lix/doc/manual/src/advanced-topics/diff-hook.md

# Verifying Build Reproducibility

Specify a program with Nix's [???](#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.

For purposes of demonstration, we'll use the following Nix file,
`deterministic.nix` for testing:

    let
      inherit (import <nixpkgs> {}) runCommand;
    in {
      stable = runCommand "stable" {} ''
        touch $out
      '';
    
      unstable = runCommand "unstable" {} ''
        echo $RANDOM > $out
      '';
    }

Additionally, `nix.conf` contains:

    diff-hook = /etc/nix/my-diff-hook
    run-diff-hook = true

where `/etc/nix/my-diff-hook` is an executable file containing:

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

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.

# Spot-Checking Build Determinism

Verify a path which already exists in the Nix store by passing `--check`
to the build command.

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

    $ nix-build ./deterministic.nix -A stable
    this derivation 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

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

    $ nix-build ./deterministic.nix -A unstable
    this derivation 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

In the Nix daemon's log, we will now see:

    For derivation /nix/store/cgl13lbj1w368r5z8gywipl1ifli7dhk-unstable.drv:
    1c1
    < 8108
    ---
    > 30204

Using `--check` with `--keep-failed` will cause Nix to keep the second
build's output in a special, `.check` path:

    $ 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'

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

> **Note**
> 
> Check paths are not protected against garbage collection, and this
> path will be deleted on the next garbage collection.
> 
> The path is guaranteed to be alive for the duration of
> [???](#conf-diff-hook)'s execution, but may be deleted any time after.
> 
> 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.

`--check` 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:

    error: some outputs of '/nix/store/hzi1h60z2qf0nb85iwnpvrai3j2w7rr6-unstable.drv' are not valid, so checking is not possible

Run the build without `--check`, and then try with `--check` again.

# Automatic and Optionally Enforced Determinism Verification

Automatically verify every build at build time by executing the build
multiple times.

Setting [???](#conf-repeat) and [???](#conf-enforce-determinism) in your
`nix.conf` permits the automated verification of every build Nix
performs.

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

    enforce-determinism = true
    repeat = 2

Setting [???](#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:

    enforce-determinism = false
    repeat = 1

An example output of this configuration:

    $ nix-build ./test.nix -A unstable
    this derivation 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