buildEnv: relax constraints about readdir contents

[h0nIg]

We were wondering why nix builds were failing, where the nix store is located on a virtual filesystem / fuse. In our case it is a store located on a lazy pulling enabled OCI image. We use estargz / zstd:chunked images, where fuse is used to start the container right away (and pulling the rest of the image in the background)

# nix profile upgrade runtime 
error: Cannot build '/nix/store/nccvdkcbvrl9y35f7ngw9x8qpcvbp7kp-devtools.drv'.
       Reason: builder failed with exit code 2.
       Output paths:
         /nix/store/2sj7yjv2756y16nz5f1azszgms4vix8z-devtools
       Last 1 log lines:
       > pkgs.buildEnv error: Died at /nix/store/4z0grds97rmripxps6b0kby1fr3g35cp-builder.pl line 77.
       For full logs, run:
         nix log /nix/store/nccvdkcbvrl9y35f7ngw9x8qpcvbp7kp-devtools.drv
error: build of '/nix/store/nccvdkcbvrl9y35f7ngw9x8qpcvbp7kp-devtools.drv^out' failed

This boils down to the POSIX standard, where "." and ".." are optional and SHALL get displayed (not MUST).

see https://pubs.opengroup.org/onlinepubs/9799919799/functions/readdir.html

The nixpkgs code has an assumption that empty directories at least have the result "." and "..", but this is not valid for all FS.

https://perldoc.perl.org/functions/readdir

Returns the next directory entry for a directory opened by opendir.

The nixpkgs code should work in a more relaxed mode, since opendir already checks that the directory is valid and readable. We would appreciate a backport to 25.11. staging as well (I can not set the tags)

# ls -lsa /nix/store/5r2skpf8xr3wjkgix63993cnp99nngsl-file-5.45-man/share/man/
total 0
0 dr-xr-xr-x 2 root root 0 Jan  1  1970 man1
0 dr-xr-xr-x 2 root root 0 Jan  1  1970 man3
0 dr-xr-xr-x 2 root root 0 Jan  1  1970 man4
0 dr-xr-xr-x 2 root root 0 Jan  1  1970 man5
# ls -lsa /nix/store/5r2skpf8xr3wjkgix63993cnp99nngsl-file-5.45-man/share/man/man5
total 0

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

---

Add a 👍 reaction to pull requests you find important.

[nixpkgs-ci]

Successfully created backport PR for staging-25.11:

• [Backport staging-25.11] buildEnv: relax constraints about readdir contents #464591