07-working-derivation.xml

<chapter xmlns="http://docbook.org/ns/docbook"
         xmlns:xlink="http://www.w3.org/1999/xlink"
         xmlns:xi="http://www.w3.org/2001/XInclude"
         version="5.0"
         xml:id="working-derivation">

  <title>Working Derivation</title>

  <section>
    <title>Introduction</title>
    <para>
      Welcome to the seventh nix pill. In the previous
      <link linkend="our-first-derivation">sixth pill</link> we introduced the
      notion of derivation in the Nix language — how to define a raw derivation
      and how to (try to) build it.
    </para>
    <para>
      In this post we continue along the path, by creating a derivation that
      actually builds something. Then, we try to package a real program: we
      compile a simple C file and create a derivation out of it, given a blessed
      toolchain.
    </para>

    <para>
      I remind you how to enter the Nix environment:
      <command>source ~/.nix-profile/etc/profile.d/nix.sh</command>
    </para>
  </section>

  <section>
    <title>Using a script as a builder</title>

    <para>
      What's the easiest way to run a sequence of commands for building
      something? A bash script. We write a custom bash script, and we want it to
      be our builder. Given a <filename>builder.sh</filename>, we want the
      derivation to run <command>bash builder.sh</command>.
    </para>

    <para>
      We don't use hash bangs in <filename>builder.sh</filename>, because at the
      time we are writing it we do not know the path to
      <application>bash</application> in the nix store. Yes, even bash is in the
      nix store, everything is there.
    </para>

    <para>
      We don't even use <application>/usr/bin/env</application>, because then we
      lose the cool stateless property of Nix. Not to mention that
      <envar>PATH</envar> gets cleared when building, so it wouldn't find
      <application>bash</application> anyway.
    </para>

    <para>
      In summary, we want the builder to be <application>bash</application>, and
      pass it an argument, <literal>builder.sh</literal>. Turns out the
      <function>derivation</function> function accepts an optional
      <parameter>args</parameter> attribute which is used to pass arguments to
      the builder executable.
    </para>

    <para>
      First of all, let's write our <filename>builder.sh</filename> in the
      current directory:

      <programlisting><xi:include href="./07/builder.sh.txt" parse="text" /></programlisting>

      The command <literal>declare -xp</literal>
      lists exported variables.
      (<literal>declare</literal> is a builtin bash function.)
      As we covered in the previous pill, Nix computes the output path of the
      derivation. The resulting <literal>.drv</literal> file contains a list of
      environment variables passed to the builder. One of these is
      <varname>$out</varname>.
    </para>
    <para>
      What we have to do is create something in the path
      <varname>$out</varname>, be it a file or a directory. In this case we are
      creating a file.
    </para>

    <para>
      In addition, we print out the environment variables during the build
      process. We cannot use <application>env</application> for this, because
      <application>env</application> is part of
      <application>coreutils</application> and we don't have a dependency to it
      yet. We only have <application>bash</application> for now.
    </para>

    <para>
      Like for coreutils in the previous pill, we get a blessed bash for free
      from our magic nixpkgs stuff:

      <xi:include href="./07/bash.xml" />

      So with the usual trick, we can refer to
      <application>bin/bash</application> and create our derivation:

      <xi:include href="./07/simple-derivation.xml" />

      We did it! The contents of
      <filename>/nix/store/w024zci0x1hh1wj6gjq0jagkc1sgrf5r-<emphasis>foo</emphasis></filename>
      is really foo. We've built our first derivation.
    </para>
    <para>
      Note that we used <code>./builder.sh</code> and not
      <code>"./builder.sh"</code>. This way, it is parsed as a path, and Nix
      performs some magic which we will cover later. Try using the string
      version and you will find that it cannot find
      <filename>builder.sh</filename>. This is because it tries to find it
      relative to the temporary build directory.
    </para>
  </section>

