or what it is
used for---e.g., the synopsis for GNU@tie{}grep is ``Print lines
matching a pattern''.

Keep in mind that the synopsis must be meaningful for a very wide
audience.  For example, ``Manipulate alignments in the SAM format''
might make sense for a seasoned bioinformatics researcher, but might be
fairly unhelpful or even misleading to a non-specialized audience.  It
is a good idea to come up with a synopsis that gives an idea of the
application domain of the package.  In this example, this might give
something like ``Manipulate nucleotide sequence alignments'', which
hopefully gives the user a better idea of whether this is what they are
looking for.

Descriptions should take between five and ten lines.  Use full
sentences, and avoid using acronyms without first introducing them.
Please avoid marketing phrases such as ``world-leading'',
``industrial-strength'', and ``next-generation'', and avoid superlatives
like ``the most advanced''---they are not helpful to users looking for a
package and may even sound suspicious.  Instead, try to be factual,
mentioning use cases and features.

@cindex Texinfo markup, in package descriptions
Descriptions can include Texinfo markup, which is useful to introduce
ornaments such as @code{@@code} or @code{@@dfn}, bullet lists, or
hyperlinks (@pxref{Overview,,, texinfo, GNU Texinfo}).  However you
should be careful when using some characters for example @samp{@@} and
curly braces which are the basic special characters in Texinfo
(@pxref{Special Characters,,, texinfo, GNU Texinfo}).  User interfaces
such as @command{guix package --show} take care of rendering it
appropriately.

Synopses and descriptions are translated by volunteers
@uref{https://translate.fedoraproject.org/projects/guix/packages, at
Weblate} so that as many users as possible can read them in
their native language.  User interfaces search them and display them in
the language specified by the current locale.

To allow @command{xgettext} to extract them as translatable strings,
synopses and descriptions @emph{must be literal strings}.  This means
that you cannot use @code{string-append} or @code{format} to construct
these strings:

@lisp
(package
  ;; @dots{}
  (synopsis "This is translatable")
  (description (string-append "This is " "*not*" " translatable.")))
@end lisp

Translation is a lot of work so, as a packager, please pay even more
attention to your synopses and descriptions as every change may entail
additional work for translators.  In order to help them, it is possible
to make recommendations or instructions visible to them by inserting
special comments like this (@pxref{xgettext Invocation,,, gettext, GNU
Gettext}):

@lisp
;; TRANSLATORS: "X11 resize-and-rotate" should not be translated.
(description "ARandR is designed to provide a simple visual front end
for the X11 resize-and-rotate (RandR) extension. @dots{}")
@end lisp

@node Snippets versus Phases
@subsection Snippets versus Phases

@cindex snippets, when to use
The boundary between using an origin snippet versus a build phase to
modify the sources of a package can be elusive.  Origin snippets are
typically used to remove unwanted files such as bundled libraries,
nonfree sources, or to apply simple substitutions.  The source derived
from an origin should produce a source that can be used to build the
package on any system that the upstream package supports (i.e., act as
the corresponding source).  In particular, origin snippets must not
embed store items in the sources; such patching should rather be done
using build phases.  Refer to the @code{origin} record documentation for
more information (@pxref{origin Reference}).

@node Emacs Packages
@subsection Emacs Packages

@cindex emacs, packaging
@cindex elisp, packaging
Emacs packages should preferably use the Emacs build system
(@pxref{emacs-build-system}), for uniformity and the benefits provided
by its build phases, such as the auto-generation of the autoloads file
and the byte compilation of the sources.  Because there is no
standardized way to run a test suite for Emacs packages, tests are
disabled by default.  When a test suite is available, it should be
enabled by setting the @code{#:tests?} argument to @code{#true}.  By
default, the command to run the test is @command{make check}, but any
command can be specified via the @code{#:test-command} argument.  The
@code{#:test-command} argument expects a list containing a command and
its arguments, to be invoked during the @code{check} phase.

