* Finished GNU Hello walkthrough.
This commit is contained in:
Eelco Dolstra
parent
8b934694f2
commit
6ca9c7f0a9
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@ -4,7 +4,8 @@ XMLLINT = $(ENV) $(xmllint) $(xmlflags) --catalogs
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XSLTPROC = $(ENV) $(xsltproc) $(xmlflags) --catalogs \
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--param section.autolabel 1 \
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--param section.label.includes.component.label 1 \
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--param html.stylesheet \'style.css\'
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--param html.stylesheet \'style.css\' \
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--param xref.with.number.and.title 0
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man1_MANS = nix-env.1 nix-store.1 nix-instantiate.1 \
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nix-collect-garbage.1 nix-push.1 nix-pull.1 \
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@ -52,17 +52,6 @@
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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 current garbage collector is a hack. It should be
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integrated into <command>nix-store</command>. It should
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delete derivations in an order determined by topologically
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sorting derivations under the points-to relation. This
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ensures that no store paths ever exist that point to
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non-existant store paths.
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</para>
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</listitem>
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<listitem>
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<para>
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There are race conditions between the garbage collector and
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@ -44,7 +44,8 @@ need to do three things:
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<sect2><title>The Nix expression</title>
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<example id='ex-hello-nix'><title>Nix expression for GNU Hello</title>
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<example id='ex-hello-nix'><title>Nix expression for GNU Hello
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(<filename>default.nix</filename>)</title>
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<programlisting>
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{stdenv, fetchurl, perl}: <co id='ex-hello-nix-co-1' />
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@ -189,24 +190,108 @@ perl = perl;</programlisting>
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<sect2><title>The builder</title>
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<example id='ex-hello-builder'><title>Build script for GNU Hello</title>
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<example id='ex-hello-builder'><title>Build script for GNU Hello
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(<filename>builder.sh</filename>)</title>
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<programlisting>
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. $stdenv/setup
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. $stdenv/setup <co id='ex-hello-builder-co-1' />
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PATH=$perl/bin:$PATH
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PATH=$perl/bin:$PATH <co id='ex-hello-builder-co-2' />
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tar xvfz $src
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tar xvfz $src <co id='ex-hello-builder-co-3' />
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cd hello-*
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./configure --prefix=$out
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make
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./configure --prefix=$out <co id='ex-hello-builder-co-4' />
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make <co id='ex-hello-builder-co-5' />
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make install</programlisting>
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</example>
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<para><xref linkend='ex-hello-builder' /> shows the builder referenced
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from Hello's Nix expression (stored in
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<filename>pkgs/applications/misc/hello/ex-1/builder.sh</filename>).</para>
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<filename>pkgs/applications/misc/hello/ex-1/builder.sh</filename>).
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The builder can actually be made a lot shorter by using the
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<emphasis>generic builder</emphasis> functions provided by
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<varname>stdenv</varname>, but here we write out the build steps to
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elucidate what a builder does. It performs the following
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steps:</para>
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<para>TODO</para>
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<calloutlist>
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<callout arearefs='ex-hello-builder-co-1'>
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<para>When Nix runs a builder, it initially completely clears the
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environment. For instance, the <envar>PATH</envar> variable is
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empty<footnote><para>Actually, it's initialised to
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<filename>/path-not-set</filename> to prevent Bash from setting it
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to a default value.</para></footnote>. This is done to prevent
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undeclared inputs from being used in the build process. If for
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example the <envar>PATH</envar> contained
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<filename>/usr/bin</filename>, then you might accidentally use
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<filename>/usr/bin/gcc</filename>.</para>
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<para>So the first step is to set up the environment. This is
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done by calling the <filename>setup</filename> script of the
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standard environment. The environment variable
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<envar>stdenv</envar> points to the location of the standard
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environment being used. (It wasn't specified explicitly as an
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attribute in <xref linkend='ex-hello-nix' />, but
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<varname>mkDerivation</varname> adds it automatically.)</para>
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</callout>
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<callout arearefs='ex-hello-builder-co-2'>
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<para>Since Hello needs Perl, we have to make sure that Perl is in
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the <envar>PATH</envar>. The <envar>perl</envar> environment
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variable points to the location of the Perl component (since it
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was passed in as an attribute to the derivation), so
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<filename><replaceable>$perl</replaceable>/bin</filename> is the
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directory containing the Perl interpreter.</para>
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</callout>
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<callout arearefs='ex-hello-builder-co-3'>
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<para>Now we have to unpack the sources. The
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<varname>src</varname> attribute was bound to the result of
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fetching the Hello source tarball from the network, so the
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<envar>src</envar> environment variable points to the location in
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the Nix store to which the tarball was downloaded. After
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unpacking, we <command>cd</command> to the resulting source
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directory.</para>
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<para>The whole build is performed in a temporary directory
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created in <varname>/tmp</varname>, by the way. This directory is
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removed after the builder finishes, so there is no need to clean
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up the sources afterwards. Also, the temporary directory is
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always newly created, so you don't have to worry about files from
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previous builds interfering with the current build.</para>
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</callout>
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<callout arearefs='ex-hello-builder-co-4'>
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<para>GNU Hello is a typical Autoconf-based package, so we first
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have to run its <filename>configure</filename> script. In Nix
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every component is stored in a separate location in the Nix store,
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for instance
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<filename>/nix/store/9a54ba97fb71b65fda531012d0443ce2-hello-2.1.1</filename>.
