en/glep/glep-0056.txt

GLEP: 56
Title: USE flag descriptions in metadata
Version: $Revision: 1.1 $
Last-Modified: $Date: 2008/06/05 05:54:10 $
Author: Doug Goldstein <cardoe@gentoo.org>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 03-Jun-2008
Post-History: 13-Jun-2008

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? It's 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 Guide - 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 Developer Manual 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] http://www.gentoo.org/proj/en/devrel/handbook/handbook.xml?part=2&chap=4

.. [#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 document is placed into the public domain.

.. vim: set ft=glep tw=72 :

1	GLEP: 56
2	Title: USE flag descriptions in metadata
3	Version: $Revision: 1.1 $
4	Last-Modified: $Date: 2008/06/05 05:54:10 $
5	Author: Doug Goldstein <cardoe@gentoo.org>
6	Status: Draft
7	Type: Standards Track
8	Content-Type: text/x-rst
9	Created: 03-Jun-2008
10	Post-History: 13-Jun-2008
11
12	Abstract
13	========
14
15	This GLEP proposes to add per-package USE flag descriptions to each package's
16	metadata.
17
18
19	Motivation
20	==========
21
22	Gives Gentoo users the ability to better identify how USE flags affect their
23	installations of a given package. For example, many global USE flags have very
24	generic descriptions but no specifics on how it affects a certain package.
25	Specifically speaking, an example would be net-print/cups and the 'jpeg' USE
26	flag. Does this flag mean you won't be able to print jpeg files? You can print
27	them directly? It's interface won't use jpeg files.
28
29	- Motivator References: [#motivators1]_, [#motivators2]_, [#motivators3]_,
30	and [#motivators4]_
31
32
33	Specification
34	=============
35
36	This GLEP proposes the addition of ``<use>`` XML tag that is only allowed to
37	appear inside of a ``<pkgmetadata>`` XML tag.
38
39	- Inside the ``<use>`` XML tag, the ``<flag>`` XML tag is allowed to appear
40	once per USE flag as specified by the ``'name'`` attribute with the
41	following exception:
42
43	* The ``'restrict'`` atttribute can limit to specific versions of the
44	package, where the attribute value must be a valid CPV as defined by the
45	`Gentoo Developer Handbook` [#devhandbook]_. This follows the current
46	behavior of the ``'restrict'`` attribute in metadata.xml.
47
48	- e.g. A USE flag may have one behavior for version 0.1 of a package,
49	while version 0.2, the USE flag may differ slightly.
50
51	- Each ``<flag>`` XML tag requires a 'name' attribute which is the full USE
52	flag name as it would appear in the IUSE section of the ebuild.
53
54	* e.g. "video_cards_i810" or "alsa"
55
56	- Each ``<flag>`` XML tag allows 0 or more nested ``<pkg>`` XML tags whose
57	character data is a valid CP or CPV as defined by the
58	`Gentoo Development Guide - Ebuild File Format` [#devmanual]_.
59
60	- Each ``<flag>`` XML tag allows 0 or more nested ``<cat>`` XML tags whose
61	character data is a valid category.
62
63	- The ``<use>`` XML tag may appear multiple times inside of the
64	``<pkgmetadata>`` XML tag if and only if it contains a different ``'lang'``
65	attribute value.
66
67	* The ``lang`` attribute follows the documented ``lang`` attribute in the
68	`Gentoo Developer Handbook` [#devhandbook]_.
69
70	Documentation for the Developer Manual and the metadata.dtd can be found in
71	Gentoo's Bugzilla [#use-flag-metadata-bug]_ bug #199788.
72
73	The following are two concrete examples in tree, [#use-flag-metadata-example1]_
74	and [#use-flag-metadata-example2]_.
75
76	And the following is an embedded example and not from a real package::
77
78	<use>
79	<flag name='acpi'>Enables HAL to attempt to read from
80	/proc/acpi/event, if unavailable, HAL will read events from
81	<pkg>sys-power/acpid</pkg>. If you need multiple acpi readers,
82	ensure acpid is in your default runlevel
83	(rc-update add acpid default) along with HAL. This will also
84	enable HAL to read Toshiba and IBM acpi events which do not
85	get sent via /proc/acpi/event</flag>
86	<flag name='spell'>Enables spell checking capability using
87	dictionaries found in <cat>app-dict</cat></flag>
88	</use>
89
90
91
92	Credits
93	=======
94
95	Thanks to the following persons for their input on or related to this GLEP
96	(even though they might not have known it):
97	Diego Pettenò (flameeyes), Alec Warner (antarus), Joshua Nichols (nichoj),
98	Steve Dibb (beandog), and Tiziano Müller (dev-zero)
99
100
101	References
102	==========
103
104	.. [#use-flag-metadata-bug] http://bugs.gentoo.org/show_bug.cgi?id=199788
105
106	.. [#use-flag-metadata-example1] http://sources.gentoo.org/viewcvs.py/gentoo-x86/sys-apps/hal/metadata.xml?view=markup
107
108	.. [#use-flag-metadata-example2] http://sources.gentoo.org/viewcvs.py/gentoo-x86/media-tv/mythtv/metadata.xml?view=markup
109
110	.. [#devhandbook] http://www.gentoo.org/proj/en/devrel/handbook/handbook.xml?part=2&chap=4
111
112	.. [#devmanual] http://devmanual.gentoo.org/ebuild-writing/file-format/index.html
113
114	.. [#motivators1] http://blog.flameeyes.eu/articles/2007/11/19/lets-actually-get-some-metadata
115
116	.. [#motivators2] http://blog.cardoe.com/archives/2007/11/19/use-flag-metadata/
117
118	.. [#motivators3] http://blog.cardoe.com/archives/2007/11/23/metadataxml-updates-examples/
119
120	.. [#motivators4] http://technicalpickles.com/posts/pidgin-idle-time
121
122
123	Backwards Compatibility
124	=======================
125
126	No changes are necessary to existing ``metadata.xml`` files. Information in
127	the new tags is not mandatory. Tools that currently read ``metadata.xml``
128	files may break if written poorly, while well written tools should just ignore
129	the additional elements. Tools which are capable of handling the new tags
130	should prefer their data over ``use.desc`` and ``use.local.desc``.
131
132	USE flags still must be defined in ``use.desc`` or ``use.local.desc``. If the
133	USE flag is not found in either ``use.desc`` or ``use.local.desc``, the
134	information contained within the new tags in ``metadata.xml`` must be ignored
135	and QA tools should warn as they currently do.
136
137	Once this GLEP is approved, the Gentoo Infrastructure Team will work to remove
138	the ``use.local.desc`` file from CVS and it will be auto-generated for rsync.
139	This will ensure that backwards compatibility is not broken for users of
140	non-CVS trees. At this time, QA tools will need to be updated to verify the
141	contents of ``metadata.xml`` containing the necessary tags which would appear
142	in ``use.local.desc``.
143
144
145	Copyright
146	=========
147
148	This document is placed into the public domain.
149
150	.. vim: set ft=glep tw=72 :
151