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

Verifying Build Reproducibility

You can use Nix's diff-hook setting to compare build results. Note that this hook is only executed if the results differ; it 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.

[]{#check-dirs-are-unregistered} 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 the 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.