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Nix computes this path by cryptographically hashing all attributes
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of the derivation. The path is passed to the builder through the
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<envar>out</envar> environment variable. So here we give
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<filename>configure</filename> the parameter
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<literal>--prefix=$out</literal> to cause Hello to be installed in
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the expected location.</para>
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</callout>
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<callout arearefs='ex-hello-builder-co-5'>
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<para>Finally we build Hello (<literal>make</literal>) and install
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it into the location specified by <envar>out</envar>
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(<literal>make install</literal>).</para>
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</callout>
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</calloutlist>
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<para>If you are wondering about the absence of error checking on the
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result of various commands called in the builder: this is because the
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@ -217,6 +302,203 @@ error check.</para>
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</sect2>
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<sect2><title>Composition</title>
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<example id='ex-hello-composition'><title>Composing GNU Hello
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(<filename>all-packages-generic.nix</filename>)</title>
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<programlisting>
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...
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rec { <co id='ex-hello-composition-co-1' />
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hello = (import ../applications/misc/hello/ex-1 <co id='ex-hello-composition-co-2' />) { <co id='ex-hello-composition-co-3' />
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inherit fetchurl stdenv perl;
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};
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perl = (import ../development/interpreters/perl) { <co id='ex-hello-composition-co-4' />
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inherit fetchurl stdenv;
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};
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fetchurl = (import ../build-support/fetchurl) {
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inherit stdenv; ...
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};
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stdenv = ...;
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}
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</programlisting>
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</example>
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<para>The Nix expression in <xref linkend='ex-hello-nix' /> is a
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function; it is missing some arguments that have to be filled in
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somewhere. In the Nix Packages collection this is done in the file
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<filename>pkgs/system/all-packages-generic.nix</filename>, where all
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Nix expressions for components are imported and called with the
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appropriate arguments. <xref linkend='ex-hello-composition' /> shows
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some fragments of
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<filename>all-packages-generic.nix</filename>.</para>
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<calloutlist>
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<callout arearefs='ex-hello-composition-co-1'>
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<para>This file defines a set of attributes, all of which are
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concrete derivations (i.e., not functions). In fact, we define a
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<emphasis>mutually recursive</emphasis> set of attributes. That
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is, the attributes can refer to each other. This is precisely
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what we want since we want to <quote>plug</quote> the
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various components into each other.</para>
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</callout>
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<callout arearefs='ex-hello-composition-co-2'>
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<para>Here we <emphasis>import</emphasis> the Nix expression for
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GNU Hello. The import operation just loads and returns the
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specified Nix expression. In fact, we could just have put the
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contents of <xref linkend='ex-hello-nix' /> in
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<filename>all-packages-generic.nix</filename> at this point. That
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would be completely equivalent, but it would make the file rather
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bulky.</para>
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<para>Note that we refer to
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<filename>../applications/misc/hello/ex-1</filename>, not
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<filename>../applications/misc/hello/ex-1/default.nix</filename>.
