GLEP:21

Status
Abandoned. Package set support has been added in portage-2.2, but it doesn't match the description in this document in many cases, and the document has several major gaps regarding the behavior and restrictions of package sets.

Abstract
In Portage, package sets (formerly known as 'classes' or 'targets') are mere groups of packages, grouped together to allow easier updating and handling of them. Currently it is impossible to define sets further than the two default ones: "system" and "world".

Motivation
Over the months, quite a few requests for user-defined sets were made by users and developers, either by posting bugs, messages to mailing lists or on IRC. Usually the response is that this is an awesome idea, but no one ever took the time to actually define it properly and implement it.

This document offers a specification for the implementation of user-defined sets using configuration files similar to the current world/system sets use.

Specification
The proposed implementation uses a one file per set approach, meaning each package set is defined in a single file. All set definition files will reside in a directory  and each set's name will be its file name. Therefore, if one defines a set in /etc/portage/sets/foo-set, the set name will be 'foo-set'. Usual package naming rules also apply to sets.

As it is impossible to create two or more files with identical names in the same directory, a theoretic conflict between two different sets sharing the same name is impossible. However, users may define a package set whose name conflicts with one more or packages (for ambiguity resolution, see below).

Syntax for the package list file is the same as the world file syntax, as described in the Portage manpage, with one addition: sets may not be 'inherited' by other sets, only packages may be listed. There is no limitation to the number of packages in a set or to the number of sets a package may belong to.

Using User-defined Sets With Emerge
The user-defined sets will be available either directly or using the --package-set option, As in: # Basically the same: emerge foo-set emerge --package-set foo-set

The --package-set option is introduced to bypass ambiguities, as illustrated in the next example:

emerge foo 		 # Where foo is both a set and a one or more # existing packages. This will cause emerge to show # the ambiguity, ask us to be more # specific, and stop. emerge --package-set foo # So we specify that what we actually # meant was the package set. emerge cat-bar/foo	 # Or we specify the exact package name.

When running emerge with the --pretend option, sets will be expanded to the packages they are comprised off in the output, as with the current system-defined sets.

Only one set may be passed to portage at time, and sets can not be mixed with ordinary packages. Thus, the following snippets are all invalid and will result in an error (assuming   and   are defined as sets):

emerge foo-set glibc emerge bar-set system emerge world foo-set gcc

Compatibility With Other Portage Features

 * Dependencies: Package sets (both system-defined and user-defined) may not be depended on by ordinary packages and eclasses.
 * package.mask: Masking a package set through the  file is forbidden. In order to 'mask' a package set, one should move it away from the sets directory.
 * package.use: USE flags may not be defined for sets in the  file.

Implementation
The implementation of the package sets concept in Portage should be mostly done in portage.py, and only the interface parts should be added to emerge itself, to keep the separation between interface and logic.

The amount of work needed for implementation is not trivial, but not huge either.

Rationale
The one file per set approach makes it easy to list the sets which are defined on a system by just listing the  directory contents. Additionally, it makes the set lookup process more efficient as it only requires to check if a file exists.

I chose the --package-set option over the --set option for explicitly telling portage to emerge a set mostly because --set implies setting an environment variable, or such.

Allowing sets' USE flags to be manipulated through the  file would have done more harm than good, for several reasons:


 * If a USE flag is turned on (i.e. 'foo') for a set and the same USE flag is turned off (i.e. '-foo'), for a package which is part of the set, it is unclear which setting should take precedence.
 * Similarly, if a USE flag is turned on for a set and the same USE flag is turned off for a set that is a subset of the original set, it is unclear which setting should take precedence.
 * If a USE flag is defined (either off or on) for a set and a package that belongs in the set is to be emerged, it is unclear whether the USE flag should be defined when emerging the package in question.

Therefore, I have decided it would be better to disallow setting USE flags for sets.

Backwards Compatibility
Backwards compatibility with the current situation, in which only two system-defined sets exist can be kept in one of two ways:


 * 1) Leaving the situation as is - the 'world' and 'system' sets are hard-coded in Portage.
 * 2) Distributing default 'system' and 'world' files under the   directory.

Other than that, there are no other backwards compatibility concerns involved.

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/.