path: root/glep-0068.rst

---
GLEP: 68
Title: Package and category metadata
Author: Michał Górny <mgorny@gentoo.org>
Type: Standards Track
Status: Final
Version: 1.3
Created: 2016-03-14
Last-Modified: 2022-10-14
Post-History: 2016-03-16, 2018-02-20, 2022-05-22, 2022-10-07
Content-Type: text/x-rst
Requires: 67
Replaces: 34, 46, 56
---

Abstract
========

This GLEP specifies the format of files used to describe category and package
metadata (``metadata.xml``).


Motivation
==========

At the moment of writing this GLEP, category and package ``metadata.xml``
lacked proper specification. PMS Appendix A [#PMS-A]_ specified that
the format of this file is beyond its scope, deferring the specification
to the DTD file.

The original metadata.dtd file [#METADATA-DTD]_ (the version before cleanups
related to this spec) did not serve well as the specification. Due to
the technical limitations on DTD format, it was both unable to enforce
the specification fully and explain it in a readable form. Furthermore,
it lacked some important details such as the format of ``<pkg/>`` entries.

Besides that, there were numerous alterations to the format. GLEP 34 added
metadata files for category descriptions, GLEP 46 added upstream information,
GLEP 56 added USE flag descriptions, GLEP 67 altered the maintainer
descriptions. Furthermore, there were additions and removals done without
a formal specification, e.g. addition of slot descriptions.

Sadly, some of those GLEPs are partially in conflict with other specifications
— for example, the ``<pkg/>`` element as described in GLEP 56 is different
than the one originally proposed and used in metadata.xml.

Therefore, the motivation for this GLEP is to provide unified, clear
and complete specification for both category-wide and package-wide
metadata.xml files. It is meant to combine previous GLEPs, relevant
discussions and implementation in order to provide the specification that is
closest to the originally intended meaning while preserving best compatibility
with existing tools and data.


Specification
=============

Metadata files
--------------

This specification provides two kinds of metadata files: category metadata
files and package metadata files. Both kinds of files use the XML 1.0 file
format [#XML10]_. They must not use external markup declarations, as defined
in the XML specification. While they may reference or include a DTD, the parser
must not fetch or process it.

The data structure of metadata files is defined in this GLEP. The elements
and attributes do not use namespaces. Conforming files must not contain
any elements or attributes that are not defined in this specification.
However, parsers should ignore any unknown elements or attributes in order
to permit future extension.

Category metadata files are named ``metadata.xml`` and located inside category
directories in an ebuild repository. Their structure is described
in `Category metadata`_ section.

Package metadata files are named ``metadata.xml`` and located inside package
directories in an ebuild repository. Their structure is described
in `Package metadata`_ section.

Text data
---------

The following text data types are used:

- text data,
- multi-line text data.

In case of text data, all whitespace inside the element is normalized
(consecutive whitespace sequences are replaced by a single SP). Trailing
and leading whitespace is stripped.

In case of multi-line text data, all whitespace except for newline characters
is normalized. Newlines are used to delimit lines of text. Leading
and trailing lines of text that are either empty or consist purely of
whitespace are stripped. Afterwards, the whitespace belonging to
the indentation common to all non-empty lines of text is stripped.

Optionally, interspersing text with ``<cat/>`` and ``<pkg/>`` elements can be
allowed. In this case, ``<cat/>`` element is used to reference a category
inside the repository, and must contain a valid category name. ``<pkg/>``
is used to reference a package, and must contain a valid qualified package
name.

Common attributes
-----------------

The following common attributes are allowed on multiple elements:

- language specifiers,
- restriction specifiers.

Language specifiers are used whenever an element supports variants
in different languages. In this case, each occurrence of the element may
contain an optional ``lang=""`` attribute that contains an IETF language tag
[#BCP-47]_. In case no ``lang=""`` attribute is provided, an implicit default
of ``en`` is assumed.