The Elisp dependencies of Emacs packages are typically provided as
@code{propagated-inputs} when required at run time.  As for other
packages, build or test dependencies should be specified as
@code{native-inputs}.

Emacs packages sometimes depend on resources directories that should be
installed along the Elisp files.  The @code{#:include} argument can be
used for that purpose, by specifying a list of regexps to match.  The
best practice when using the @code{#:include} argument is to extend
rather than override its default value (accessible via the
@code{%default-include} variable).  As an example, a yasnippet extension
package typically include a @file{snippets} directory, which could be
copied to the installation directory using:

@lisp
#:include (cons "^snippets/" %default-include))
@end lisp

When encountering problems, it is wise to check for the presence of the
@code{Package-Requires} extension header in the package main source
file, and whether any dependencies and their versions listed therein are
satisfied.

@node Python Modules
@subsection Python Modules

@cindex python
We currently package Python 2 and Python 3, under the Scheme variable names
@code{python-2} and @code{python} as explained in @ref{Version Numbers}.
To avoid confusion and naming clashes with other programming languages, it
seems desirable that the name of a package for a Python module contains
the word @code{python}.

Some modules are compatible with only one version of Python, others with
both.  If the package Foo is compiled with Python 3, we name it
@code{python-foo}.  If it is compiled with Python 2, we name it
@code{python2-foo}.  Packages should be added when they are necessary;
we don't add Python 2 variants of the package unless we are going to use
them.

If a project already contains the word @code{python}, we drop this;
for instance, the module python-dateutil is packaged under the names
@code{python-dateutil} and @code{python2-dateutil}.  If the project name
starts with @code{py} (e.g.@: @code{pytz}), we keep it and prefix it as
described above.

@subsubsection Specifying Dependencies
@cindex inputs, for Python packages

Dependency information for Python packages is usually available in the
package source tree, with varying degrees of accuracy: in the
@file{setup.py} file, in @file{requirements.txt}, or in @file{tox.ini}.

Your mission, when writing a recipe for a Python package, is to map
these dependencies to the appropriate type of ``input'' (@pxref{package
Reference, inputs}).  Although the @code{pypi} importer normally does a
good job (@pxref{Invoking guix import}), you may want to check the
following check list to determine which dependency goes where.

@itemize

@item
We currently package Python 2 with @code{setuptools} and @code{pip}
installed like Python 3.4 has per default.  Thus you don't need to
specify either of these as an input.  @command{guix lint} will warn you
if you do.

@item
Python dependencies required at run time go into
@code{propagated-inputs}.  They are typically defined with the
@code{install_requires} keyword in @file{setup.py}, or in the
@file{requirements.txt} file.

@item
Python packages required only at build time---e.g., those listed with
the @code{setup_requires} keyword in @file{setup.py}---or only for
testing---e.g., those in @code{tests_require}---go into
@code{native-inputs}.  The rationale is that (1) they do not need to be
propagated because they are not needed at run time, and (2) in a
cross-compilation context, it's the ``native'' input that we'd want.

Examples are the @code{pytest}, @code{mock}, and @code{nose} test
frameworks.  Of course if any of these packages is also required at
run-time, it needs to go to @code{propagated-inputs}.

@item
Anything that does not fall in the previous categories goes to
@code{inputs}, for example programs or C libraries required for building
Python packages containing C extensions.

@item
If a Python package has optional dependencies (@code{extras_require}),
it is up to you to decide whether to add them or not, based on their
usefulness/overhead ratio (@pxref{Submitting Patches, @command{guix
size}}).

@end itemize


@node Perl Modules
@subsection Perl Modules

@cindex perl
Perl programs standing for themselves are named as any other package,
using the lowercase upstream name.
For Perl packages containing a single class, we use the lowercase class name,
replace all occurrences of @code{::} by dashes and prepend the prefix
@code{perl-}.
So the class @code{XML::Parser} becomes @code{perl-xml-parser}.
Modules containing several classes keep their lowercase upstream name and
are also prepended by @code{perl-}.  Such modules tend to have the word
@code{perl} somewhere in their name, which gets dropped in favor of the
prefix.  For instance, @code{libwww-perl} becomes @code{perl-libwww}.


