GLEP:47
Title:Creating 'safe' environment variables
Version:1.8
Last-Modified:2006/01/26 20:56:55
Author:Diego Pettenò, Fabian rroffen <{flameeyes,grobian}@gentoo.org>
Status:Active
Type:Proposal
Content-Type:text/x-rst
Created:14-Oct-2005
Post-History:

Contents

Credits

The text of this GLEP is a result of a discussion and input of the following persons, in no particular order: Mike Frysinger, Diego Pettenò, Fabian Groffen and Finn Thain.

Abstract

In order for ebuilds and eclasses to be able to make host specific decisions, it is necessary to have a number of environmental variables which allow for such decisions. This GLEP introduces some measures that need to be made to make these decisions 'safe', by making sure the variables the decisions are based on are 'safe'. A small overlap with GLEP 22 [1] is being handled in this GLEP where the use of 2-tuple keywords are being kept instead of 4-tuple keywords. Additionally, the ELIBC, KERNEL and ARCH get auto filled starting from CHOST and the 2-tuple keyword, instead of solely from they 4-tuple keyword as proposed in GLEP 22.

The destiny of the USERLAND variable is out of the scope of this GLEP. Depending on its presence in the tree, it may be decided to set this variable the same way we propose to set ELIBC, KERNEL and ARCH, or alternatively, e.g. via the profiles.

Motivation

The Gentoo/Alt project is in an emerging state to get ready to serve a plethora of 'alternative' configurations such as FreeBSD, NetBSD, DragonflyBSD, GNU/kFreeBSD, Mac OS X, (Open)Darwin, (Open)Solaris and so on. As such, the project is in need for a better grip on the actual host being built on. This information on the host environment is necessary to make proper (automated) decisions on settings that are highly dependant on the build environment, such as platform or as in [2].

Rationale

Gentoo's unique Portage system allows easy installation of applications from source packages. Compiling sources is prone to many environmental settings and availability of certain tools. Only recently the Gentoo for FreeBSD project has started, as second Gentoo project that operates on a foreign host operating system using foreign (non-GNU) C-libraries and userland utilities. Such projects suffer from the current implicit assumption made within Gentoo Portage's ebuilds that there is a single type of operating system, C-libraries and system utilities. In order to enable ebuilds -- and also eclasses -- to be aware of these environmental differences, information regarding it should be supplied. Since decisions based on this information can be vital, it is of high importance that this information can be trusted and the values can be considered 'safe' and correct.

Backwards Compatibility

The proposed keywording scheme in this GLEP is fully compatible with the current situation of the portage tree, this in contrast to GLEP 22. The variables provided by GLEP 22 can't be extracted from the new keyword, but since GLEP 22-style keywords aren't in the tree at the moment, that is not a problem. The same information can be extracted from the CHOST variable, if necessary. No modifications to ebuilds will have to be made.

Specification

Unlike GLEP 22 the current keyword scheme as used in practice is not changed. Instead of proposing a 4-tuple [3] keyword, a 2-tuple keyword is chosen for archs that require them. Archs for which a 1-tuple keyword suffices, keep that keyword. Since this doesn't change anything to the current situation in the tree, it is considered to be a big advantage over the 4-tuple keyword from GLEP 22. This GLEP is an official specification of the syntax of the keyword.

Keywords will consist out of two parts separated by a hyphen ('-'). The left hand part of the keyword 2-tuple is the architecture, such as ppc64, sparc and x86. The right hand part indicates the operating system or distribution, such as linux, macos, darwin, obsd, etc. If the right hand part is omitted, it implies the operating system/distribution type is GNU/Linux. In such case the hyphen is also omitted. Examples of such keywords are ppc-darwin and x86. This is fully compatible with the current keywords used in the tree.

The variables ELIBC, KERNEL and ARCH are currently set in the profiles when other than their defaults for a GNU/Linux system. They can as such easily be overridden and defined by the user. To prevent this from happening, the variables should be auto filled by Portage itself, based on the CHOST variable.

A map file can be used to have the various CHOST values being translated to the correct values for the four variables. This change is invisible for ebuilds and eclasses, but allows to rely on these variables as they are based on a 'safe' value -- the CHOST variable. Ebuilds should not be sensitive to the keyword value, but use the aforementioned four variables instead. They allow specific tests for properties. If this is undesirable, the full CHOST variable can be used to match a complete operating system.

Current USE-expansion is being maintained, for backwards compatibility. Since the expansion is based on the variables mentioned above which do not change, but only in the way they are generated, there should be no problem in maintaining them.

Variables

The ELIBC, KERNEL, ARCH variables are filled from a profile file. The file can be overlaid, such that the following entries in the map file (on the left of the arrow) will result in the assigned variables on the right hand side of the arrow:

*-*-linux-*      -> KERNEL="linux"  
*-*-*-gnu        -> ELIBC="glibc"
*-*-kfreebsd-gnu -> KERNEL="FreeBSD" ELIBC="glibc"
*-*-freebsd*     -> KERNEL="FreeBSD" ELIBC="FreeBSD"
*-*-darwin*      -> KERNEL="Darwin" ELIBC="Darwin"
*-*-netbsd*      -> KERNEL="NetBSD" ELIBC="NetBSD"
*-*-solaris*     -> KERNEL="Solaris" ELIBC="Solaris"

A way to achieve this is proposed by Mike Frysinger [4], which suggests to have a env-map file, for instance filled with:

% cat env-map
*-linux-* KERNEL=linux
*-gnu ELIBC=glibc
x86_64-* ARCH=amd64

then the following bash script can be used to set the four variables to their correct values:

% cat readmap 
#!/bin/bash 

CBUILD=${CBUILD:-${CHOST=${CHOST:-$1}}} 
[[ -z ${CHOST} ]] && echo need chost 

unset KERNEL ELIBC ARCH 

while read LINE ; do 
    set -- ${LINE} 
    targ=$1 
    shift 
    [[ ${CBUILD} == ${targ} ]] && eval $@ 
done < env-map 

echo ARCH=${ARCH} KERNEL=${KERNEL} ELIBC=${ELIBC}

Given the example env-map file, this script would result in:

% ./readmap x86_64-pc-linux-gnu 
ARCH=amd64 KERNEL=linux ELIBC=glibc

It should be noted, however, that the bash script is a proof of concept implementation. It cannot be used as Portage will need this information, which is written in Python. Hence, an equivalent of this bash script should be written in Python to be able to use it within Portage. This is considered to be a non-issue coding wise.

References

[1]GLEP 22, New "keyword" system to incorporate various userlands/kernels/archs, Goodyear, (http://glep.gentoo.org/glep-0022.html)
[2]

For example in the perl ebuild, it is necessary to fill in the C-library part, which on a FreeBSD system is other than on a Linux system and currently is handled as follows:

[[ ${ELIBC} == "FreeBSD" ]] && myconf="${myconf} -Dlibc=/usr/lib/libc.a"
[3]For the purpose of readability, we will refer to 1, 2 and 4-tuples, even though tuple in itself suggest a field consisting of two values. For clarity: a 1-tuple describes a single value field, while a 4-tuple decribes a field consisting out of four values.
[4]mailto:vapier [at) gentoo .dot org