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<title>GLEP 37 -- Virtuals Deprecation</title>
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[<b><a href="http://www.gentoo.org/proj/en/glep">GLEP Index</a></b>]
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[<b><a href="./glep-0037.txt">GLEP Source</a></b>]
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<col class="field-body" />
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<tbody valign="top">
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<tr class="field"><th class="field-name">GLEP:</th><td class="field-body">37</td>
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</tr>
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<tr class="field"><th class="field-name">Title:</th><td class="field-body">Virtuals Deprecation</td>
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</tr>
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<tr class="field"><th class="field-name">Version:</th><td class="field-body">1.1</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" href="http://www.gentoo.org/cgi-bin/viewcvs/xml/htdocs/proj/en/glep/glep-0037.txt?cvsroot=gentoo">2005/04/30 22:35:58</a></td>
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</tr>
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<tr class="field"><th class="field-name">Author:</th><td class="field-body">Jason Stubbs <jstubbs 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>
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<tr class="field"><th class="field-name">Content-Type:</th><td class="field-body"><a class="reference" href="glep-0012.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">30-April-2005</td>
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</tr>
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<tr class="field"><th class="field-name">Post-History:</th><td class="field-body">30-April-2005</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"><a name="contents">Contents</a></p>
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<ul class="simple">
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<li><a class="reference" href="#credits" id="id2" name="id2">Credits</a></li>
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<li><a class="reference" href="#abstract" id="id3" name="id3">Abstract</a></li>
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<li><a class="reference" href="#motivation" id="id4" name="id4">Motivation</a></li>
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<li><a class="reference" href="#specification" id="id5" name="id5">Specification</a><ul>
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<li><a class="reference" href="#consistency" id="id6" name="id6">Consistency</a></li>
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<li><a class="reference" href="#overrides" id="id7" name="id7">Overrides</a></li>
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</ul>
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</li>
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<li><a class="reference" href="#rationale" id="id8" name="id8">Rationale</a></li>
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<li><a class="reference" href="#backwards-compatibility" id="id9" name="id9">Backwards Compatibility</a></li>
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<li><a class="reference" href="#copyright" id="id10" name="id10">Copyright</a></li>
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</ul>
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</div>
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<div class="section" id="credits">
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<h1><a class="toc-backref" href="#id2" name="credits">Credits</a></h1>
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<p>Most ideas in this GLEP came out of discussion with Thomas de Grenier de
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Latour. Ciaran McCreesh, Brian Harring and Stephen Bennett have also provided
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help in fleshing out the idea.</p>
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</div>
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<div class="section" id="abstract">
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<h1><a class="toc-backref" href="#id3" name="abstract">Abstract</a></h1>
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<p>This GLEP covers the pitfalls of the current virtuals system, the benefits of
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using regular ebuilds to serve the purpose of virtuals and what needs to be
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supported to make it viable.</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="#id4" name="motivation">Motivation</a></h1>
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<p>The current virtuals system is decentralized; that is there is no way to find
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information about a specific virtual other than to scan all packages for what
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they provide. There is also no way to tell whether an atom is a virtual or
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not - yes, the "virtual/" prefix could have been used but it isn't which has
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led to its abuse.</p>
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<p>What this means is that portage must scan all installed packages for the
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virtuals they provide, that profiles must provide a default for every single
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virtual that portage might encounter and that every single atom that portage
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processes must be checked against the list of virtuals. Needless to say that
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this causes quite a performance decrease.</p>
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<p>The current virtuals system also has some other major shortcomings. The most
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well known case is virtual/jdk and kaffe. Kaffe-1.1.4 implements the Java 1.4
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API but can not satisfy a package that requires >=virtual/jdk-1.4 because
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kaffe's versioning scheme differs. (ED: Need to add some more here. ;)</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="#id5" name="specification">Specification</a></h1>
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<p>This GLEP recommends that virtuals become no different to regular packages.