@node Java Packages
@subsection Java Packages

@cindex java
Java programs standing for themselves are named as any other package,
using the lowercase upstream name.

To avoid confusion and naming clashes with other programming languages,
it is desirable that the name of a package for a Java package is
prefixed with @code{java-}.  If a project already contains the word
@code{java}, we drop this; for instance, the package @code{ngsjava} is
packaged under the name @code{java-ngs}.

For Java packages containing a single class or a small class hierarchy,
we use the lowercase class name, replace all occurrences of @code{.} by
dashes and prepend the prefix @code{java-}.  So the class
@code{apache.commons.cli} becomes package
@code{java-apache-commons-cli}.


@node Rust Crates
@subsection Rust Crates

@cindex rust
Rust programs standing for themselves are named as any other package, using the
lowercase upstream name.

To prevent namespace collisions we prefix all other Rust packages with the
@code{rust-} prefix.  The name should be changed to lowercase as appropriate and
dashes should remain in place.

In the rust ecosystem it is common for multiple incompatible versions of a
package to be used at any given time, so all package definitions should have a
versioned suffix.  The versioned suffix is the left-most non-zero digit (and
any leading zeros, of course).  This follows the ``caret'' version scheme
intended by Cargo.  Examples@: @code{rust-clap-2}, @code{rust-rand-0.6}.

Because of the difficulty in reusing rust packages as pre-compiled inputs for
other packages the Cargo build system (@pxref{Build Systems,
@code{cargo-build-system}}) presents the @code{#:cargo-inputs} and
@code{cargo-development-inputs} keywords as build system arguments.  It would be
helpful to think of these as similar to @code{propagated-inputs} and
@code{native-inputs}.  Rust @code{dependencies} and @code{build-dependencies}
should go in @code{#:cargo-inputs}, and @code{dev-dependencies} should go in
@code{#:cargo-development-inputs}.  If a Rust package links to other libraries
then the standard placement in @code{inputs} and the like should be used.

Care should be taken to ensure the correct version of dependencies are used; to
this end we try to refrain from skipping the tests or using @code{#:skip-build?}
when possible.  Of course this is not always possible, as the package may be
developed for a different Operating System, depend on features from the Nightly
Rust compiler, or the test suite may have atrophied since it was released.


@node Fonts
@subsection Fonts

@cindex fonts
For fonts that are in general not installed by a user for typesetting
purposes, or that are distributed as part of a larger software package,
we rely on the general packaging rules for software; for instance, this
applies to the fonts delivered as part of the X.Org system or fonts that
are part of TeX Live.

To make it easier for a user to search for fonts, names for other packages
containing only fonts are constructed as follows, independently of the
upstream package name.

The name of a package containing only one font family starts with
@code{font-}; it is followed by the foundry name and a dash @code{-}
if the foundry is known, and the font family name, in which spaces are
replaced by dashes (and as usual, all upper case letters are transformed
to lower case).
For example, the Gentium font family by SIL is packaged under the name
@code{font-sil-gentium}.

For a package containing several font families, the name of the collection
is used in the place of the font family name.
For instance, the Liberation fonts consist of three families,
Liberation Sans, Liberation Serif and Liberation Mono.
These could be packaged separately under the names
@code{font-liberation-sans} and so on; but as they are distributed together
under a common name, we prefer to package them together as
@code{font-liberation}.

In the case where several formats of the same font family or font collection
are packaged separately, a short form of the format, prepended by a dash,
is added to the package name.  We use @code{-ttf} for TrueType fonts,
@code{-otf} for OpenType fonts and @code{-type1} for PostScript Type 1
fonts.


@node Coding Style
@section Coding Style

In general our code follows the GNU Coding Standards (@pxref{Top,,,
standards, GNU Coding Standards}).  However, they do not say much about
Scheme, so here are some additional rules.

