* Finished GNU Hello walkthrough.

This commit is contained in:
Eelco Dolstra 2004-11-05 13:10:08 +00:00
parent 8b934694f2
commit 6ca9c7f0a9
3 changed files with 293 additions and 21 deletions

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@ -4,7 +4,8 @@ XMLLINT = $(ENV) $(xmllint) $(xmlflags) --catalogs
XSLTPROC = $(ENV) $(xsltproc) $(xmlflags) --catalogs \
--param section.autolabel 1 \
--param section.label.includes.component.label 1 \
--param html.stylesheet \'style.css\'
--param html.stylesheet \'style.css\' \
--param xref.with.number.and.title 0
man1_MANS = nix-env.1 nix-store.1 nix-instantiate.1 \
nix-collect-garbage.1 nix-push.1 nix-pull.1 \

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@ -52,17 +52,6 @@
</para>
</listitem>
<listitem>
<para>
The current garbage collector is a hack. It should be
integrated into <command>nix-store</command>. It should
delete derivations in an order determined by topologically
sorting derivations under the points-to relation. This
ensures that no store paths ever exist that point to
non-existant store paths.
</para>
</listitem>
<listitem>
<para>
There are race conditions between the garbage collector and

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