lix/doc/manual/advanced-topics/post-build-hook.xml
regnat 7c5596734f
Add a post-build-hook
Passing `--post-build-hook /foo/bar` to a nix-* command will cause
`/foo/bar` to be executed after each build with the following
environment variables set:

    DRV_PATH=/nix/store/drv-that-has-been-built.drv
    OUT_PATHS=/nix/store/...build /nix/store/...build-bin /nix/store/...build-dev

This can be useful in particular to upload all the builded artifacts to
the cache (including the ones that don't appear in the runtime closure
of the final derivation or are built because of IFD).

This new feature prints the stderr/stdout output to the `nix-build`
and `nix build` client, and the output is printed in a Nix 2
compatible format:

    [nix]$ ./inst/bin/nix-build ./test.nix
    these derivations will be built:
      /nix/store/ishzj9ni17xq4hgrjvlyjkfvm00b0ch9-my-example-derivation.drv
    building '/nix/store/ishzj9ni17xq4hgrjvlyjkfvm00b0ch9-my-example-derivation.drv'...
    hello!
    bye!
    running post-build-hook '/home/grahamc/projects/github.com/NixOS/nix/post-hook.sh'...
    post-build-hook: + sleep 1
    post-build-hook: + echo 'Signing paths' /nix/store/qr213vjmibrqwnyp5fw678y7whbkqyny-my-example-derivation
    post-build-hook: Signing paths /nix/store/qr213vjmibrqwnyp5fw678y7whbkqyny-my-example-derivation
    post-build-hook: + sleep 1
    post-build-hook: + echo 'Uploading paths' /nix/store/qr213vjmibrqwnyp5fw678y7whbkqyny-my-example-derivation
    post-build-hook: Uploading paths /nix/store/qr213vjmibrqwnyp5fw678y7whbkqyny-my-example-derivation
    post-build-hook: + sleep 1
    post-build-hook: + printf 'very important stuff'
    /nix/store/qr213vjmibrqwnyp5fw678y7whbkqyny-my-example-derivation

    [nix-shell:~/projects/github.com/NixOS/nix]$ ./inst/bin/nix build -L -f ./test.nix
    my-example-derivation> hello!
    my-example-derivation> bye!
    my-example-derivation (post)> + sleep 1
    my-example-derivation (post)> + echo 'Signing paths' /nix/store/c263gzj2kb2609mz8wrbmh53l14wzmfs-my-example-derivation
    my-example-derivation (post)> Signing paths /nix/store/c263gzj2kb2609mz8wrbmh53l14wzmfs-my-example-derivation
    my-example-derivation (post)> + sleep 1
    my-example-derivation (post)> + echo 'Uploading paths' /nix/store/c263gzj2kb2609mz8wrbmh53l14wzmfs-my-example-derivation
    my-example-derivation (post)> Uploading paths /nix/store/c263gzj2kb2609mz8wrbmh53l14wzmfs-my-example-derivation
    my-example-derivation (post)> + sleep 1
    my-example-derivation (post)> + printf 'very important stuff'
    [1 built, 0.0 MiB DL]

Co-authored-by: Graham Christensen <graham@grahamc.com>
Co-authored-by: Eelco Dolstra <edolstra@gmail.com>
2019-08-02 10:48:15 -04:00

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<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 <xref linkend="conf-post-build-hook" /></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
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 only contain characters which are safe for word
splitting, and free of any globs.
</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>