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diff --git a/doc/manual/advanced-topics/diff-hook.xml b/doc/manual/advanced-topics/diff-hook.xml new file mode 100644 index 000000000000..fb4bf819f94b --- /dev/null +++ b/doc/manual/advanced-topics/diff-hook.xml @@ -0,0 +1,205 @@ +<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 <nixpkgs> {}) 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 >&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 +< 8108 +--- +> 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> |