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324 lines
11 KiB
XML
324 lines
11 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE chapter [
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<!ENTITY ndash "–"> <!-- @vcunat likes to use this one ;-) -->
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]>
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<chapter xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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xml:id="chap-multiple-output">
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<title>Multiple-output packages</title>
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<section>
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<title>Introduction</title>
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<para>
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The Nix language allows a derivation to produce multiple outputs, which is
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similar to what is utilized by other Linux distribution packaging systems.
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The outputs reside in separate nix store paths, so they can be mostly
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handled independently of each other, including passing to build inputs,
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garbage collection or binary substitution. The exception is that building
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from source always produces all the outputs.
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</para>
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<para>
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The main motivation is to save disk space by reducing runtime closure sizes;
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consequently also sizes of substituted binaries get reduced. Splitting can
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be used to have more granular runtime dependencies, for example the typical
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reduction is to split away development-only files, as those are typically
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not needed during runtime. As a result, closure sizes of many packages can
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get reduced to a half or even much less.
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</para>
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<note>
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<para>
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The reduction effects could be instead achieved by building the parts in
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completely separate derivations. That would often additionally reduce
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build-time closures, but it tends to be much harder to write such
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derivations, as build systems typically assume all parts are being built at
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once. This compromise approach of single source package producing multiple
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binary packages is also utilized often by rpm and deb.
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</para>
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</note>
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</section>
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<section>
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<title>Installing a split package</title>
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<para>
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When installing a package via <varname>systemPackages</varname> or
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<command>nix-env</command> you have several options:
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</para>
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<itemizedlist>
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<listitem>
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<para>
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You can install particular outputs explicitly, as each is available in the
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Nix language as an attribute of the package. The
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<varname>outputs</varname> attribute contains a list of output names.
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</para>
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</listitem>
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<listitem>
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<para>
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You can let it use the default outputs. These are handled by
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<varname>meta.outputsToInstall</varname> attribute that contains a list of
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output names.
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</para>
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<para>
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TODO: more about tweaking the attribute, etc.
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</para>
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</listitem>
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<listitem>
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<para>
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NixOS provides configuration option
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<varname>environment.extraOutputsToInstall</varname> that allows adding
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extra outputs of <varname>environment.systemPackages</varname> atop the
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default ones. It's mainly meant for documentation and debug symbols, and
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it's also modified by specific options.
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</para>
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<note>
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<para>
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At this moment there is no similar configurability for packages installed
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by <command>nix-env</command>. You can still use approach from
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<xref linkend="sec-modify-via-packageOverrides" /> to override
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<varname>meta.outputsToInstall</varname> attributes, but that's a rather
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inconvenient way.
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</para>
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</note>
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</listitem>
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</itemizedlist>
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</section>
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<section>
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<title>Using a split package</title>
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<para>
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In the Nix language the individual outputs can be reached explicitly as
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attributes, e.g. <varname>coreutils.info</varname>, but the typical case is
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just using packages as build inputs.
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</para>
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<para>
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When a multiple-output derivation gets into a build input of another
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derivation, the <varname>dev</varname> output is added if it exists,
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otherwise the first output is added. In addition to that,
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<varname>propagatedBuildOutputs</varname> of that package which by default
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contain <varname>$outputBin</varname> and <varname>$outputLib</varname> are
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also added. (See <xref linkend="multiple-output-file-type-groups" />.)
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</para>
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</section>
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<section>
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<title>Writing a split derivation</title>
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<para>
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Here you find how to write a derivation that produces multiple outputs.
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</para>
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<para>
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In nixpkgs there is a framework supporting multiple-output derivations. It
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tries to cover most cases by default behavior. You can find the source
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separated in
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<<filename>nixpkgs/pkgs/build-support/setup-hooks/multiple-outputs.sh</filename>>;
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it's relatively well-readable. The whole machinery is triggered by defining
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the <varname>outputs</varname> attribute to contain the list of desired
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output names (strings).
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</para>
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<programlisting>outputs = [ "bin" "dev" "out" "doc" ];</programlisting>
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<para>
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Often such a single line is enough. For each output an equally named
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environment variable is passed to the builder and contains the path in nix
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store for that output. Typically you also want to have the main
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<varname>out</varname> output, as it catches any files that didn't get
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elsewhere.
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</para>
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<note>
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<para>
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There is a special handling of the <varname>debug</varname> output,
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described at <xref linkend="stdenv-separateDebugInfo" />.
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</para>
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</note>
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<section xml:id="multiple-output-file-binaries-first-convention">
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<title><quote>Binaries first</quote></title>
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<para>
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A commonly adopted convention in <literal>nixpkgs</literal> is that
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executables provided by the package are contained within its first output.
