121 lines
2.7 KiB
Markdown
121 lines
2.7 KiB
Markdown
---
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title: "Lets Learn Nix: Reproducibility"
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date: 2020-03-17T12:06:47Z
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draft: true
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---
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I am dedicating this page to defining and disambiguating some terminology. I
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think it is important to use these terms precisely, so it may be worthwhile to
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memorize these definitions and ensure that you are clarifying the discourse
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rather than muddying it.
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## Terms
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- repeatable build:
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- reproducible build:
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- deterministic build:
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- pure function:
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- impure function:
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- idempotent function:
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TODO(wpcarro): Consistently and deliberately use reproducible and
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deterministic.
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## Repeatable vs. Reproducible
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Is NixOS reproducible? Visit [@grhmc][who-grhmc]'s website,
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[r13y.com](https://r13y.com), to find out.
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At the time of this writing, 1519 of 1568 (i.e. 96.9%) of the paths in the
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`nixos.iso_minimal.x86_64-linux` installation image are reproducible.
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## What hinders reproducibility?
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Timestamps.
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If package A encodes a timestamp into its build artifact, then we can
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demonstrate that package A is *not reproducible* simply by building it at two
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different times and doing a byte-for-byte comparison of the build artifacts.
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## Does Nix protect developers against non-determinism
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Yes. But not entirely. How?
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## Deterministic Nix derivation
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```nix
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{ pkgs ? import <nixpkgs> {}, ... }:
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with pkgs;
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stdenv.mkDerivation {
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name = "reproducible";
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phases = [ "buildPhase" ];
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buildPhase = "echo reproducible >$out";
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}
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```
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## Non-deterministic Nix derivation
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We can introduce some non-determinism into our build using the `date` function.
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```nix
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# file: /tmp/test.nix
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{ pkgs ? import <nixpkgs> {}, ... }:
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with pkgs;
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stdenv.mkDerivation {
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name = "non-reproducible";
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phases = [ "buildPhase" ];
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buildPhase = "date >$out";
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}
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```
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Then run...
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```shell
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$ nix-build /tmp/test.nix
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$ nix-build /tmp/test.nix --check --keep-failed
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```
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## How do you test reproducibility?
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We can use `cmp` to compare files byte-for-byte. The following comparison should
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fail:
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```shell
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$ echo foo >/tmp/a
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$ echo bar >/tmp/b
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$ cmp --silent /tmp/{a,b}
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$ echo $?
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```
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And the following comparison should succeed:
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```shell
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$ echo hello >/tmp/a
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$ echo hello >/tmp/b
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$ cmp --silent /tmp/{a,b}
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$ echo $?
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```
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## Reproducible vs. deterministic
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Reproducible builds *are* deterministic builds and deterministic build
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## Deterministic, Reproducible, Pure, Idempotent, oh my
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- A pure function has no side-effects.
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- An idempotent function can be executed more than once with the same arguments
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without altering the side-effects.
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- A deterministic function ensures that
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## Deterministic vs. Reproducible
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I can check if a build is reproducible using [these tools][wtf-repro-tools].
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[wtf-repro-tools]: https://reproducible-builds.org/tools/
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[who-grhmc]: https://twitter.com/grhmc
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