Restriction specifiers are used whenever an element supports restricting to
specific package versions. In this case, each occurence of the element may
contain an optional ``restrict=""`` attribute that contains an EAPI 0
dependency specification that has to match one or more versions of the
package. In this case, the metadata provided by the element applies only to
the package versions matching the restriction.

Category metadata
-----------------

The category metadata file uses ``<catmetadata/>`` top-level element. This
element can contain, in any order:

- zero or more ``<longdescription/>`` elements containing category
  descriptions in different languages (at most one for each language).
  The category description is formed of multi-line text, optionally
  interspersed with ``<cat/>`` and ``<pkg/>`` elements.

Package metadata
----------------
Top-level structure
~~~~~~~~~~~~~~~~~~~
The package metadata file uses ``<pkgmetadata/>`` top-level element. This
element can contain, in any order:

- zero or more ``<longdescription/>`` elements containing package descriptions
  in different languages, possibly restricted to specific package versions
  (at most one for each combination of language and package version).
  The package description is formed of multi-line text, optionally
  interspersed with ``<cat/>`` and ``<pkg/>`` elements.

- zero or more ``<maintainer/>`` elements listing package maintainers,
  optionally restricted to specific package versions. The maintainer format
  is detailed in `Maintainer descriptions`_.

- zero or more ``<slots/>`` elements containing slot descriptions in different
  languages (at most one for each language), as detailed
  in `Slot descriptions`_.

- zero or more ``<stabilize-allarches/>`` elements, possibly restricted
  to specific package versions (at most one for each version) whose presence
  indicates that the appropriate ebuilds are suitable for simultaneously
  marking stable on all architectures where a previous version is stable
  after arch testing on one of them (i.e. if the package is known to be fully
  arch-independent).

- zero or more ``<use/>`` elements containing USE flag descriptions
  in different languages (at most one for each language), as detailed
  in `USE flag descriptions`_.

- at most one ``<upstream/>`` element providing information on upstream
  of the package, as detailed in `Upstream descriptions`_.

Maintainer descriptions
~~~~~~~~~~~~~~~~~~~~~~~
Each ``<maintainer/>`` element describes a single maintainer.

The ``<maintainer/>`` element has an obligatory ``type=""`` attribute whose
value can be either ``person`` or ``project``.

The ``<maintainer/>`` element contains the following elements, in any order:

- exactly one ``<email/>`` element that contains the maintainer's e-mail
  address (used as unique identifier),

- at most one ``<name/>`` element that contains the maintainer's
  human-readable name (real name or nickname),

- zero or more ``<description/>`` elements that explain the role
  of the maintainer in different languages (at most one ``<description/>``
  for each language).

Slot descriptions
~~~~~~~~~~~~~~~~~
Each ``<slots/>`` element describes slots of a package (in specific language).

The ``<slots/>`` element can contain the following elements:

- zero or more ``<slot/>`` elements describing specific ebuild slots
  (at most one for each slot name).
  The ``<slot/>`` element contains an obligatory ``name=""`` attribute stating
  the slot to which the description applies, and contains slot description as
  text. Alternatively, a slot name of ``*`` can be used to indicate a single
  description applying to all slots (no other ``<slot/>`` elements may be used
  in this case).

- at most one ``<subslots/>`` element describing the role of subslots (all
  of them) as text.

USE flag descriptions
~~~~~~~~~~~~~~~~~~~~~
Each ``<use/>`` element describes USE flags of a package (in specific
language).

The ``<use/>`` element can contain the following elements:

- zero or more ``<flag/>`` elements describing specific USE flags, optionally
  restricted to specific package versions (at most one entry for a combination
  of USE flag name and package version). The ``<flag/>`` element contains
  an obligatory ``name=""`` attribute stating the name of the USE flag to
  which the description applies, and contains text, optionally interspersed
  with ``<cat/>`` and ``<pkg/>`` elements.

