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GLEP: 62
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Title: Optional runtime dependencies via runtime-switchable USE flags
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Version: $Revision: 1.1 $
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Last-Modified: $Date: 2012/07/11 20:21:27 $
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Author: Michał Górny <mgorny@gentoo.org>
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Status: Draft
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Type: Standards Track
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Content-Type: text/x-rst
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Created: 17 Jun 2012
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Post-History: 11 Jul 2012
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Abstract
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========
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This GLEP addresses the issue of referencing optional runtime
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dependencies in Gentoo packages and ebuilds. It does introduce
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a concept of runtime-switchable USE flags to achieve that goal.
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Motivation
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==========
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Optional runtime dependencies are often found in packages installing
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various scripts (shell, python, perl). These are not strictly required
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for the particular package to work but installing them enables
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additional functionality.
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Unlike in compiled programs, enabling or disabling those features
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(dependencies) does not affect the files installed by the package.
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They can be installed and uninstalled independently of the package,
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resulting in changes of functionality without a need to rebuild
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the package.
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Currently such dependencies are usually expressed only through
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``pkg_postinst()`` messages. This forces user to manually install
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the necessary dependencies, and uninstall them when they are no longer
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necessary.
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Another solution is to use regular USE flags. Those flags do not strictly
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follow the principles of USE flags because they do not affect files
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installed by the package and are not entirely effective to the package
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(a disabled feature will still be available if necessary dependency is
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installed). Additionally, it requires unnecessary rebuilds
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of the package in order to change the dependencies.
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Specification
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=============
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The ebuilds aiming to provide features enabled through optional runtime
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dependencies should:
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1. create regular USE flags for all those features, following
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appropriate specifications for Gentoo ebuilds, and including
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the flags in the ``IUSE`` variable;
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2. introduce additional ``IUSE_RUNTIME`` variable listing names of USE
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flags related to optional runtime dependencies (without prefixes
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related to IUSE defaults).
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Additionally, the ebuilds must obey the following rules:
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1. all flags listed in ``IUSE_RUNTIME`` have to be listed in ``IUSE``
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as well,
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2. flags listed in ``IUSE_RUNTIME`` can be referenced in ``RDEPEND``,
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``PDEPEND`` and ``REQUIRED_USE`` variables,
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3. flags listed in ``IUSE_RUNTIME`` must not be referenced in phase
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functions, ``DEPEND``, ``LICENSE`` or ``SRC_URI``,
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4. flags listed in ``IUSE_RUNTIME`` can be referenced through USE
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dependencies by other packages' ``DEPEND``, ``RDEPEND``
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and ``PDEPEND`` variables but it is unallowed to request disabling
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those flags (only ``[flag]`` and ``[flag?]`` forms are allowed),
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5. flags listed in ``IUSE_RUNTIME`` can be referenced through
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``has_version`` and ``best_version`` yet the caller must not rely
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upon those flags being disabled.
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The package manager should treat flags listed in ``IUSE_RUNTIME``
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as regular USE flags, except for the following:
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1. enabling or disabling any of the flags must not involve rebuilding
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the package,
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2. it should be possible for a package manager to change those flags
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on a installed package without using the original ebuild [1]_,
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3. when queried on a installed package, the package manager must
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consider a particular flag enabled only if its dependencies
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are satisfied already [2]_,
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4. the flags may be listed in the visual output in a distinct way
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to inform the user that they affect runtime dependencies only.
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.. [1] The package manager has to ensure that all relevant information
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is stored in the installed package metadata.
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.. [2] The result of this check can be cached when updating the metadata
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of installed package, and it is not strictly required that
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a package manager must ensure that the dependency graph is still
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consistent afterwards.
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Rationale
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=========
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The proposed solution tries to solve the issue of handling runtime
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dependencies while reusing the existing infrastructure. Most
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importantly, users will be able to reuse the existing tools
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and configuration files to enable and disable optional runtime
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and build-time dependencies alike.
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The remaining reused features include:
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- dependency syntax (USE-conditionals),
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- ability to use ``REQUIRED_USE``, USE dependencies,
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- ability to describe flags in `metadata.xml`,
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- global flag names (and descriptions).
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Alternative proposed solution involved creating additional ``SDEPEND``
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variable. That proposition had the following disadvantages:
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- being package-oriented rather than feature-oriented,
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- lack of ability to express multiple packages required by a single
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feature,
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- lack of ability to express cross-feature dependencies,
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- lack of ability to describe features provided by enabled packages,
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- necessity of implementing a new user interface parts to control
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the dependencies,
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- lack of backwards compatibility.
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Those disadvantages could be fixed by either extending dependency
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syntax alike Exherbo (grouping, annotations) or using USE flags.
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The latter is practically equivalent to this solution yet introduces
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another variable unnecessarily.
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Reference implementation
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========================
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In order to support runtime-switchable USE flag changes on installed
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packages, a package manager should store the following information
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in the installed package metadata:
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1. a list of runtime-switchable flags (``IUSE_RUNTIME``),
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2. a list of runtime dependencies conditional to runtime-switchable
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flags, not subjected to USE expansion (relevant part of ``RDEPEND``),
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3. a list of ``REQUIRED_USE`` constraints relevant to runtime-switchable
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USE flags.
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The package manager should be also able to update the list of effective
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USE flags in installed package metadata (``USE``) without rebuilding
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the package.
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The following procedure should apply when a dependency atom is
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considered (either as a package dependency or user-requested atom):
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1. if the dependency is not satisfied with an installed package,
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perform the dependency resolution on ebuilds as usual (in order to
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install a new package);
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2. if runtime-switchable USE updates are disabled, continue to
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the next dependency (with this one satisfied);
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3. ``current_use`` = installed package metadata . ``USE``;
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4. ``effective_use`` = requested ``USE`` for the package;
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5. ``use_changes`` = ``current_use`` xor ``effective_use``;
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6. if ``use_changes`` is empty, continue to the next dependency;
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7. ``iuse_runtime`` = installed package metadata . ``IUSE_RUNTIME``;
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8. ``reg_use_changes`` = ``use_changes`` and not ``iuse_runtime``;
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9. if ``reg_use_changes`` is not empty and regular USE updates
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are enabled, perform the dependency resolution on ebuilds (in order
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to rebuild the package);
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10. ``run_use_changes`` = ``use_changes`` and ``iuse_runtime``;
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11. if ``run_use_changes`` is empty, continue to the next dependency;
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12. ``required_use`` = installed package metadata . ``REQUIRED_USE``;
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13. perform a check for ``required_use`` constraints being satisfied
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by ``effective_use``;
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14. ``run_rdepend`` = installed package metadata . ``RDEPEND`` (with
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unexpanded ``IUSE_RUNTIME`` conditionals);
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15. perform the dependency resolution for ``run_rdepend`` with
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``effective_use``,
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16. queue the package for runtime-switchable USE update.
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Furthermore, after installing all dependencies introduced by the above
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procedure (``run_rdepend`` resolution), if a package was queued for
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runtime-switchable USE update, the package manager should write a new
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value for ``USE`` key in installed package metadata.
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Backwards compatibility
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=======================
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Package managers not implementing this GLEP will consider
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the ``IUSE_RUNTIME`` variable as an irrelevant bash variable and treat
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runtime-switchable USE flags as regular USE flags. The dependency tree
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will still be consistent yet packages may be rebuilt unnecessarily.
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Copyright
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=========
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This document has been placed in the public domain.
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