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---
GLEP: 56
Title: USE flag descriptions in metadata
Author: Doug Goldstein <cardoe@gentoo.org>
Type: Standards Track
Status: Replaced
Version: 1
Created: 2008-06-03
Last-Modified: 2017-10-13
Post-History: 2008-06-05, 2008-06-13
Content-Type: text/x-rst
Replaced-By: 68
---
Abstract
========
This GLEP proposes to add per-package USE flag descriptions to each package's
metadata.
Motivation
==========
Gives Gentoo users the ability to better identify how USE flags affect their
installations of a given package. For example, many global USE flags have very
generic descriptions but no specifics on how it affects a certain package.
Specifically speaking, an example would be net-print/cups and the 'jpeg' USE
flag. Does this flag mean you won't be able to print jpeg files? You can print
them directly? Its interface won't use jpeg files.
- Motivator References: [#motivators1]_, [#motivators2]_, [#motivators3]_,
and [#motivators4]_
Specification
=============
This GLEP proposes the addition of ``<use>`` XML tag that is only allowed to
appear inside of a ``<pkgmetadata>`` XML tag.
- Inside the ``<use>`` XML tag, the ``<flag>`` XML tag is allowed to appear
once per USE flag as specified by the ``'name'`` attribute with the
following exception:
* The ``'restrict'`` atttribute can limit to specific versions of the
package, where the attribute value must be a valid CPV as defined by the
`Gentoo Developer Handbook` [#devhandbook]_. This follows the current
behavior of the ``'restrict'`` attribute in metadata.xml.
- e.g. A USE flag may have one behavior for version 0.1 of a package,
while version 0.2, the USE flag may differ slightly.
- Each ``<flag>`` XML tag requires a 'name' attribute which is the full USE
flag name as it would appear in the IUSE section of the ebuild.
* e.g. "video_cards_i810" or "alsa"
- Each ``<flag>`` XML tag allows 0 or more nested ``<pkg>`` XML tags whose
character data is a valid CP or CPV as defined by the
`Gentoo Development Manual - Ebuild File Format` [#devmanual]_.
- Each ``<flag>`` XML tag allows 0 or more nested ``<cat>`` XML tags whose
character data is a valid category.
- The ``<use>`` XML tag may appear multiple times inside of the
``<pkgmetadata>`` XML tag if and only if it contains a different ``'lang'``
attribute value.
* The ``lang`` attribute follows the documented ``lang`` attribute in the
`Gentoo Developer Handbook` [#devhandbook]_.
Documentation for the `Gentoo Developer Handbook` [#devhandbook]_ and the
metadata.dtd can be found in Gentoo's Bugzilla [#use-flag-metadata-bug]_
bug #199788.
The following are two concrete examples in tree, [#use-flag-metadata-example1]_
and [#use-flag-metadata-example2]_.
And the following is an embedded example and not from a real package::
<use>
<flag name='acpi'>Enables HAL to attempt to read from
/proc/acpi/event, if unavailable, HAL will read events from
<pkg>sys-power/acpid</pkg>. If you need multiple acpi readers,
ensure acpid is in your default runlevel
(rc-update add acpid default) along with HAL. This will also
enable HAL to read Toshiba and IBM acpi events which do not
get sent via /proc/acpi/event</flag>
<flag name='spell'>Enables spell checking capability using
dictionaries found in <cat>app-dict</cat></flag>
</use>
Credits
=======
Thanks to the following persons for their input on or related to this GLEP
(even though they might not have known it):
Diego Pettenò (flameeyes), Alec Warner (antarus), Joshua Nichols (nichoj),
Steve Dibb (beandog), and Tiziano Müller (dev-zero)
References
==========
.. [#use-flag-metadata-bug] http://bugs.gentoo.org/show_bug.cgi?id=199788
.. [#use-flag-metadata-example1] http://sources.gentoo.org/viewcvs.py/gentoo-x86/sys-apps/hal/metadata.xml?view=markup
.. [#use-flag-metadata-example2] http://sources.gentoo.org/viewcvs.py/gentoo-x86/media-tv/mythtv/metadata.xml?view=markup
.. [#devhandbook] https://devmanual.gentoo.org/ebuild-writing/misc-files/metadata/index.html
.. [#devmanual] http://devmanual.gentoo.org/ebuild-writing/file-format/index.html
.. [#motivators1] http://blog.flameeyes.eu/articles/2007/11/19/lets-actually-get-some-metadata
.. [#motivators2] http://blog.cardoe.com/archives/2007/11/19/use-flag-metadata/
.. [#motivators3] http://blog.cardoe.com/archives/2007/11/23/metadataxml-updates-examples/
.. [#motivators4] http://technicalpickles.com/posts/pidgin-idle-time
Backwards Compatibility
=======================
No changes are necessary to existing ``metadata.xml`` files. Information in
the new tags is not mandatory. Tools that currently read ``metadata.xml``
files may break if written poorly, while well written tools should just ignore
the additional elements. Tools which are capable of handling the new tags
should prefer their data over ``use.desc`` and ``use.local.desc``.
USE flags still must be defined in ``use.desc`` or ``use.local.desc``. If the
USE flag is not found in either ``use.desc`` or ``use.local.desc``, the
information contained within the new tags in ``metadata.xml`` must be ignored
and QA tools should warn as they currently do.
Once this GLEP is approved, the Gentoo Infrastructure Team will work to remove
the ``use.local.desc`` file from CVS and it will be auto-generated for rsync.
This will ensure that backwards compatibility is not broken for users of
non-CVS trees. At this time, QA tools will need to be updated to verify the
contents of ``metadata.xml`` containing the necessary tags which would appear
in ``use.local.desc``.
Copyright
=========
This work is licensed under the Creative Commons Attribution-ShareAlike 3.0
Unported License. To view a copy of this license, visit
http://creativecommons.org/licenses/by-sa/3.0/.
.. vim: set ft=glep tw=72 :
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