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When you try to import a directory, Nix automatically appends
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<filename>/default.nix</filename> to the file name.</para>
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</callout>
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<callout arearefs='ex-hello-composition-co-3'>
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<para>This is where the actual composition takes place. Here we
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<emphasis>call</emphasis> the function imported from
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<filename>../applications/misc/hello/ex-1</filename> with an
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attribute set containing the things that the function expects,
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namely <varname>fetchurl</varname>, <varname>stdenv</varname>, and
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<varname>perl</varname>. We use inherit again to use the
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attributes defined in the surrounding scope (we could also have
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written <literal>fetchurl = fetchurl;</literal>, etc.).</para>
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<para>The result of this function call is an actual derivation
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that can be built by Nix (since when we fill in the arguments of
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the function, what we get is its body, which is the call to
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<varname>stdenv.mkDerivation</varname> in <xref
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linkend='ex-hello-nix ' />).</para>
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</callout>
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<callout arearefs='ex-hello-composition-co-4'>
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<para>Likewise, we have to instantiate Perl,
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<varname>fetchurl</varname>, and the standard environment.</para>
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</callout>
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</calloutlist>
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</sect2>
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<sect2><title>Testing</title>
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<para>You can now try to build Hello. The simplest way to do that is
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by using <command>nix-env</command>:
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<screen>
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$ nix-env -f pkgs/system/i686-linux.nix -i hello
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installing `hello-2.1.1'
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building path `/nix/store/632d2b22514dcebe704887c3da15448d-hello-2.1.1'
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hello-2.1.1/
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hello-2.1.1/intl/
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hello-2.1.1/intl/ChangeLog
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<replaceable>...</replaceable>
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</screen>
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This will build Hello and install it into your profile. The file
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<filename>i686-linux</filename> is just a simple Nix expression that
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imports <filename>all-packages-generic.nix</filename> and instantiates
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it for Linux on the x86 platform.</para>
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<para>Note that the <literal>hello</literal> argument here refers to
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the symbolic name given to the Hello derivation (the
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<varname>name</varname> attribute in <xref linkend='ex-hello-nix' />),
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<emphasis>not</emphasis> the <varname>hello</varname> attribute in
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<filename>all-packages-generic.nix</filename>.
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<command>nix-env</command> simply walks through all derivations
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defined in the latter file, looking for one with a
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<varname>name</varname> attribute matching the command-line
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argument.</para>
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<para>You can test whether it works:
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<screen>
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$ hello
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Hello, world!</screen>
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</para>
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<para>Generally, however, using <command>nix-env</command> is not the
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best way to test components, since you may not want to install them
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into your profile right away (they might not work properly, after
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all). A better way is to write a short file containging the
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following:
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<programlisting>
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(import pkgs/system/i686-linux.nix).hello</programlisting>
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Call it <filename>test.nix</filename>. Then you can build it without
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installing it using the command <command>nix-build</command>:
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<screen>
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$ nix-build ./test.nix
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...
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/nix/store/632d2b22514dcebe704887c3da15448d-hello-2.1.1</screen>
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<command>nix-build</command> will build the derivation and print the
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output path. It also creates a symlink to the output path called
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<filename>result</filename> in the current directory. This is
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convenient for testing the component:
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<screen>
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$ ./result/bin/hello
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Hello, world!</screen>
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</para>
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<para>Nix has a transactional semantics. Once a build finishes
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succesfully, Nix makes a note of this in its database: it registers
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that the path denoted by <envar>out</envar> is now
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<quote>valid</quote>. If you try to build the derivation again, Nix
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will see that the path is already valid and finish immediately. If a
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build fails, either because it returns a non-zero exit code, because
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Nix or the builder are killed, or because the machine crashes, then
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the output path will not be registered as valid. If you try to build
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the derivation again, Nix will remove the output path if it exists
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(e.g., because the builder died half-way through <literal>make
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install</literal>) and try again. Note that there is no
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<quote>negative caching</quote>: Nix doesn't remember that a build
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failed, and so a failed build can always be repeated. This is because
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Nix cannot distinguish between permanent failures (e.g., a compiler
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error due to a syntax error in the source) and transient failures
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(e.g., a disk full condition).</para>
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<para>Nix also performs locking. If you run multiple Nix builds
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simultaneously, and they try to build the same derivation, the first
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Nix instance that gets there will perform the build, while the others
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block (or perform other derivations if available) until the build
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finishes. So it is always safe to run multiple instances of Nix in
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parallel (contrary to, say, <command>make</command>).</para>
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<para>If you have a system with multiple CPUs, you may want to have
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Nix build different derivations in parallel (insofar as possible).
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Just pass the option <option>-j <replaceable>N</replaceable></option>,
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where <replaceable>N</replaceable> is the maximum number of jobs to be
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run in parallel. Typically this should be the number of CPUs.</para>
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</sect2>
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</sect1>
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