@menu
* Programming Paradigm::        How to compose your elements.
* Modules::                     Where to store your code?
* Data Types and Pattern Matching::  Implementing data structures.
* Formatting Code::             Writing conventions.
@end menu

@node Programming Paradigm
@subsection Programming Paradigm

Scheme code in Guix is written in a purely functional style.  One
exception is code that involves input/output, and procedures that
implement low-level concepts, such as the @code{memoize} procedure.

@node Modules
@subsection Modules

Guile modules that are meant to be used on the builder side must live in
the @code{(guix build @dots{})} name space.  They must not refer to
other Guix or GNU modules.  However, it is OK for a ``host-side'' module
to use a build-side module.

Modules that deal with the broader GNU system should be in the
@code{(gnu @dots{})} name space rather than @code{(guix @dots{})}.

@node Data Types and Pattern Matching
@subsection Data Types and Pattern Matching

The tendency in classical Lisp is to use lists to represent everything,
and then to browse them ``by hand'' using @code{car}, @code{cdr},
@code{cadr}, and co.  There are several problems with that style,
notably the fact that it is hard to read, error-prone, and a hindrance
to proper type error reports.

Guix code should define appropriate data types (for instance, using
@code{define-record-type*}) rather than abuse lists.  In addition, it
should use pattern matching, via Guile’s @code{(ice-9 match)} module,
especially when matching lists (@pxref{Pattern Matching,,, guile, GNU
Guile Reference Manual}).

@node Formatting Code
@subsection Formatting Code

@cindex formatting code
@cindex coding style
When writing Scheme code, we follow common wisdom among Scheme
programmers.  In general, we follow the
@url{https://mumble.net/~campbell/scheme/style.txt, Riastradh's Lisp
Style Rules}.  This document happens to describe the conventions mostly
used in Guile’s code too.  It is very thoughtful and well written, so
please do read it.

Some special forms introduced in Guix, such as the @code{substitute*}
macro, have special indentation rules.  These are defined in the
@file{.dir-locals.el} file, which Emacs automatically uses.  Also note
that Emacs-Guix provides @code{guix-devel-mode} mode that indents and
highlights Guix code properly (@pxref{Development,,, emacs-guix, The
Emacs-Guix Reference Manual}).

@cindex indentation, of code
@cindex formatting, of code
If you do not use Emacs, please make sure to let your editor knows these
rules.  To automatically indent a package definition, you can also run:

@example
./etc/indent-code.el gnu/packages/@var{file}.scm @var{package}
@end example

@noindent
This automatically indents the definition of @var{package} in
@file{gnu/packages/@var{file}.scm} by running Emacs in batch mode.  To
indent a whole file, omit the second argument:

@example
./etc/indent-code.el gnu/services/@var{file}.scm
@end example

@cindex Vim, Scheme code editing
If you are editing code with Vim, we recommend that you run @code{:set
autoindent} so that your code is automatically indented as you type.
Additionally,
@uref{https://www.vim.org/scripts/script.php?script_id=3998,
@code{paredit.vim}} may help you deal with all these parentheses.

We require all top-level procedures to carry a docstring.  This
requirement can be relaxed for simple private procedures in the
@code{(guix build @dots{})} name space, though.

Procedures should not have more than four positional parameters.  Use
keyword parameters for procedures that take more than four parameters.


@node Submitting Patches
@section Submitting Patches

Development is done using the Git distributed version control system.
Thus, access to the repository is not strictly necessary.  We welcome
contributions in the form of patches as produced by @code{git
format-patch} sent to the @email{guix-patches@@gnu.org} mailing list.
Seasoned Guix developers may also want to look at the section on commit
access (@pxref{Commit Access}).

This mailing list is backed by a Debbugs instance, which allows us to
keep track of submissions (@pxref{Tracking Bugs and Patches}).  Each
message sent to that mailing list gets a new tracking number assigned;
people can then follow up on the submission by sending email to
@code{@var{NNN}@@debbugs.gnu.org}, where @var{NNN} is the tracking
number (@pxref{Sending a Patch Series}).