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This convention allows the dependent packages to reference the executables
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provided by packages in a uniform manner. For instance, provided with the
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knowledge that the <literal>perl</literal> package contains a
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<literal>perl</literal> executable it can be referenced as
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<literal>${pkgs.perl}/bin/perl</literal> within a Nix derivation that needs
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to execute a Perl script.
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</para>
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<para>
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The <literal>glibc</literal> package is a deliberate single exception to
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the <quote>binaries first</quote> convention. The <literal>glibc</literal>
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has <literal>libs</literal> as its first output allowing the libraries
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provided by <literal>glibc</literal> to be referenced directly (e.g.
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<literal>${stdenv.glibc}/lib/ld-linux-x86-64.so.2</literal>). The
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executables provided by <literal>glibc</literal> can be accessed via its
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<literal>bin</literal> attribute (e.g.
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<literal>${stdenv.glibc.bin}/bin/ldd</literal>).
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</para>
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<para>
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The reason for why <literal>glibc</literal> deviates from the convention is
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because referencing a library provided by <literal>glibc</literal> is a
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very common operation among Nix packages. For instance, third-party
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executables packaged by Nix are typically patched and relinked with the
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relevant version of <literal>glibc</literal> libraries from Nix packages
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(please see the documentation on
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<link xlink:href="https://nixos.org/patchelf.html">patchelf</link> for more
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details).
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</para>
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</section>
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<section xml:id="multiple-output-file-type-groups">
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<title>File type groups</title>
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<para>
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The support code currently recognizes some particular kinds of outputs and
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either instructs the build system of the package to put files into their
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desired outputs or it moves the files during the fixup phase. Each group of
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file types has an <varname>outputFoo</varname> variable specifying the
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output name where they should go. If that variable isn't defined by the
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derivation writer, it is guessed – a default output name is defined,
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falling back to other possibilities if the output isn't defined.
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</para>
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<variablelist>
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<varlistentry>
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<term><varname>
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$outputDev</varname>
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</term>
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<listitem>
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<para>
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is for development-only files. These include C(++) headers, pkg-config,
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cmake and aclocal files. They go to <varname>dev</varname> or
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<varname>out</varname> by default.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>
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$outputBin</varname>
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</term>
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<listitem>
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<para>
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is meant for user-facing binaries, typically residing in bin/. They go
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to <varname>bin</varname> or <varname>out</varname> by default.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>
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$outputLib</varname>
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</term>
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<listitem>
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<para>
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is meant for libraries, typically residing in <filename>lib/</filename>
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and <filename>libexec/</filename>. They go to <varname>lib</varname> or
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<varname>out</varname> by default.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>
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$outputDoc</varname>
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</term>
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<listitem>
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<para>
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is for user documentation, typically residing in
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<filename>share/doc/</filename>. It goes to <varname>doc</varname> or
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<varname>out</varname> by default.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>
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$outputDevdoc</varname>
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</term>
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<listitem>
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<para>
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is for <emphasis>developer</emphasis> documentation. Currently we count
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gtk-doc and devhelp books in there. It goes to <varname>devdoc</varname>
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or is removed (!) by default. This is because e.g. gtk-doc tends to be
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rather large and completely unused by nixpkgs users.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>
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$outputMan</varname>
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</term>
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<listitem>
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<para>
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is for man pages (except for section 3). They go to
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<varname>man</varname> or <varname>$outputBin</varname> by default.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>
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$outputDevman</varname>
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</term>
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<listitem>
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<para>
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is for section 3 man pages. They go to <varname>devman</varname> or
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<varname>$outputMan</varname> by default.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>
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$outputInfo</varname>
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</term>
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<listitem>
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<para>
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is for info pages. They go to <varname>info</varname> or
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<varname>$outputBin</varname> by default.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</section>
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<section>
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<title>Common caveats</title>
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<itemizedlist>
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<listitem>
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<para>
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Some configure scripts don't like some of the parameters passed by
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default by the framework, e.g. <literal>--docdir=/foo/bar</literal>. You
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can disable this by setting <literal>setOutputFlags = false;</literal>.
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</para>
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</listitem>
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<listitem>
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<para>
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The outputs of a single derivation can retain references to each other,
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but note that circular references are not allowed. (And each
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strongly-connected component would act as a single output anyway.)
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</para>
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</listitem>
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<listitem>
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<para>
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Most of split packages contain their core functionality in libraries.
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These libraries tend to refer to various kind of data that typically gets
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into <varname>out</varname>, e.g. locale strings, so there is often no
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advantage in separating the libraries into <varname>lib</varname>, as
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keeping them in <varname>out</varname> is easier.
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</para>
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</listitem>
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<listitem>
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<para>
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Some packages have hidden assumptions on install paths, which complicates
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splitting.
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</para>
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</listitem>
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</itemizedlist>
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</section>
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</section>
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<!--Writing a split derivation-->
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</chapter>
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