about summary refs log tree commit diff
path: root/doc/manual/advanced-topics/diff-hook.xml
blob: fb4bf819f94b77892abc927bd538d244361cb852 (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
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 &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>