Please write commit logs in the ChangeLog format (@pxref{Change Logs,,,
standards, GNU Coding Standards}); you can check the commit history for
examples.

Before submitting a patch that adds or modifies a package definition,
please run through this check list:

@enumerate
@item
If the authors of the packaged software provide a cryptographic
signature for the release tarball, make an effort to verify the
authenticity of the archive.  For a detached GPG signature file this
would be done with the @code{gpg --verify} command.

@item
Take some time to provide an adequate synopsis and description for the
package.  @xref{Synopses and Descriptions}, for some guidelines.

@item
Run @code{guix lint @var{package}}, where @var{package} is the
name of the new or modified package, and fix any errors it reports
(@pxref{Invoking guix lint}).

@item
Make sure the package builds on your platform, using @code{guix build
@var{package}}.

@item
We recommend you also try building the package on other supported
platforms.  As you may not have access to actual hardware platforms, we
recommend using the @code{qemu-binfmt-service-type} to emulate them.  In
order to enable it, add the following service to the list of services in
your @code{operating-system} configuration:

@lisp
(service qemu-binfmt-service-type
 (qemu-binfmt-configuration
   (platforms (lookup-qemu-platforms "arm" "aarch64"))
@end lisp

Then reconfigure your system.

You can then build packages for different platforms by specifying the
@code{--system} option.  For example, to build the "hello" package for
the armhf, aarch64, or mips64 architectures, you would run the following
commands, respectively:
@example
guix build --system=armhf-linux --rounds=2 hello
guix build --system=aarch64-linux --rounds=2 hello
@end example

@item
@cindex bundling
Make sure the package does not use bundled copies of software already
available as separate packages.

Sometimes, packages include copies of the source code of their
dependencies as a convenience for users.  However, as a distribution, we
want to make sure that such packages end up using the copy we already
have in the distribution, if there is one.  This improves resource usage
(the dependency is built and stored only once), and allows the
distribution to make transverse changes such as applying security
updates for a given software package in a single place and have them
affect the whole system---something that bundled copies prevent.

@item
Take a look at the profile reported by @command{guix size}
(@pxref{Invoking guix size}).  This will allow you to notice references
to other packages unwillingly retained.  It may also help determine
whether to split the package (@pxref{Packages with Multiple Outputs}),
and which optional dependencies should be used.  In particular, avoid adding
@code{texlive} as a dependency: because of its extreme size, use
the @code{texlive-tiny} package or @code{texlive-union} procedure instead.

@item
For important changes, check that dependent package (if applicable) are
not affected by the change; @code{guix refresh --list-dependent
@var{package}} will help you do that (@pxref{Invoking guix refresh}).

@c See <https://lists.gnu.org/archive/html/guix-devel/2016-10/msg00933.html>.
@cindex branching strategy
@cindex rebuild scheduling strategy
Depending on the number of dependent packages and thus the amount of
rebuilding induced, commits go to different branches, along these lines:

@table @asis
@item 300 dependent packages or less
@code{master} branch (non-disruptive changes).

@item between 300 and 1,800 dependent packages
@code{staging} branch (non-disruptive changes).  This branch is intended
to be merged in @code{master} every 6 weeks or so.  Topical changes
(e.g., an update of the GNOME stack) can instead go to a specific branch
(say, @code{gnome-updates}).

@item more than 1,800 dependent packages
@code{core-updates} branch (may include major and potentially disruptive
changes).  This branch is intended to be merged in @code{master} every
6 months or so.
@end table

All these branches are @uref{@value{SUBSTITUTE-URL},
tracked by our build farm} and merged into @code{master} once
everything has been successfully built.  This allows us to fix issues
before they hit users, and to reduce the window during which pre-built
binaries are not available.