Upstream descriptions
~~~~~~~~~~~~~~~~~~~~~
The ``<upstream/>`` element provides information on the upstream of a package.
It contains the following elements:

- zero or more ``<maintainer/>`` elements listing package's upstream
  maintainers, as described in `Upstream maintainer descriptions`_,

- at most one ``<changelog/>`` element containing URL to an on-line copy
  of upstream changelog,

- zero or more ``<doc/>`` elements containing URLs to on-line copies
  of upstream documentation in different languages (at most one for each
  language),

- at most one ``<bugs-to/>`` element containing upstream bug reporting URL,
  that can optionally be a ``mailto:`` URL,

- zero or more ``<remote-id/>`` elements listing package identities on package
  identification trackers. Each of those elements has an obligatory
  ``type=""`` attribute that matches a pre-defined name of package
  identification tracker, and a value that is an identifier specific to
  the tracker. The list of available trackers and their specific identifiers
  are outside scope of this specification.

Upstream maintainer descriptions
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Each ``<maintainer/>`` element inside ``<upstream/>`` describes a single
upstream maintainer.

The ``<maintainer/>`` element has an optional ``status=""`` attribute whose
value can be either ``active`` or ``inactive``. If not specified, an implicit
``unknown`` value is assumed.

The ``<maintainer/>`` element has the following attributes, in any order:

- at most one ``<email/>`` element that contains the maintainer's e-mail
  address,

- exactly one ``<name/>`` element that contains the maintainer's
  human-readable name (real name or nickname).


Rationale
=========

Information sources
-------------------

The basic source of information on current metadata.xml format was
``metadata.dtd`` as of 2016-03-02 [#ORIGINAL-METADATA-XML]_. Whenever the DTD
was unclear, appropriate GLEPs were referenced in order to deduce the original
intent. Whenever the GLEPs were unclear or the elements missed GLEPs, original
mailing list discussions were referenced.

Removed elements
----------------

Compared to the original DTD, the following elements were removed (both
in the spec and in the updated DTD file):

- package-scope ``<changelog/>`` element was removed. It dates back to the
  original metadata.xml proposal [#ORIGINAL-METADATA-XML]_ but it was never
  implemented — instead, plain text ChangeLogs were used. Furthermore,
  GLEP 46 introduced ``<changelog/>`` inside ``<upstream/>`` with
  different type which collided with the global declaration due to DTD
  limitations.

- package-scope ``<natural-name/>`` element was removed. It was available for
  1.5yr and after that time, it reached four packages providing it and no
  known tool supporting/using it. It was used only to provide a copy of
  package name with correct case (e.g. libressl -> LibreSSL), therefore
  the information provided by it was considered redundant.

- top-level ``<packages/>`` variant was removed. It was never used and it was
  really unclear what its use would be. In any case, this made the DTD
  simpler.

<pkg/> value format
-------------------

A debate on valid format of ``<pkg/>`` element values preceded the writing of
this GLEP. The DTD did not specify a value format restriction on this, only
suggested that it is used *for cross-linking*. Further on, GLEP 56 redefined
its value to *a valid CP or CPV*. The practical uses did not include
the latter case; however, it was common to include EAPI 1 slot specifiers or
even EAPI 5 slot operators following the qualified package names.

After finding the Doug Goldstein's blog post on introduction of <pkg/>
elements [#USE-FLAG-METADATA]_, it turned out that the original intent was to
*allow cross-linking/referencing from packages.gentoo.org*. Since the latter
uses qualified package names as identifiers, it was decided to restrict
``<pkg/>`` elements to reference those. For entries that include slot
specifiers, it is recommended to move the slot specifiers out of ``<pkg/>``
element.

Language identifiers
--------------------

Originally, the DTD used implicit default value of ``C``. However, this value
was not in line with real language specifiers found in ``metadata.xml``.
The latter usually took form of ISO 639-1 language codes which do not form
a valid (complete) locale identifiers, while the former is not a valid