  <section>
    <title>The builder environment</title>
    <para>
      Let's inspect those environment variables printed during the build process.
      <itemizedlist>
        <listitem><para>
          <envar>$HOME</envar> is not your home directory, and
          <filename>/homeless-shelter</filename> doesn't exist at all. We force
          packages not to depend on <envar>$HOME</envar> during the build
          process.
        </para></listitem>
        <listitem><para>
          <envar>$PATH</envar> plays the same game as <envar>$HOME</envar>
        </para></listitem>
        <listitem><para>
          <envar>$NIX_BUILD_CORES</envar> and <envar>$NIX_STORE</envar> are
          <link xlink:href="http://nixos.org/nix/manual/#sec-conf-file">nix
          configuration options</link>
        </para></listitem>
        <listitem><para>
          <envar>$PWD</envar> and <envar>$TMP</envar> clearly show that nix
          created a temporary build directory
        </para></listitem>
        <listitem><para>
          Then <envar>$builder</envar>, <envar>$name</envar>,
          <envar>$out</envar>, and <envar>$system</envar> are variables set due
          to the .drv file's contents.
        </para></listitem>
      </itemizedlist>
    </para>
    <para>
      And that's how we were able to use <envar>$out</envar> in our derivation
      and put stuff in it. It's like Nix reserved a slot in the nix store for
      us, and we must fill it.
    </para>
    <para>
      <!-- Not really sure what this paragraph is meant to mean, particularly
      wrt the insistence that this is not make's DESTDIR. I've left it as is but
      it should probably be written more clearly. -->
      In terms of autotools, <envar>$out</envar> will be the
      <option>--prefix</option> path. Yes, not the make
      <option>DESTDIR</option>, but the <option>--prefix</option>. That's the
      essence of stateless packaging. You don't install the package in a global
      common path under <filename>/</filename>, you install it in a local
      isolated path under your nix store slot.
    </para>
  </section>
  <section>
    <title>The .drv contents</title>
    <para>
      We added something else to the derivation this time: the args attribute.
      Let's see how this changed the .drv compared to the previous pill:
      <xi:include href="./07/foo.drv.xml" />

      Much like the usual .drv, except that there's a list of arguments in there
      passed to the builder (<application>bash</application>) with
      <filename>builder.sh</filename>… In the nix store..? Nix automatically
      copies files or directories needed for the build into the store to ensure
      that they are not changed during the build process and that the deployment
      is stateless and independent of the building machine.
      <filename>builder.sh</filename> is not only in the arguments passed to the
      builder, it's also in the input derivations.
    </para>
    <para>
      Given that <filename>builder.sh</filename> is a plain file, it has no .drv
      associated with it. The store path is computed based on the filename and
      on the hash of its contents. Store paths are covered in detail in <link
        linkend="nix-store-paths">a later pill</link>.
    </para>
  </section>
  <section>
    <title>Packaging a simple C program</title>
    <para>
      Start off by writing a simple C program called <filename>simple.c</filename>:

      <programlisting><xi:include href="./07/simple.c.txt" parse="text" /></programlisting>

      And its <filename>simple_builder.sh</filename>:

      <programlisting><xi:include href="./07/simple_builder.sh.txt" parse="text" /></programlisting>

      Don't worry too much about where those variables come from yet; let's
      write the derivation and build it:

