<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-post-build-hook"
      version="5.0"
      >

<title>Using the <option linkend="conf-post-build-hook">post-build-hook</option></title>
<subtitle>Uploading to an S3-compatible binary cache after each build</subtitle>


<section xml:id="chap-post-build-hook-caveats">
  <title>Implementation Caveats</title>
  <para>Here we use the post-build hook to upload to a binary cache.
  This is a simple and working example, but it is not suitable for all
  use cases.</para>

  <para>The post build hook program runs after each executed build,
  and blocks the build loop. The build loop exits if the hook program
  fails.</para>

  <para>Concretely, this implementation will make Nix slow or unusable
  when the internet is slow or unreliable.</para>

  <para>A more advanced implementation might pass the store paths to a
  user-supplied daemon or queue for processing the store paths outside
  of the build loop.</para>
</section>

<section>
  <title>Prerequisites</title>

  <para>
    This tutorial assumes you have configured an S3-compatible binary cache
    according to the instructions at
    <xref linkend="ssec-s3-substituter-authenticated-writes" />, and
    that the <literal>root</literal> user's default AWS profile can
    upload to the bucket.
  </para>
</section>

<section>
  <title>Set up a Signing Key</title>
  <para>Use <command>nix-store --generate-binary-cache-key</command> to
  create our public and private signing keys. We will sign paths
  with the private key, and distribute the public key for verifying
  the authenticity of the paths.</para>

  <screen>
# nix-store --generate-binary-cache-key example-nix-cache-1 /etc/nix/key.private /etc/nix/key.public
# cat /etc/nix/key.public
example-nix-cache-1:1/cKDz3QCCOmwcztD2eV6Coggp6rqc9DGjWv7C0G+rM=
</screen>

<para>Then, add the public key and the cache URL to your
<filename>nix.conf</filename>'s <xref linkend="conf-trusted-public-keys" />
and <xref linkend="conf-substituters" /> like:</para>

<programlisting>
substituters = https://cache.nixos.org/ s3://example-nix-cache
trusted-public-keys = cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY= example-nix-cache-1:1/cKDz3QCCOmwcztD2eV6Coggp6rqc9DGjWv7C0G+rM=
</programlisting>

<para>we will restart the Nix daemon a later step.</para>
</section>

<section>
  <title>Implementing the build hook</title>
  <para>Write the following script to
  <filename>/etc/nix/upload-to-cache.sh</filename>:
  </para>

  <programlisting>
#!/bin/sh

set -eu
set -f # disable globbing
export IFS=' '

echo "Signing paths" $OUT_PATHS
nix sign-paths --key-file /etc/nix/key.private $OUT_PATHS
echo "Uploading paths" $OUT_PATHS
exec nix copy --to 's3://example-nix-cache' $OUT_PATHS
</programlisting>

  <note>
    <title>Should <literal>$OUT_PATHS</literal> be quoted?</title>
    <para>
      The <literal>$OUT_PATHS</literal> variable is a space-separated
      list of Nix store paths. In this case, we expect and want the
      shell to perform word splitting to make each output path its
      own argument to <command>nix sign-paths</command>. Nix guarantees
      the paths will not contain any spaces, however a store path
      might contain glob characters. The <command>set -f</command>
      disables globbing in the shell.
    </para>
  </note>
  <para>
    Then make sure the hook program is executable by the <literal>root</literal> user:
    <screen>
# chmod +x /etc/nix/upload-to-cache.sh
</screen></para>
</section>

<section>
  <title>Updating Nix Configuration</title>

  <para>Edit <filename>/etc/nix/nix.conf</filename> to run our hook,
  by adding the following configuration snippet at the end:</para>

  <programlisting>
post-build-hook = /etc/nix/upload-to-cache.sh
</programlisting>

<para>Then, restart the <command>nix-daemon</command>.</para>
</section>

<section>
  <title>Testing</title>

  <para>Build any derivation, for example:</para>

  <screen>
$ nix-build -E '(import &lt;nixpkgs&gt; {}).writeText "example" (builtins.toString builtins.currentTime)'
these derivations will be built:
  /nix/store/s4pnfbkalzy5qz57qs6yybna8wylkig6-example.drv
building '/nix/store/s4pnfbkalzy5qz57qs6yybna8wylkig6-example.drv'...
running post-build-hook '/home/grahamc/projects/github.com/NixOS/nix/post-hook.sh'...
post-build-hook: Signing paths /nix/store/ibcyipq5gf91838ldx40mjsp0b8w9n18-example
post-build-hook: Uploading paths /nix/store/ibcyipq5gf91838ldx40mjsp0b8w9n18-example
/nix/store/ibcyipq5gf91838ldx40mjsp0b8w9n18-example
</screen>

  <para>Then delete the path from the store, and try substituting it from the binary cache:</para>
  <screen>
$ rm ./result
$ nix-store --delete /nix/store/ibcyipq5gf91838ldx40mjsp0b8w9n18-example
</screen>

<para>Now, copy the path back from the cache:</para>
<screen>
$ nix store --realize /nix/store/ibcyipq5gf91838ldx40mjsp0b8w9n18-example
copying path '/nix/store/m8bmqwrch6l3h8s0k3d673xpmipcdpsa-example from 's3://example-nix-cache'...
warning: you did not specify '--add-root'; the result might be removed by the garbage collector
/nix/store/m8bmqwrch6l3h8s0k3d673xpmipcdpsa-example
</screen>
</section>
<section>
  <title>Conclusion</title>
  <para>
    We now have a Nix installation configured to automatically sign and
    upload every local build to a remote binary cache.
  </para>

  <para>
    Before deploying this to production, be sure to consider the
    implementation caveats in <xref linkend="chap-post-build-hook-caveats" />.
  </para>
</section>
</chapter>
