By default, Perl versions since 5.8.1 use randomization to make hashes
resistant to complexity attacks.
That randomization makes building VM images such as ubuntu1804x86_64
non-deterministic because the (imported) derivations built by
deb/deb-closure.pl are not stable.
This can easily be observed by repeating the following sequence of
commands and noting the path of the image's .drv:
nix-instantiate -E '(import <nixpkgs> {}).vmTools.diskImageFuns.ubuntu1804x86_64 {}'
nix-store --delete /nix/store/*ubuntu-18.04-bionic-amd64.nix
One source of non-determinism is the handling of Provides/Replaces,
which depends on the order of iteration over %packages. Here is a
diff showing the corresponding change in output:
>>> awk
-virtual awk: using original-awk
- original-awk: libc6 (>= 2.14)
+virtual awk: using mawk
+ mawk: libc6 (>= 2.14)
- mawk: libc6 (>= 2.14)
->>> libc6
This patch sorts packages by name for Provides/Replaces processing,
which seems to result in stable output.
(If the above turns out not to be sufficient, one could also set the
PERL_HASH_SEED and PERL_PERTURB_KEYS environment variables, documented
in 'perlrun', to disable Perl's built-in randomization. Complexity
attacks are not an issue as we control and trust all inputs.)