When we decide to start building the @code{staging} or
@code{core-updates} branches, they will be forked and renamed with the
suffix @code{-frozen}, at which time only bug fixes may be pushed to the
frozen branches.  The @code{core-updates} and @code{staging} branches
will remain open to accept patches for the next cycle.  Please ask on
the mailing list or IRC if unsure where to place a patch.
@c TODO: It would be good with badges on the website that tracks these
@c branches.  Or maybe even a status page.

@item
@cindex determinism, of build processes
@cindex reproducible builds, checking
Check whether the package's build process is deterministic.  This
typically means checking whether an independent build of the package
yields the exact same result that you obtained, bit for bit.

A simple way to do that is by building the same package several times in
a row on your machine (@pxref{Invoking guix build}):

@example
guix build --rounds=2 my-package
@end example

This is enough to catch a class of common non-determinism issues, such
as timestamps or randomly-generated output in the build result.

Another option is to use @command{guix challenge} (@pxref{Invoking guix
challenge}).  You may run it once the package has been committed and
built by @code{@value{SUBSTITUTE-SERVER}} to check whether it obtains the same
result as you did.  Better yet: Find another machine that can build it
and run @command{guix publish}.  Since the remote build machine is
likely different from yours, this can catch non-determinism issues
related to the hardware---e.g., use of different instruction set
extensions---or to the operating system kernel---e.g., reliance on
@code{uname} or @file{/proc} files.

@item
When writing documentation, please use gender-neutral wording when
referring to people, such as
@uref{https://en.wikipedia.org/wiki/Singular_they, singular
``they''@comma{} ``their''@comma{} ``them''}, and so forth.

@item
Verify that your patch contains only one set of related changes.
Bundling unrelated changes together makes reviewing harder and slower.

Examples of unrelated changes include the addition of several packages,
or a package update along with fixes to that package.

@item
Please follow our code formatting rules, possibly running the
@command{etc/indent-code.el} script to do that automatically for you
(@pxref{Formatting Code}).

@item
When possible, use mirrors in the source URL (@pxref{Invoking guix download}).
Use reliable URLs, not generated ones.  For instance, GitHub archives are not
necessarily identical from one generation to the next, so in this case it's
often better to clone the repository.  Don't use the @command{name} field in
the URL: it is not very useful and if the name changes, the URL will probably
be wrong.

@item
Check if Guix builds (@pxref{Building from Git}) and address the
warnings, especially those about use of undefined symbols.

@item
Make sure your changes do not break Guix and simulate a @code{guix pull} with:
@example
guix pull --url=/path/to/your/checkout --profile=/tmp/guix.master
@end example

@end enumerate

When posting a patch to the mailing list, use @samp{[PATCH] @dots{}} as
a subject, if your patch is to be applied on a branch other than
@code{master}, say @code{core-updates}, specify it in the subject like
@samp{[PATCH core-updates] @dots{}}.  You may use your email client or
the @command{git send-email} command (@pxref{Sending a Patch Series}).
We prefer to get patches in plain text messages, either inline or as
MIME attachments.  You are advised to pay attention if your email client
changes anything like line breaks or indentation which could potentially
break the patches.

When a bug is resolved, please close the thread by sending an email to
@email{@var{NNN}-done@@debbugs.gnu.org}.

@unnumberedsubsec Sending a Patch Series
@anchor{Sending a Patch Series}
@cindex patch series
@cindex @code{git send-email}
@cindex @code{git-send-email}

When sending a patch series (e.g., using @code{git send-email}), please
first send one message to @email{guix-patches@@gnu.org}, and then send
subsequent patches to @email{@var{NNN}@@debbugs.gnu.org} to make sure
they are kept together.  See
@uref{https://debbugs.gnu.org/Advanced.html, the Debbugs documentation}
for more information.  You can install @command{git send-email} with
@command{guix install git:send-email}.
@c Debbugs bug: https://debbugs.gnu.org/db/15/15361.html

@node Tracking Bugs and Patches
@section Tracking Bugs and Patches

@cindex bug reports, tracking
@cindex patch submissions, tracking
@cindex issue tracking
@cindex Debbugs, issue tracking system
Bug reports and patch submissions are currently tracked using the
Debbugs instance at @uref{https://bugs.gnu.org}.  Bug reports are filed
against the @code{guix} ``package'' (in Debbugs parlance), by sending
email to @email{bug-guix@@gnu.org}, while patch submissions are filed
against the @code{guix-patches} package by sending email to
@email{guix-patches@@gnu.org} (@pxref{Submitting Patches}).