language identifier in any of the considered standards. Furthermore, since
``en`` was commonly used to identify English in metadata.xml files,
and no tools relied on the implicit default defined in the DTD, it was decided
to change the implicit default to ``en``.

Language identifiers were later updated to allow full IETF language tags,
so that codes like ``pt-BR`` or ``zh-Hant`` can be represented.

Package restrictions
--------------------

Originally, the DTD described the ``restrict=""`` attribute as: *the format
of this attribute is equal to the format of DEPEND lines in ebuilds.* This
specification is based upon this definition. However, for practical reasons it
added three clarifications to it:

- only package dependency specifications are allowed (i.e. no USE-conditionals
  or multiple dependency specifications),

- only EAPI=0 dependency specifications are allowed, since ``metadata.xml``
  provides no EAPI identification mechanism and it predates EAPI,

- only dependencies referencing the same package are allowed.

Furthermore, DTD added a special case for ``*`` value that *applies if there
are no other tags that apply*. This behavior was not used at all, and being
at least a bit confusing (compared to the common use of ``*`` to imply
matching everything), it was removed.

Upstream block
--------------

The upstream block was defined by GLEP 46. However, this GLEP is ambiguous
at the best. Tiziano Müller (one of the original authors) has explained
the intent behind most of the elements of the GLEP.

In particular, he confirmed that the GLEP lists all elements that are allowed
explicitly, and no implicit inclusions were meant to be allowed. This means
that the ``<maintainer/>`` element does not allow a ``<description/>``.

He also confirmed that unless noted otherwise, elements were not allowed to
be used more than once. This affects ``<bugs-to/>`` and ``<changelog/>``
elements. Repetitions of ``<doc/>`` were only allowed because DTD technically
didn't permit restricting them while allowing uses of different languages.

At the time of writing this GLEP, only a single Gentoo package was using
multiple ``<bugs-to/>`` elements, and no packages were using multiple
``<changelog/>`` or ``<doc/>`` elements (or non-English docs). For this
reason, this GLEP enforces the original intent of *at most one* element.

Rationale for upstream maintainer descriptions
----------------------------------------------

The proper contents of the ``<maintainer/>`` elements in ``<upstream/>``
blocks were unclear in the DTD since the technical file format limitation
implied that all elements and attributes added for the Gentoo maintainers
also applied to upstream maintainers, and vice versa.

The comments in the DTD clearly separated attributes between the two —
i.e.  stated that the ``type`` attribute is used only for Gentoo maintainers,
while the ``status`` attribute is used only for upstream maintainers. However,
package version restrictions and maintainer descriptions were also implicitly
allowed on them. Since neither of the two was allowed by GLEP 46, this
specification disallows them.


Backwards Compatibility
=======================

This specification does not introduce any new elements or attributes compared
to the current DTD. Therefore, all ``metadata.xml`` files created in its
compliance will be read correctly by the existing tools and will conform
to the current DTD.

However, this specification is more strict than the rules enforced by the DTD.
Therefore, not all existing ``metadata.xml`` will be conforming to the spec,
even though they would be correct according to the DTD. New tools will
consider the files incorrect and request developers to fix them.


Reference implementation
========================

Parsing metadata.xml
--------------------

Since the metadata.xml format provided by this specification is compatible
with existing tool, no new implementation is required for reading those files.

Checking metadata.xml validity
------------------------------

To provide more strict checking of metadata.xml files, XML schema file is
provided in the Gentoo xml-schema repository [#XML-SCHEMA]_. This schema
provides:

- element structure checks,

- data duplication checks (e.g. multiple descriptions for the same flag
  but see below),

- partial value correctness checks.

The limitations of the schema are:

- values are verified using simple regular expressions, so not all format
  violations will be caught (e.g. the rule will consider ``app-foo/bar-1``
  a valid qualified package name when the version suffix is disallowed),

- cross-references can not be checked (package references, category
  references, URLs, project identifiers),

- ``<maintainer type=""/>`` correctness can not be checked,

- data duplication checks are done per ``restrict=""`` value rather than
  per every package version matched by the restriction. Therefore, multiple
  definitions that are applied to a single package by two different
  ``restrict=""`` rules will not be caught.

Example metadata.xml file
-------------------------

.. code:: xml

    <?xml version='1.0' encoding='UTF-8'?>
    <pkgmetadata>
      <maintainer type='person'>
        <email>developer@example.com</email>
        <name>Example Developer</name>
      </maintainer>
      <maintainer type='project'>
        <email>project@example.com</email>
        <name>Example Project</name>
      </maintainer>
      <maintainer type='person'>
        <email>upstream@example.com</email>
        <name>Upstream Developer</name>
        <description>Upstream developer, wishing to be CC-ed on bugs</description>
      </maintainer>
      <longdescription>
        First paragraph of extensive description.

        Second paragraph.
      </longdescription>
      <longdescription lang='de'>
        Erster Absatz mit detaillierter Beschreibung.

        Zweiter Absatz.
      </longdescription>
      <slots>
        <slot name='11'>Compatibility slot providing libfoo.so.11 only.</slot>
        <subslots>
          Match SONAME of libfoo.so.
        </subslots>
      </slots>
      <slots lang='de'>
        <slot name='11'>Kompatibilitäts-Slot, installiert ausschließlich libfoo.so.11.</slot>
        <subslots>
          Subslot ist stets identisch mit dem SONAME von libfoo.so.
        </subslots>
      </slots>
      <use>
        <flag name='foo'>Enables foo feature</flag>
        <flag name='bar' restrict='&lt;dev-libs/foo-12'>Enables bar feature (requires <pkg>dev-libs/bar</pkg>)</flag>
        <flag name='bar' restrict='&gt;=dev-libs/foo-12'>Enables bar feature</flag>
      </use>
      <use lang='de'>
        <flag name='foo'>Konfiguriert das Paket mit Unterstützung für foo</flag>
        <flag name='bar' restrict='&lt;dev-libs/foo-12'>Konfiguriert das Paket mit Unterstützung für bar (benötigt <pkg>dev-libs/bar</pkg>)</flag>
        <flag name='bar' restrict='&gt;=dev-libs/foo-12'>Konfiguriert das Paket mit Unterstützung für bar</flag>
      </use>
      <upstream>
        <maintainer status='active'>
          <email>upstream@example.com</email>
          <name>Upstream Developer</name>
        </maintainer>
        <maintainer status='inactive'>
          <!-- e-mail unknown -->
          <name>John Smith</name>
        </maintainer>
        <changelog>http://www.example.com/releases.html</changelog>
        <doc>http://www.example.com/doc.html</doc>
        <doc lang='de'>http://www.example.com/doc.de.html</doc>
        <bugs-to>http://www.example.com/issues.html</bugs-to>
        <remote-id type='foohub'>example/foo</remote-id>
      </upstream>
    </pkgmetadata>

German translations provided by tamiko.


References
==========

.. [#PMS-A] PMS Appendix A
   https://projects.gentoo.org/pms/5/pms.html#x1-163000A

.. [#METADATA-DTD] The original metadata.dtd file
   https://gitweb.gentoo.org/data/dtd.git/tree/metadata.dtd?id=a908a93b5afe295359e0a01814c9bef8b5268bcd

.. [#XML10] Extensible Markup Language (XML) 1.0 (Fifth Edition)
   https://www.w3.org/TR/xml/

.. [#BCP-47] BCP 47: "Tags for identifying languages",
   https://tools.ietf.org/rfc/bcp/bcp47.txt

.. [#ORIGINAL-METADATA-XML] The original metadata.xml proposal:
   Paul de Vrieze. "IMPORTANT: The proposal for the metadata.xml file".
   gentoo-dev mailing list, 2003-06-27,
   Message-ID 200306272248.38169.pauldv\@gentoo.org,
   https://archives.gentoo.org/gentoo-dev/message/cbcc15e9906c0165976ad66d4343ba7a

.. [#USE-FLAG-METADATA] Doug Goldstein: USE flag metadata
   https://cardoe.wordpress.com/2007/11/19/use-flag-metadata/

.. [#XML-SCHEMA] Gentoo XML schema
   https://gitweb.gentoo.org/data/xml-schema.git/


Copyright
=========

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International License.  To view a copy of this license, visit
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