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Adding GLEP 47: Creating 'safe' environment variables

1 grobian 1.1 GLEP: 47
2     Title: Creating 'safe' environment variables
3     Version: $Revision: 1.8 $
4     Last-Modified: $Date: 2006/01/26 20:56:55 $
5     Author: Diego Pettenò, Fabian Groffen <{flameeyes,grobian}@gentoo.org>
6     Status: Active
7     Type: Standards Track
8     Content-Type: text/x-rst
9     Created: 14-Oct-2005
10     Post-History:
13     Credits
14     =======
16     The text of this GLEP is a result of a discussion and input of the
17     following persons, in no particular order: Mike Frysinger, Diego
18     Pettenò, Fabian Groffen and Finn Thain.
21     Abstract
22     ========
24     In order for ebuilds and eclasses to be able to make host specific
25     decisions, it is necessary to have a number of environmental variables
26     which allow for such decisions. This GLEP introduces some measures that
27     need to be made to make these decisions 'safe', by making sure the
28     variables the decisions are based on are 'safe'. A small overlap with
29     GLEP 22 [1]_ is being handled in this GLEP where the use of 2-tuple
30     keywords are being kept instead of 4-tuple keywords. Additionally, the
31     ``ELIBC``, ``KERNEL`` and ``ARCH`` get auto filled starting from
32     ``CHOST`` and the 2-tuple keyword, instead of solely from they 4-tuple
33     keyword as proposed in GLEP 22.
35     The destiny of the ``USERLAND`` variable is out of the scope of this
36     GLEP. Depending on its presence in the tree, it may be decided to set
37     this variable the same way we propose to set ``ELIBC``, ``KERNEL`` and
38     ``ARCH``, or alternatively, e.g. via the profiles.
41     Motivation
42     ==========
44     The Gentoo/Alt project is in an emerging state to get ready to serve a
45     plethora of 'alternative' configurations such as FreeBSD, NetBSD,
46     DragonflyBSD, GNU/kFreeBSD, Mac OS X, (Open)Darwin, (Open)Solaris and so
47     on. As such, the project is in need for a better grip on the actual
48     host being built on. This information on the host environment is
49     necessary to make proper (automated) decisions on settings that are
50     highly dependant on the build environment, such as platform or as in
51     [2]_.
54     Rationale
55     =========
57     Gentoo's unique Portage system allows easy installation of applications
58     from source packages. Compiling sources is prone to many environmental
59     settings and availability of certain tools. Only recently the Gentoo
60     for FreeBSD project has started, as second Gentoo project that operates
61     on a foreign host operating system using foreign (non-GNU) C-libraries
62     and userland utilities. Such projects suffer from the current implicit
63     assumption made within Gentoo Portage's ebuilds that there is a single
64     type of operating system, C-libraries and system utilities. In order to
65     enable ebuilds -- and also eclasses -- to be aware of these
66     environmental differences, information regarding it should be supplied.
67     Since decisions based on this information can be vital, it is of high
68     importance that this information can be trusted and the values can be
69     considered 'safe' and correct.
72     Backwards Compatibility
73     =======================
75     The proposed keywording scheme in this GLEP is fully compatible with the
76     current situation of the portage tree, this in contrast to GLEP 22. The
77     variables provided by GLEP 22 can't be extracted from the new keyword,
78     but since GLEP 22-style keywords aren't in the tree at the moment, that
79     is not a problem. The same information can be extracted from the CHOST
80     variable, if necessary. No modifications to ebuilds will have to be
81     made.
84     Specification
85     =============
87     Unlike GLEP 22 the current keyword scheme as used in practice is not
88     changed. Instead of proposing a 4-tuple [3]_ keyword, a 2-tuple
89     keyword is chosen for archs that require them. Archs for which a
90     1-tuple keyword suffices, keep that keyword. Since this doesn't change
91     anything to the current situation in the tree, it is considered to be a
92     big advantage over the 4-tuple keyword from GLEP 22. This GLEP is an
93     official specification of the syntax of the keyword.
95     Keywords will consist out of two parts separated by a hyphen ('-'). The
96     left hand part of the keyword 2-tuple is the architecture, such as
97     ppc64, sparc and x86. The right hand part indicates the operating
98     system or distribution, such as linux, macos, darwin, obsd, etc. If the
99     right hand part is omitted, it implies the operating system/distribution
100     type is GNU/Linux. In such case the hyphen is also omitted. Examples
101     of such keywords are ppc-darwin and x86. This is fully compatible with
102     the current keywords used in the tree.
