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<?xml version="1.0"?>
<guide self="general-concepts/virtuals/">
<chapter>
<title>Virtuals</title>
<body>
<p>
Virtuals are merely packages that are in the category of <c>virtual</c>. They
use their dependency string to specify the providers for the virtual and should
not install any files. Since they are regular ebuilds, there can be several
versions of a virtual (which can be helpful when a package may be provided by
another in some versions, and not others <d/> see the perl virtuals for an
example of this). One other difference (besides not installing any files) is
that a virtual does not define <c>HOMEPAGE</c> and <c>LICENSE</c> variables.
Since it installs no files, it really does not have a license.
</p>
<p>
Before adding a new virtual, it should be discussed on <c>gentoo-dev</c>.
</p>
<p>
An example of a virtual:
<codesample lang="ebuild">
EAPI=4
DESCRIPTION="Virtual for C++ tr1 <type_traits>"
SLOT="0"
KEYWORDS="alpha amd64 arm hppa ia64 ~mips ppc ppc64 ~s390 sparc x86 ~x86-fbsd"
RDEPEND="|| ( >=sys-devel/gcc-4.1 dev-libs/boost )"
</codesample>
Looks familar...right? It should since its going to look just like a regular
ebuild.
</p>
<note>
The so-called <e>old-style</e> or <c>PROVIDE</c> type virtuals have been banned
from the Gentoo repository.
</note>
</body>
<section>
<title>KEYWORDS in virtual packages</title>
<body>
<p>
Since virtual packages do not install any files, they do not follow the regular
arch testing procedure. Instead, the developer can immediately set
the <c>KEYWORDS</c> of a virtual to the union of <c>KEYWORDS</c> of its
providers. In particular, if a new virtual is created for a stable package,
the virtual is committed straight to stable.
</p>
<p>
For example, if you have two packages: <c>dev-libs/liblinux</c> with
<c>KEYWORDS="amd64 ~x86"</c> and <c>dev-libs/libbsd</c> with
<c>KEYWORDS="~amd64-fbsd ~x86-fbsd"</c>, the resulting virtual will
have:
</p>
<codesample lang="ebuild">
KEYWORDS="amd64 ~x86 ~amd64-fbsd ~x86-fbsd"
RDEPEND="|| ( dev-libs/liblinux dev-libs/libbsd )"
</codesample>
</body>
</section>
<section>
<title>Virtuals and subslots</title>
<body>
<warning>
This section is only applicable if virtual providers include versions that
are ABI-compatible with one another (and use matching SONAMEs) and/or
the incompatible providers are being obsoleted.
</warning>
<p>
Like regular packages, virtuals can define subslots that can be used
to trigger rebuilds of their reverse dependencies. For this to work, a new
version of the virtual is created for each subslot of the providers,
where each version contains dependencies on a specific subslot.
</p>
<p>
For example, a virtual for different packages providing ABI-compatible
<c>libfoo.so.1</c> libraries could look like the following:
</p>
<codesample lang="ebuild">
EAPI=6
DESCRIPTION="Virtual for libfoo.so.1"
SLOT="0/1"
RDEPEND="
|| (
dev-libs/libfoo:0/1
dev-libs/libfoo-alternate:0/1
dev-libs/bigpack:0/libfoo-1+libbar-0
dev-libs/bigpack:0/libfoo-1+libbar-1
)
"
</codesample>
<p>
Virtuals can also be used when one of the providers is being obsoleted in favor
of another that breaks ABI compatibility while remaining API-compatible. In this
case, multiple versions of the virtual are created, each specifying a single
provider and a unique subslot.
</p>
<p>
For example, if <c>dev-libs/libfoo</c> (<c>libfoo.so.0</c>) is being replaced
by <c>dev-libs/newfoo</c> (<c>libfoo.so.1</c>), <c>virtual/libfoo-0.ebuild</c>
would contain:
</p>
<codesample lang="ebuild">
EAPI=6
DESCRIPTION="Virtual for libfoo.so.0"
SLOT="0/0"
RDEPEND="dev-libs/libfoo:0/0"
</codesample>
<p>
while <c>virtual/libfoo-1.ebuild</c> would contain:
</p>
<codesample lang="ebuild">
EAPI=6
DESCRIPTION="Virtual for libfoo.so.1"
SLOT="0/1"
RDEPEND="dev-libs/newfoo:0/1"
</codesample>
<note>
In this case, the package manager will naturally want to upgrade
to <c>dev-libs/newfoo</c> whenever possible. Therefore, this solution
is not viable if clean choice between the two providers is desired.
</note>
</body>
</section>
</chapter>
</guide>
|
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