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<title>GLEP 62 -- Optional runtime dependencies via runtime-switchable USE flags</title> |
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<col class="field-body" /> |
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<tr class="field"><th class="field-name">GLEP:</th><td class="field-body">62</td> |
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</tr> |
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<tr class="field"><th class="field-name">Title:</th><td class="field-body">Optional runtime dependencies via runtime-switchable USE flags</td> |
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</tr> |
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<tr class="field"><th class="field-name">Version:</th><td class="field-body">1.2</td> |
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</tr> |
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<tr class="field"><th class="field-name">Last-Modified:</th><td class="field-body"><a class="reference external" href="http://www.gentoo.org/cgi-bin/viewcvs.cgi/xml/htdocs/proj/en/glep/glep-0062.txt?cvsroot=gentoo">2012/07/11 20:24:37</a></td> |
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</tr> |
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<tr class="field"><th class="field-name">Author:</th><td class="field-body">Michał Górny <mgorny at gentoo.org></td> |
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</tr> |
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<tr class="field"><th class="field-name">Status:</th><td class="field-body">Draft</td> |
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</tr> |
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<tr class="field"><th class="field-name">Type:</th><td class="field-body">Standards Track</td> |
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<tr class="field"><th class="field-name">Content-Type:</th><td class="field-body"><a class="reference external" href="glep-0002.html">text/x-rst</a></td> |
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</tr> |
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<tr class="field"><th class="field-name">Created:</th><td class="field-body">17 Jun 2012</td> |
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</tr> |
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<tr class="field"><th class="field-name">Post-History:</th><td class="field-body">11 Jul 2012</td> |
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</tr> |
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</tbody> |
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</table> |
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<hr /> |
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<div class="contents topic" id="contents"> |
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<p class="topic-title first">Contents</p> |
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<ul class="simple"> |
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<li><a class="reference internal" href="#abstract" id="id6">Abstract</a></li> |
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<li><a class="reference internal" href="#motivation" id="id7">Motivation</a></li> |
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<li><a class="reference internal" href="#specification" id="id8">Specification</a></li> |
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<li><a class="reference internal" href="#rationale" id="id9">Rationale</a></li> |
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<li><a class="reference internal" href="#reference-implementation" id="id10">Reference implementation</a></li> |
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<li><a class="reference internal" href="#backwards-compatibility" id="id11">Backwards compatibility</a></li> |
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<li><a class="reference internal" href="#copyright" id="id12">Copyright</a></li> |
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</ul> |
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</div> |
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<div class="section" id="abstract"> |
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<h1><a class="toc-backref" href="#id6">Abstract</a></h1> |
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<p>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.</p> |
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</div> |
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<div class="section" id="motivation"> |
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<h1><a class="toc-backref" href="#id7">Motivation</a></h1> |
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<p>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.</p> |
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<p>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.</p> |
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<p>Currently such dependencies are usually expressed only through |
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<tt class="docutils literal">pkg_postinst()</tt> 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.</p> |
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<p>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.</p> |
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</div> |
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<div class="section" id="specification"> |
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<h1><a class="toc-backref" href="#id8">Specification</a></h1> |
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<p>The ebuilds aiming to provide features enabled through optional runtime |
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dependencies should:</p> |
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<ol class="arabic simple"> |
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<li>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 <tt class="docutils literal">IUSE</tt> variable;</li> |
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<li>introduce additional <tt class="docutils literal">IUSE_RUNTIME</tt> 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).</li> |
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</ol> |
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<p>Additionally, the ebuilds must obey the following rules:</p> |
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<ol class="arabic simple"> |
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<li>all flags listed in <tt class="docutils literal">IUSE_RUNTIME</tt> have to be listed in <tt class="docutils literal">IUSE</tt> |
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as well,</li> |
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<li>flags listed in <tt class="docutils literal">IUSE_RUNTIME</tt> can be referenced in <tt class="docutils literal">RDEPEND</tt>, |
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<tt class="docutils literal">PDEPEND</tt> and <tt class="docutils literal">REQUIRED_USE</tt> variables,</li> |
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<li>flags listed in <tt class="docutils literal">IUSE_RUNTIME</tt> must not be referenced in phase |
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functions, <tt class="docutils literal">DEPEND</tt>, <tt class="docutils literal">LICENSE</tt> or <tt class="docutils literal">SRC_URI</tt>,</li> |
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<li>flags listed in <tt class="docutils literal">IUSE_RUNTIME</tt> can be referenced through USE |
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dependencies by other packages' <tt class="docutils literal">DEPEND</tt>, <tt class="docutils literal">RDEPEND</tt> |
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and <tt class="docutils literal">PDEPEND</tt> variables but it is unallowed to request disabling |
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those flags (only <tt class="docutils literal">[flag]</tt> and <tt class="docutils literal"><span class="pre">[flag?]</span></tt> forms are allowed),</li> |
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<li>flags listed in <tt class="docutils literal">IUSE_RUNTIME</tt> can be referenced through |
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<tt class="docutils literal">has_version</tt> and <tt class="docutils literal">best_version</tt> yet the caller must not rely |
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upon those flags being disabled.</li> |
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</ol> |
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<p>The package manager should treat flags listed in <tt class="docutils literal">IUSE_RUNTIME</tt> |
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as regular USE flags, except for the following:</p> |
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<ol class="arabic simple"> |
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<li>enabling or disabling any of the flags must not involve rebuilding |
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the package,</li> |
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<li>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 <a class="footnote-reference" href="#id3" id="id1">[1]</a>,</li> |
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<li>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 <a class="footnote-reference" href="#id4" id="id2">[2]</a>,</li> |
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<li>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.</li> |
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</ol> |
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<table class="docutils footnote" frame="void" id="id3" rules="none"> |
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<colgroup><col class="label" /><col /></colgroup> |
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<tbody valign="top"> |
