GLEP:62

Abstract
This GLEP addresses the issue of referencing optional runtime dependencies in Gentoo packages and ebuilds. It does introduce a concept of runtime-switchable USE flags to achieve that goal.

Motivation
Optional runtime dependencies are often found in packages installing various scripts (shell, python, perl). These are not strictly required for the particular package to work but installing them enables additional functionality.

Unlike in compiled programs, enabling or disabling those features (dependencies) does not affect the files installed by the package. They can be installed and uninstalled independently of the package, resulting in changes of functionality without a need to rebuild the package.

Currently such dependencies are usually expressed only through  messages. This forces user to manually install the necessary dependencies, and uninstall them when they are no longer necessary.

Another solution is to use regular USE flags. Those flags do not strictly follow the principles of USE flags because they do not affect files installed by the package and are not entirely effective to the package (a disabled feature will still be available if necessary dependency is installed). Additionally, it requires unnecessary rebuilds of the package in order to change the dependencies.

Specification
The ebuilds aiming to provide features enabled through optional runtime dependencies should:


 * 1) create regular USE flags for all those features, following appropriate specifications for Gentoo ebuilds, and including the flags in the   variable;
 * 2) introduce additional   variable listing names of USE flags related to optional runtime dependencies (without prefixes related to IUSE defaults).

Additionally, the ebuilds must obey the following rules:


 * 1) all flags listed in   have to be listed in   as well,
 * 2) flags listed in   can be referenced in ,   and   variables,
 * 3) flags listed in   must not be referenced in phase functions, ,   or  ,
 * 4) flags listed in   can be referenced through USE dependencies by other packages' ,   and   variables but it is unallowed to request disabling those flags (only   and   forms are allowed),
 * 5) flags listed in   can be referenced through   and   yet the caller must not rely upon those flags being disabled.

The package manager should treat flags listed in  as regular USE flags, except for the following:


 * 1) enabling or disabling any of the flags must not involve rebuilding the package,
 * 2) it should be possible for a package manager to change those flags on a installed package without using the original ebuild
 * 3) when queried on a installed package, the package manager must consider a particular flag enabled only if its dependencies are satisfied already ,
 * 4) the flags may be listed in the visual output in a distinct way to inform the user that they affect runtime dependencies only.

Rationale
The proposed solution tries to solve the issue of handling runtime dependencies while reusing the existing infrastructure. Most importantly, users will be able to reuse the existing tools and configuration files to enable and disable optional runtime and build-time dependencies alike.

The remaining reused features include:


 * dependency syntax (USE-conditionals),
 * ability to use, USE dependencies,
 * ability to describe flags in metadata.xml,
 * global flag names (and descriptions).

Alternative proposed solution involved creating additional  variable. That proposition had the following disadvantages:


 * being package-oriented rather than feature-oriented,
 * lack of ability to express multiple packages required by a single feature,
 * lack of ability to express cross-feature dependencies,
 * lack of ability to describe features provided by enabled packages,
 * necessity of implementing a new user interface parts to control the dependencies,
 * lack of backwards compatibility.

Those disadvantages could be fixed by either extending dependency syntax alike Exherbo (grouping, annotations) or using USE flags. The latter is practically equivalent to this solution yet introduces another variable unnecessarily.

Reference implementation
In order to support runtime-switchable USE flag changes on installed packages, a package manager should store the following information in the installed package metadata:


 * 1) a list of runtime-switchable flags ,
 * 2) a list of runtime dependencies conditional to runtime-switchable flags, not subjected to USE expansion (relevant part of  ),
 * 3) a list of   constraints relevant to runtime-switchable USE flags.

The package manager should be also able to update the list of effective USE flags in installed package metadata without rebuilding the package.

The following procedure should apply when a dependency atom is considered (either as a package dependency or user-requested atom):


 * 1) if the dependency is not satisfied with an installed package, perform the dependency resolution on ebuilds as usual (in order to install a new package);
 * 2) if runtime-switchable USE updates are disabled, continue to the next dependency (with this one satisfied);
 * 3)   = installed package metadata .  ;
 * 4)   = requested   for the package;
 * 5)   =   xor  ;
 * 6) if   is empty, continue to the next dependency;
 * 7)   = installed package metadata .  ;
 * 8)   =   and not  ;
 * 9) if   is not empty and regular USE updates are enabled, perform the dependency resolution on ebuilds (in order to rebuild the package);
 * 10)   =   and  ;
 * 11) if   is empty, continue to the next dependency;
 * 12)   = installed package metadata .  ;
 * 13) perform a check for   constraints being satisfied  by  ;
 * 14)   = installed package metadata .   (with unexpanded   conditionals);
 * 15) perform the dependency resolution for   with  ,
 * 16) queue the package for runtime-switchable USE update.

Furthermore, after installing all dependencies introduced by the above procedure ( resolution), if a package was queued for runtime-switchable USE update, the package manager should write a new value for   key in installed package metadata.

Backwards compatibility
Package managers not implementing this GLEP will consider the  variable as an irrelevant bash variable and treat runtime-switchable USE flags as regular USE flags. The dependency tree will still be consistent yet packages may be rebuilt unnecessarily.

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/.