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Specifically, the standard virtual would include the DESCRIPTION, KEYWORDS,
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IUSE and RDEPEND metadata. An example would be something like this:</p>
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<pre class="literal-block">
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DESCRIPTION="Java Development Kit 1.4"
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KEYWORDS="amd64 hppa ia64 ppc ppc64 sparc x86"
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RDEPEND="|| (
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=dev-java/blackdown-jdk-1.4\*
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=dev-java/ibm-jdk-bin-1.4*
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=dev-java/jrockit-jdk-bin-1.4*
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=dev-java/kaffe-1.1.4*
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=dev-java/sun-jdk-1.4*
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)"
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IUSE=""
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</pre>
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<p>However, there are some issues that have been brought up with doing this.</p>
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<div class="section" id="consistency">
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<h2><a class="toc-backref" href="#id6" name="consistency">Consistency</a></h2>
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<p>Presently, it is very easy to remove packages even if others are dependent
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on them, which can lead to broken emerges when packages rely on indirect
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dependencies. For example, if kdelibs is merged bringing in qt and then
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qt is unmerged, attempting to merge kdebase will likely fail. This becomes
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a much bigger problem with virtuals as packages because the dependencies
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are always indirect.</p>
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<p>The resolution for this issue will be to add full dependency tracking and
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verification to portage. The details of how it will be done are outside the
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scope of this GLEP, but essentially this means that portage will need to be
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forced to unmerge a package that is depended on by another and will also be
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able to scan and fix any broken dependencies.</p>
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</div>
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<div class="section" id="overrides">
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<h2><a class="toc-backref" href="#id7" name="overrides">Overrides</a></h2>
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<p>Profiles currently specify the default provider of each virtual and users are
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able to override these defaults using /etc/portage/profile/virtuals. If
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virtuals are replaced by regular packages and thus able to have arbitrarily
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complex DEPENDs, the current method of overriding default virtuals can not
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be extended to support this.</p>
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<p>Before looking at a solution, lets look at how the current system works. When
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portage initializes, it searches installed packages for available virtuals.
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It then searches profiles and user overrides and adds them to the available
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providers list and/or changes the order of the providers so that overrides are
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listed earlier. Portage then expands any virtual atom it finds into an OR
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list using the order decided upon at initialization.</p>
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<p>To keep this behaviour available, this GLEP proposes a new file named
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package.prefer. In its basic form, this is just a list of package names
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ordered by preference. Portage would use it by reordering the atoms of any
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OR list it processes to fit the order given by package.prefer. For example,
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if package.prefer contained "dev-java/kaffe" then:</p>
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<pre class="literal-block">
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|| (
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dev-java/blackdown-jdk
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dev-java/sun-jdk
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dev-java/kaffe
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)
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</pre>
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<p>would be processed as:</p>
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<pre class="literal-block">
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|| (
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dev-java/kaffe
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dev-java/blackdown-jdk
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dev-java/sun-jdk
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)
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</pre>
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<p>In its basic form, package.prefer already covers profile and user overrides.
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However, this GLEP proposes that any type of atom be usable. This will be
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accomplished by checking for intersections of the atoms in the OR list and
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atoms in the preferred list. When an intersection is found, both atoms
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would be specified in a sublist, which would then be treated as a ranged dep.
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For example, if package.prefer contained "<=dev-java/sun-jdk-1.4" then:</p>
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<pre class="literal-block">
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|| (
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>=dev-java/blackdown-jdk-1.3
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>=dev-java/sun-jdk-1.3
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)
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</pre>
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<p>would be processed as:</p>
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<pre class="literal-block">
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|| (
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(
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<=dev-java/sun-jdk-1.4
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>=dev-java/sun-jdk-1.3
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)
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>=dev-java/blackdown-jdk-1.3
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>=dev-java/sun-jdk-1.3
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)
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</pre>
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<p>Ranged deps are outside of the scope of this GLEP.</p>
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</div>
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</div>
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<div class="section" id="rationale">
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<h1><a class="toc-backref" href="#id8" name="rationale">Rationale</a></h1>
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<p>The number one advantage is that it offers more power to both the user and
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the developer. Flexibility of virtuals is far greater in this scheme and
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fulfills requirements that exist already. It also means that the maintainers
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of profiles will not need to list a default for every virtual. The user
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benefits by being able to easily gather a list of providers of a virtual as
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well as their control being extended to allow selection where there is a
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choice within any package.</p>
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<p>Portage code also benefits from this scheme as virtuals will no longer
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require special handling or dual implementations of essentially the same
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feature, for example USE-based PROVIDEs. This scheme is also much easier to
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optimize which will benefit the processing of all packages. It also means
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that any additions to the DEPEND vocabulary become available for use in the
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definitions of virtuals.</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="#id9" name="backwards-compatibility">Backwards Compatibility</a></h1>
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<p>Compatibility will begin by making 2.0.51.20 treat unknown virtuals like
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regular packages. When the tree is stripped of PROVIDEs and "virtuals"
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override files, the only virtuals that these portages will use are those that
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the user has specified and those gleaned from installed packages. Any
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unknown virtual will be treated like a regular package and looked for in the
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tree.</p>
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<p>The next major version of portage (2.1.0) will support consistency
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checking. The only remaining issue is that of user overrides. The old
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method will work even with new style virtuals. The only catch is that
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complex virtuals - that is virtuals that will install more than one package -
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may not be overridable satisfactorally.</p>
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<p>Dropping of support of current style virtuals is planned for the following
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major version of portage (2.2.0). When the time comes to release it, scripts
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will be written to create packages from the existing virtuals system as well
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as to create appropriate package.prefer overrides within the profiles.</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="#id10" name="copyright">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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