      <xi:include href="./07/c-program-derivation.xml" />

      Now you can run
      <filename>/nix/store/ni66p4jfqksbmsl616llx3fbs1d232d4-simple/simple</filename>
      in your shell.
    </para>
  </section>
  <section>
    <title>Explanation</title>
    <para>
      We added two new attributes to the derivation call, <varname>gcc</varname>
      and <varname>coreutils</varname>. In <code>gcc = gcc;</code>, the name on
      the left is the name in the derivation set, and the name on the right
      refers to the gcc derivation from nixpkgs. The same applies for coreutils.
    </para>
    <para>
      We also added the <varname>src</varname> attribute, nothing magical — it's
      just a name, to which the path <filename>./simple.c</filename> is
      assigned. Like <filename>simple-builder.sh</filename>,
      <filename>simple.c</filename> will be added to the store.
    </para>
    <para>
      The trick: every attribute in the set passed to
      <function>derivation</function> will be converted to a string and passed
      to the builder as an environment variable. This is how the builder gains
      access to <application>coreutils</application> and
      <application>gcc</application>: when converted to strings, the derivations
      evaluate to their output paths, and appending <literal>/bin</literal> to
      these leads us to their binaries.
    </para>
    <para>
      The same goes for the <varname>src</varname> variable. <envar>$src</envar>
      is the path to <filename>simple.c</filename> in the nix store. As an
      exercise, pretty print the .drv file. You'll see
      <filename>simple_builder.sh</filename> and <filename>simple.c</filename>
      listed in the input derivations, along with
      <application>bash</application>, <application>gcc</application> and
      <application>coreutils</application> .drv files. The newly added
      environment variables described above will also appear.
    </para>
    <para>
      In <filename>simple_builder.sh</filename> we set the <envar>PATH</envar>
      for <application>gcc</application> and
      <application>coreutils</application> binaries, so that our build script
      can find the necessary utilities like <application>mkdir</application> and
      <application>gcc</application>.
    </para>
    <para>
      We then create <envar>$out</envar> as a directory and place the binary
      inside it. Note that <application>gcc</application> is found via the
      <envar>PATH</envar> environment variable, but it could equivalently be
      referenced explicitly using <code>$gcc/bin/gcc</code>.
    </para>
  </section>
  <section>
    <title>Enough of <literal>nix repl</literal></title>
    <para>
      Drop out of <application>nix repl</application> and write a file
      <filename>simple.nix</filename>:

      <programlisting><xi:include href="./07/simple.txt" parse="text" /></programlisting>

      Now you can build it with <command>nix-build simple.nix</command>. This
      will create a symlink <filename>result</filename> in the current
      directory, pointing to the out path of the derivation.
    </para>
    <para>
      <application>nix-build</application> does two jobs:
      <itemizedlist>
        <listitem><para>
          <link xlink:href="http://nixos.org/nix/manual/#sec-nix-instantiate">
            <application>nix-instantiate</application>
          </link>: parse and evaluate <filename>simple.nix</filename> and return
          the .drv file corresponding to the parsed derivation set
        </para></listitem>
        <listitem><para>
            <link xlink:href="http://nixos.org/nix/manual/#rsec-nix-store-realise">
              <command>nix-store -r</command>
            </link>: realise the .drv file, which actually builds it.
        </para></listitem>
      </itemizedlist>
      Finally, it creates the symlink.
    </para>
    <para>
      In the first line of <filename>simple.nix</filename>, we have an
      <function>import</function> function call nested in a <code>with</code>
      statement. Recall that <function>import</function> accepts one argument, a
      nix file to load. In this case, the contents of the file evaluated to a
      function.
    </para>
    <para>
      Afterwards, we call the function with the empty set. We saw this already
      in <link linkend="functions-and-imports">the fifth pill</link>. To
      reiterate: <code>import &lt;nixpkgs> {}</code> is calling two functions,
      not one. Reading it as <code>(import &lt;nixpkgs>) {}</code> makes this
      clearer.
    </para>
    <para>
      The value returned by the nixpkgs function is a set. More specifically,
      it's a set of derivations. Using the <code>with</code> expression we bring
      them into scope. This is equivalent to the <command>:l &lt;nixpkgs&gt;</command>
      we used in <application>nix repl</application>; it allows us to easily access derivations
      such as <varname>bash</varname>, <varname>gcc</varname>, and
      <varname>coreutils</varname>.
    </para>
    <para>
      Then we meet the
      <link xlink:href="https://nixos.org/nix/manual/#idm140737318075200"><code>inherit</code> keyword</link>.
      <code>inherit foo;</code> is equivalent to <code>foo = foo;</code>.
      Similarly, <code>inherit foo bar;</code> is equivalent to <code>foo = foo; bar = bar;</code>.
    </para>
    <para>
      This syntax only makes sense inside sets. There's no magic involved, it's
      simply a convenience to avoid repeating the same name for both the
      attribute name and the value in scope.
    </para>
  </section>
  <section>
    <title>Next pill</title>
    <para>
      We will generalize the builder. You may have noticed that we wrote two
      separate <filename>builder.sh</filename> scripts in this post. We would
      like to have a generic builder script instead, especially since each build
      script goes in the nix store: a bit of a waste.
    </para>
    <para>
      <emphasis>Is it really that hard to package stuff in Nix? No</emphasis>,
      here we're studying the fundamentals of Nix.
    </para>
  </section>
</chapter>