104     The variables ``ELIBC``, ``KERNEL`` and ``ARCH`` are currently set in
105     the profiles when other than their defaults for a GNU/Linux system.
106     They can as such easily be overridden and defined by the user. To
107     prevent this from happening, the variables should be auto filled by
108     Portage itself, based on the ``CHOST`` variable.
110     A map file can be used to have the various ``CHOST`` values being
111     translated to the correct values for the four variables. This change is
112     invisible for ebuilds and eclasses, but allows to rely on these
113     variables as they are based on a 'safe' value -- the ``CHOST`` variable.
114     Ebuilds should not be sensitive to the keyword value, but use the
115     aforementioned four variables instead. They allow specific tests for
116     properties. If this is undesirable, the full ``CHOST`` variable can be
117     used to match a complete operating system.
119     Current USE-expansion is being maintained, for backwards compatibility.
120     Since the expansion is based on the variables mentioned above which do
121     not change, but only in the way they are generated, there should be no
122     problem in maintaining them.
125     Variables
126     ---------
128     The ``ELIBC``, ``KERNEL``, ``ARCH`` variables are filled from a profile
129     file. The file can be overlaid, such that the following entries in the
130     map file (on the left of the arrow) will result in the assigned
131     variables on the right hand side of the arrow:
133     ::
135     *-*-linux-* -> KERNEL="linux"
136     *-*-*-gnu -> ELIBC="glibc"
137     *-*-kfreebsd-gnu -> KERNEL="FreeBSD" ELIBC="glibc"
138     *-*-freebsd* -> KERNEL="FreeBSD" ELIBC="FreeBSD"
139     *-*-darwin* -> KERNEL="Darwin" ELIBC="Darwin"
140     *-*-netbsd* -> KERNEL="NetBSD" ELIBC="NetBSD"
141     *-*-solaris* -> KERNEL="Solaris" ELIBC="Solaris"
143     A way to achieve this is proposed by Mike Frysinger [4]_, which
144     suggests to have a env-map file, for instance filled with:
146     ::
148     % cat env-map
149     *-linux-* KERNEL=linux
150     *-gnu ELIBC=glibc
151     x86_64-* ARCH=amd64
153     then the following bash script can be used to set the four variables to
154     their correct values:
156     ::
158     % cat readmap
159     #!/bin/bash
161     CBUILD=${CBUILD:-${CHOST=${CHOST:-$1}}}
162     [[ -z ${CHOST} ]] && echo need chost
164     unset KERNEL ELIBC ARCH
166     while read LINE ; do
167     set -- ${LINE}
168     targ=$1
169     shift
170     [[ ${CBUILD} == ${targ} ]] && eval $@
171     done < env-map
175     Given the example env-map file, this script would result in:
177     ::
179     % ./readmap x86_64-pc-linux-gnu
180     ARCH=amd64 KERNEL=linux ELIBC=glibc
182     It should be noted, however, that the bash script is a proof of concept
183     implementation. It cannot be used as Portage will need this
184     information, which is written in Python. Hence, an equivalent of this
185     bash script should be written in Python to be able to use it within
186     Portage. This is considered to be a non-issue coding wise.
189     References
190     ==========
192     .. [1] GLEP 22, New "keyword" system to incorporate various
193     userlands/kernels/archs, Goodyear,
194     (http://glep.gentoo.org/glep-0022.html)
196     .. [2] For example in the perl ebuild, it is necessary to
197     fill in the C-library part, which on a FreeBSD system is other than
198     on a Linux system and currently is handled as follows:
199     ::
201     [[ ${ELIBC} == "FreeBSD" ]] && myconf="${myconf} -Dlibc=/usr/lib/libc.a"
203     .. [3] For the purpose of readability, we will refer to 1, 2 and
204     4-tuples, even though tuple in itself suggest a field consisting of
205     two values. For clarity: a 1-tuple describes a single value field,
206     while a 4-tuple decribes a field consisting out of four values.
208     .. [4] mailto:vapier [at) gentoo .dot org
212     Copyright
213     =========
215     This document has been placed in the public domain.

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