mirror of
https://github.com/privatevoid-net/nix-super.git
synced 2024-11-15 10:46:15 +02:00
eb7d7780b1
This is needed to avoid this https://github.com/mesonbuild/meson/issues/13774 when we go back to making our subproject directory `src`.
123 lines
3.9 KiB
Markdown
123 lines
3.9 KiB
Markdown
# 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:
|
|
|
|
```nix
|
|
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:
|
|
|
|
```bash
|
|
#!/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:
|
|
|
|
```console
|
|
$ nix-build ./deterministic.nix --attr 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 --attr 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:
|
|
|
|
```console
|
|
$ nix-build ./deterministic.nix --attr 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 --attr 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:
|
|
|
|
```console
|
|
$ nix-build ./deterministic.nix --attr 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.
|