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

<title>Verifying Build Reproducibility with <option linkend="conf-diff-hook">diff-hook</option></title>

<subtitle>Check build reproducibility by running builds multiple times
and comparing their results.</subtitle>

<para>Specify a program with Nix's <xref linkend="conf-diff-hook" /> to
compare build results when two builds produce different results. Note:
this hook is only executed if the results are not the same, this hook
is not used for determining if the results are the same.</para>

<para>For purposes of demonstration, we'll use the following Nix file,
<filename>deterministic.nix</filename> for testing:</para>

<programlisting>
let
  inherit (import &lt;nixpkgs&gt; {}) runCommand;
in {
  stable = runCommand "stable" {} ''
    touch $out
  '';

  unstable = runCommand "unstable" {} ''
    echo $RANDOM > $out
  '';
}
</programlisting>

<para>Additionally, <filename>nix.conf</filename> contains:

<programlisting>
diff-hook = /etc/nix/my-diff-hook
run-diff-hook = true
</programlisting>

where <filename>/etc/nix/my-diff-hook</filename> is an executable
file containing:

<programlisting>
#!/bin/sh
exec &gt;&amp;2
echo "For derivation $3:"
/run/current-system/sw/bin/diff -r "$1" "$2"
</programlisting>

</para>

<para>The diff hook is executed by the same user and group who ran the
build. However, the diff hook does not have write access to the store
path just built.</para>

<section>
  <title>
    Spot-Checking Build Determinism
  </title>

  <para>
    Verify a path which already exists in the Nix store by passing
    <option>--check</option> to the build command.
  </para>

  <para>If the build passes and is deterministic, Nix will exit with a
  status code of 0:</para>

  <screen>
$ nix-build ./deterministic.nix -A stable
these derivations will be built:
  /nix/store/z98fasz2jqy9gs0xbvdj939p27jwda38-stable.drv
building '/nix/store/z98fasz2jqy9gs0xbvdj939p27jwda38-stable.drv'...
/nix/store/yyxlzw3vqaas7wfp04g0b1xg51f2czgq-stable

$ nix-build ./deterministic.nix -A stable --check
checking outputs of '/nix/store/z98fasz2jqy9gs0xbvdj939p27jwda38-stable.drv'...
/nix/store/yyxlzw3vqaas7wfp04g0b1xg51f2czgq-stable
</screen>

  <para>If the build is not deterministic, Nix will exit with a status
  code of 1:</para>

  <screen>
$ nix-build ./deterministic.nix -A unstable
these derivations will be built:
  /nix/store/cgl13lbj1w368r5z8gywipl1ifli7dhk-unstable.drv
building '/nix/store/cgl13lbj1w368r5z8gywipl1ifli7dhk-unstable.drv'...
/nix/store/krpqk0l9ib0ibi1d2w52z293zw455cap-unstable

$ nix-build ./deterministic.nix -A unstable --check
checking outputs of '/nix/store/cgl13lbj1w368r5z8gywipl1ifli7dhk-unstable.drv'...
error: derivation '/nix/store/cgl13lbj1w368r5z8gywipl1ifli7dhk-unstable.drv' may not be deterministic: output '/nix/store/krpqk0l9ib0ibi1d2w52z293zw455cap-unstable' differs
</screen>

<para>In the Nix daemon's log, we will now see:
<screen>
For derivation /nix/store/cgl13lbj1w368r5z8gywipl1ifli7dhk-unstable.drv:
1c1
&lt; 8108
---
&gt; 30204
</screen>
</para>

  <para>Using <option>--check</option> with <option>--keep-failed</option>
  will cause Nix to keep the second build's output in a special,
  <literal>.check</literal> path:</para>

  <screen>
$ nix-build ./deterministic.nix -A unstable --check --keep-failed
checking outputs of '/nix/store/cgl13lbj1w368r5z8gywipl1ifli7dhk-unstable.drv'...
note: keeping build directory '/tmp/nix-build-unstable.drv-0'
error: derivation '/nix/store/cgl13lbj1w368r5z8gywipl1ifli7dhk-unstable.drv' may not be deterministic: output '/nix/store/krpqk0l9ib0ibi1d2w52z293zw455cap-unstable' differs from '/nix/store/krpqk0l9ib0ibi1d2w52z293zw455cap-unstable.check'
</screen>

  <para>In particular, notice the
  <literal>/nix/store/krpqk0l9ib0ibi1d2w52z293zw455cap-unstable.check</literal>
  output. Nix has copied the build results to that directory where you
  can examine it.</para>

  <note xml:id="check-dirs-are-unregistered">
    <title><literal>.check</literal> paths are not registered store paths</title>

    <para>Check paths are not protected against garbage collection,
    and this path will be deleted on the next garbage collection.</para>

    <para>The path is guaranteed to be alive for the duration of
    <xref linkend="conf-diff-hook" />'s execution, but may be deleted
    any time after.</para>

    <para>If the comparison is performed as part of automated tooling,
    please use the diff-hook or author your tooling to handle the case
    where the build was not deterministic and also a check path does
    not exist.</para>
  </note>

  <para>
    <option>--check</option> is only usable if the derivation has
    been built on the system already. If the derivation has not been
    built Nix will fail with the error:
    <screen>
error: some outputs of '/nix/store/hzi1h60z2qf0nb85iwnpvrai3j2w7rr6-unstable.drv' are not valid, so checking is not possible
</screen>

    Run the build without <option>--check</option>, and then try with
    <option>--check</option> again.
  </para>
</section>

<section>
  <title>
    Automatic and Optionally Enforced Determinism Verification
  </title>

  <para>
    Automatically verify every build at build time by executing the
    build multiple times.
  </para>

  <para>
    Setting <xref linkend="conf-repeat" /> and
    <xref linkend="conf-enforce-determinism" /> in your
    <filename>nix.conf</filename> permits the automated verification
    of every build Nix performs.
  </para>

  <para>
    The following configuration will run each build three times, and
    will require the build to be deterministic:

    <programlisting>
enforce-determinism = true
repeat = 2
</programlisting>
  </para>

  <para>
    Setting <xref linkend="conf-enforce-determinism" /> to false as in
    the following configuration will run the build multiple times,
    execute the build hook, but will allow the build to succeed even
    if it does not build reproducibly:

    <programlisting>
enforce-determinism = false
repeat = 1
</programlisting>
  </para>

  <para>
    An example output of this configuration:
    <screen>
$ nix-build ./test.nix -A unstable
these derivations will be built:
  /nix/store/ch6llwpr2h8c3jmnf3f2ghkhx59aa97f-unstable.drv
building '/nix/store/ch6llwpr2h8c3jmnf3f2ghkhx59aa97f-unstable.drv' (round 1/2)...
building '/nix/store/ch6llwpr2h8c3jmnf3f2ghkhx59aa97f-unstable.drv' (round 2/2)...
output '/nix/store/6xg356v9gl03hpbbg8gws77n19qanh02-unstable' of '/nix/store/ch6llwpr2h8c3jmnf3f2ghkhx59aa97f-unstable.drv' differs from '/nix/store/6xg356v9gl03hpbbg8gws77n19qanh02-unstable.check' from previous round
/nix/store/6xg356v9gl03hpbbg8gws77n19qanh02-unstable
</screen>
  </para>
</section>
</chapter>