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<tr><td class="label"><a class="fn-backref" href="#id1">[1]</a></td><td>The package manager has to ensure that all relevant information |
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is stored in the installed package metadata.</td></tr> |
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</tbody> |
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</table> |
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<table class="docutils footnote" frame="void" id="id4" rules="none"> |
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<colgroup><col class="label" /><col /></colgroup> |
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<tbody valign="top"> |
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<tr><td class="label"><a class="fn-backref" href="#id2">[2]</a></td><td>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.</td></tr> |
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</tbody> |
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</table> |
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</div> |
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<div class="section" id="rationale"> |
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<h1><a class="toc-backref" href="#id9">Rationale</a></h1> |
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<p>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.</p> |
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<p>The remaining reused features include:</p> |
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<ul class="simple"> |
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<li>dependency syntax (USE-conditionals),</li> |
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<li>ability to use <tt class="docutils literal">REQUIRED_USE</tt>, USE dependencies,</li> |
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<li>ability to describe flags in <cite>metadata.xml</cite>,</li> |
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<li>global flag names (and descriptions).</li> |
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</ul> |
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<p>Alternative proposed solution involved creating additional <tt class="docutils literal">SDEPEND</tt> |
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variable. That proposition had the following disadvantages:</p> |
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<ul class="simple"> |
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<li>being package-oriented rather than feature-oriented,</li> |
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<li>lack of ability to express multiple packages required by a single |
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feature,</li> |
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<li>lack of ability to express cross-feature dependencies,</li> |
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<li>lack of ability to describe features provided by enabled packages,</li> |
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<li>necessity of implementing a new user interface parts to control |
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the dependencies,</li> |
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<li>lack of backwards compatibility.</li> |
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</ul> |
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<p>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.</p> |
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</div> |
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<div class="section" id="reference-implementation"> |
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<h1><a class="toc-backref" href="#id10">Reference implementation</a></h1> |
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<p>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:</p> |
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<ol class="arabic simple"> |
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<li>a list of runtime-switchable flags (<tt class="docutils literal">IUSE_RUNTIME</tt>),</li> |
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<li>a list of runtime dependencies conditional to runtime-switchable |
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flags, not subjected to USE expansion (relevant part of <tt class="docutils literal">RDEPEND</tt>),</li> |
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<li>a list of <tt class="docutils literal">REQUIRED_USE</tt> constraints relevant to runtime-switchable |
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USE flags.</li> |
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</ol> |
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<p>The package manager should be also able to update the list of effective |
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USE flags in installed package metadata (<tt class="docutils literal">USE</tt>) without rebuilding |
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the package.</p> |
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<p>The following procedure should apply when a dependency atom is |
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considered (either as a package dependency or user-requested atom):</p> |
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<ol class="arabic simple"> |
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<li>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);</li> |
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<li>if runtime-switchable USE updates are disabled, continue to |
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the next dependency (with this one satisfied);</li> |
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<li><tt class="docutils literal">current_use</tt> = installed package metadata . <tt class="docutils literal">USE</tt>;</li> |
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<li><tt class="docutils literal">effective_use</tt> = requested <tt class="docutils literal">USE</tt> for the package;</li> |
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<li><tt class="docutils literal">use_changes</tt> = <tt class="docutils literal">current_use</tt> xor <tt class="docutils literal">effective_use</tt>;</li> |
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<li>if <tt class="docutils literal">use_changes</tt> is empty, continue to the next dependency;</li> |
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<li><tt class="docutils literal">iuse_runtime</tt> = installed package metadata . <tt class="docutils literal">IUSE_RUNTIME</tt>;</li> |
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<li><tt class="docutils literal">reg_use_changes</tt> = <tt class="docutils literal">use_changes</tt> and not <tt class="docutils literal">iuse_runtime</tt>;</li> |
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<li>if <tt class="docutils literal">reg_use_changes</tt> 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);</li> |
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<li><tt class="docutils literal">run_use_changes</tt> = <tt class="docutils literal">use_changes</tt> and <tt class="docutils literal">iuse_runtime</tt>;</li> |
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<li>if <tt class="docutils literal">run_use_changes</tt> is empty, continue to the next dependency;</li> |
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<li><tt class="docutils literal">required_use</tt> = installed package metadata . <tt class="docutils literal">REQUIRED_USE</tt>;</li> |
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<li>perform a check for <tt class="docutils literal">required_use</tt> constraints being satisfied |
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by <tt class="docutils literal">effective_use</tt>;</li> |
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<li><tt class="docutils literal">run_rdepend</tt> = installed package metadata . <tt class="docutils literal">RDEPEND</tt> (with |
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unexpanded <tt class="docutils literal">IUSE_RUNTIME</tt> conditionals);</li> |
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<li>perform the dependency resolution for <tt class="docutils literal">run_rdepend</tt> with |
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<tt class="docutils literal">effective_use</tt>,</li> |
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<li>queue the package for runtime-switchable USE update.</li> |
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</ol> |
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<p>Furthermore, after installing all dependencies introduced by the above |
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procedure (<tt class="docutils literal">run_rdepend</tt> 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 <tt class="docutils literal">USE</tt> key in installed package metadata.</p> |
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</div> |
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<div class="section" id="backwards-compatibility"> |
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<h1><a class="toc-backref" href="#id11">Backwards compatibility</a></h1> |
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<p>Package managers not implementing this GLEP will consider |
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the <tt class="docutils literal">IUSE_RUNTIME</tt> 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.</p> |
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</div> |
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<div class="section" id="copyright"> |
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<h1><a class="toc-backref" href="#id12">Copyright</a></h1> |
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<p>This document has been placed in the public domain.</p> |
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</div> |
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