A web interface (actually @emph{two} web interfaces!) are available to
browse issues:

@itemize
@item
@url{https://issues.guix.gnu.org} provides a pleasant
interface@footnote{The web interface at
@url{https://issues.guix.gnu.org} is powered by Mumi, a nice piece of
software written in Guile, and you can help!  See
@url{https://git.elephly.net/gitweb.cgi?p=software/mumi.git}.} to browse
bug reports and patches, and to participate in discussions;
@item
@url{https://bugs.gnu.org/guix} lists bug reports;
@item
@url{https://bugs.gnu.org/guix-patches} lists patch submissions.
@end itemize

To view discussions related to issue number @var{n}, go to
@indicateurl{https://issues.guix.gnu.org/@var{n}} or
@indicateurl{https://bugs.gnu.org/@var{n}}.

If you use Emacs, you may find it more convenient to interact with
issues using @file{debbugs.el}, which you can install with:

@example
guix install emacs-debbugs
@end example

For example, to list all open issues on @code{guix-patches}, hit:

@example
@kbd{C-u} @kbd{M-x} debbugs-gnu @kbd{RET} @kbd{RET} guix-patches @kbd{RET} n y
@end example

@xref{Top,,, debbugs-ug, Debbugs User Guide}, for more information on
this nifty tool!

@node Commit Access
@section Commit Access

@cindex commit access, for developers
For frequent contributors, having write access to the repository is
convenient.  When you deem it necessary, consider applying for commit
access by following these steps:

@enumerate
@item
Find three committers who would vouch for you.  You can view the list of
committers at
@url{https://savannah.gnu.org/project/memberlist.php?group=guix}.  Each
of them should email a statement to @email{guix-maintainers@@gnu.org} (a
private alias for the collective of maintainers), signed with their
OpenPGP key.

Committers are expected to have had some interactions with you as a
contributor and to be able to judge whether you are sufficiently
familiar with the project's practices.  It is @emph{not} a judgment on
the value of your work, so a refusal should rather be interpreted as
``let's try again later''.

@item
Send @email{guix-maintainers@@gnu.org} a message stating your intent,
listing the three committers who support your application, signed with
the OpenPGP key you will use to sign commits, and giving its fingerprint
(see below).  See @uref{https://emailselfdefense.fsf.org/en/}, for an
introduction to public-key cryptography with GnuPG.

@c See <https://sha-mbles.github.io/>.
Set up GnuPG such that it never uses the SHA1 hash algorithm for digital
signatures, which is known to be unsafe since 2019, for instance by
adding the following line to @file{~/.gnupg/gpg.conf} (@pxref{GPG
Esoteric Options,,, gnupg, The GNU Privacy Guard Manual}):

@example
digest-algo sha512
@end example

@item
Maintainers ultimately decide whether to grant you commit access,
usually following your referrals' recommendation.

@item
@cindex OpenPGP, signed commits
If and once you've been given access, please send a message to
@email{guix-devel@@gnu.org} to say so, again signed with the OpenPGP key
you will use to sign commits (do that before pushing your first commit).
That way, everyone can notice and ensure you control that OpenPGP key.

@quotation Important
Before you can push for the first time, maintainers must:

@enumerate
@item
add your OpenPGP key to the @code{keyring} branch;
@item
add your OpenPGP fingerprint to the @file{.guix-authorizations} file of
the branch(es) you will commit to.
@end enumerate
@end quotation

@item
Make sure to read the rest of this section and... profit!
@end enumerate

@quotation Note
Maintainers are happy to give commit access to people who have been
contributing for some time and have a track record---don't be shy and
don't underestimate your work!

