[CA-drvs] nix-store --export fails with "error: hash of path ... has changed"

[fogti]

Describe the bug

nix-store --export /nix/store/q5dfzj5jm03asw3lzq6l0m2ivr0miixs-libunistring-0.9.10 fails on my machine with:

error: hash of path '/nix/store/q5dfzj5jm03asw3lzq6l0m2ivr0miixs-libunistring-0.9.10' has changed from 'sha256:1fl38plbsahjcnmpnrd6yqk8wzpw0wr6wxghyzm3wzg3ww571s0w' to 'sha256:1k4iikm5r2zsl5g3sf9wd02f4qf61qn022cnp82j4dc67b1zxbpa'!

Steps To Reproduce

unknown

Expected behavior

nix-store --export should succeed or at least suggest a possible solution.

nix-env --version output
nix-env (Nix) 2.4pre20210510_7f9759b

Additional context

My nix.conf:

max-jobs = auto
experimental-features = nix-command ca-derivations ca-references flakes
keep-outputs = true
keep-derivations = true
substituters = https://cache.ytrizja.de https://cache.nixos.org https://nix-community.cachix.org https://cache.ngi0.nixos.org/
trusted-public-keys = cache.ytrizja.de:iZzXw6+kwvY/6CZMufK+dul5i44nJeJ3rsYg2HNpLQc= cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY= nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs= cache.ngi0.nixos.org-1:KqH5CBLNSyX184S9BKZJo1LxrxJ9ltnY2uAs5c/f1MA=

[thufschmitt]

Mh indeed, exportPath does a blind hash of the path rather than a hash modulo (here).

It looks like nix store dump-path doesn’t do this check at all, so it should work fine in the meantime

[fogti]

I assume nix store dump-path works similiar to nix-store --dump, but then it won't be a suitable replacement to transfer files from one nix store to another (which I only do because nix copy fails with "unexpected end of file" errors)

   Synopsis
       nix-store --dump path

   Description
       The operation --dump produces a NAR (Nix ARchive) file containing the contents of the file system tree rooted at path. The archive is written to standard output.

       A  NAR archive is like a TAR or Zip archive, but it contains only the information that Nix considers important. For instance, timestamps are elided because all files in the Nix store have their timestamp set to 0 anyway. Likewise, all permissions are left out except for the execute bit, because all files in the Nix
       store have 444 or 555 permission.

       Also, a NAR archive is canonical, meaning that “equal” paths always produce the same NAR archive. For instance, directory entries are always sorted so that the actual on-disk order doesn’t influence the result.  This means that the cryptographic hash of a NAR dump of a path is usable as a fingerprint  of  the  con‐
       tents of the path. Indeed, the hashes of store paths stored in Nix’s database (see nix-store -q --hash) are SHA-256 hashes of the NAR dump of each store path.

       NAR archives support filenames of unlimited length and 64-bit file sizes. They can contain regular files, directories, and symbolic links, but not other types of files (such as device nodes).

       A Nix archive can be unpacked using nix-store --restore.

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/tweag-nix-dev-update-13/13443/1