However, note that the project is working towards a more automated patch
review and merging system, which, as a consequence, may lead us to have
fewer people with commit access to the main repository.  Stay tuned!
@end quotation

If you get commit access, please make sure to follow
the policy below (discussions of the policy can take place on
@email{guix-devel@@gnu.org}).

Non-trivial patches should always be posted to
@email{guix-patches@@gnu.org} (trivial patches include fixing typos,
etc.).  This mailing list fills the patch-tracking database
(@pxref{Tracking Bugs and Patches}).

For patches that just add a new package, and a simple one, it's OK to
commit, if you're confident (which means you successfully built it in a
chroot setup, and have done a reasonable copyright and license
auditing).  Likewise for package upgrades, except upgrades that trigger
a lot of rebuilds (for example, upgrading GnuTLS or GLib).  We have a
mailing list for commit notifications (@email{guix-commits@@gnu.org}),
so people can notice.  Before pushing your changes, make sure to run
@code{git pull --rebase}.

All commits that are pushed to the central repository on Savannah must
be signed with an OpenPGP key, and the public key should be uploaded to
your user account on Savannah and to public key servers, such as
@code{keys.openpgp.org}.  To configure Git to automatically sign
commits, run:

@example
git config commit.gpgsign true
git config user.signingkey CABBA6EA1DC0FF33
@end example

You can prevent yourself from accidentally pushing unsigned commits to
Savannah by using the pre-push Git hook called located at
@file{etc/git/pre-push}:

@example
cp etc/git/pre-push .git/hooks/pre-push
@end example

When pushing a commit on behalf of somebody else, please add a
@code{Signed-off-by} line at the end of the commit log message---e.g.,
with @command{git am --signoff}.  This improves tracking of who did
what.

When adding channel news entries (@pxref{Channels, Writing Channel
News}), make sure they are well-formed by running the following command
right before pushing:

@example
make check-channel-news
@end example

For anything else, please post to @email{guix-patches@@gnu.org} and
leave time for a review, without committing anything (@pxref{Submitting
Patches}).  If you didn’t receive any reply after two weeks, and if
you're confident, it's OK to commit.

That last part is subject to being adjusted, allowing individuals to commit
directly on non-controversial changes on parts they’re familiar with.

In order to reduce the possibility of mistakes, committers will have
their Savannah account removed from the Guix Savannah project and their
key removed from @file{.guix-authorizations} after 12 months of
inactivity; they can ask to regain commit access by emailing the
maintainers, without going through the vouching process.

One last thing: the project keeps moving forward because committers not
only push their own awesome changes, but also offer some of their time
@emph{reviewing} and pushing other people's changes.  As a committer,
you're welcome to use your expertise and commit rights to help other
contributors, too!

@node Updating the Guix Package
@section Updating the Guix Package

@cindex update-guix-package, updating the guix package
It is sometimes desirable to update the @code{guix} package itself (the
package defined in @code{(gnu packages package-management)}), for
example to make new daemon features available for use by the
@code{guix-service-type} service type.  In order to simplify this task,
the following command can be used:

@example
make update-guix-package
@end example

The @code{update-guix-package} make target will use the last known
@emph{commit} corresponding to @code{HEAD} in your Guix checkout,
compute the hash of the Guix sources corresponding to that commit and
update the @code{commit}, @code{revision} and hash of the @code{guix}
package definition.

To validate that the updated @code{guix} package hashes are correct and
that it can be built successfully, the following command can be run from
the directory of your Guix checkout:

@example
./pre-inst-env guix build guix
@end example

To guard against accidentally updating the @code{guix} package to a
commit that others can't refer to, a check is made that the commit used
has already been pushed to the Savannah-hosted Guix git repository.

This check can be disabled, @emph{at your own peril}, by setting the
@code{GUIX_ALLOW_ME_TO_USE_PRIVATE_COMMIT} environment variable.  When
this variable is set, the updated package source is also added to the
store.  This is used as part of the release process of Guix.