Regression: symbolic links in the build sandbox become regular files unexpectedly

[Atry]

Describe the bug

When I create a derivation of a symbolic link:

hello = pkgs.runCommand "hello" { } ''
  ln -s ${pkgs.hello}/bin/hello $out
'';

then print it from another derivation:

ls-hello = pkgs.runCommand "ls-hello" { } ''
  ls -l ${hello} > $out
'';

the output path of hello becomes a regular file.

I created a simple reproduction at https://github.com/Atry/nix-regression-sandbox-symbolic-links/blob/main/flake.nix

Steps To Reproduce

1. Start nix-daemon service 2.21.0
2. Run nix run github:Atry/nix-regression-sandbox-symbolic-links#print-ls-hello

The output is:

++ /nix/store/cydbsmqkxk30didm1rlz8ffk5wfa9gva-nix-2.18.2/bin/nix build --no-link --print-out-paths /nix/store/jpxm4snhyx07i4gr7r2nzq4dfqm8rfy5-rhz512l9ahfz4228n11lwqqxx4a2z1hs-source#hello
+ HELLO_FILE=/nix/store/r6lqcxqvm8q574rfba804d5gsxspnrrp-hello
+ ls -l /nix/store/r6lqcxqvm8q574rfba804d5gsxspnrrp-hello
lrwxrwxrwx 2 root root 66 Jan  1  1970 /nix/store/r6lqcxqvm8q574rfba804d5gsxspnrrp-hello -> /nix/store/rnxji3jf6fb0nx2v0svdqpj9ml53gyqh-hello-2.12.1/bin/hello
++ /nix/store/cydbsmqkxk30didm1rlz8ffk5wfa9gva-nix-2.18.2/bin/nix build --no-link --print-out-paths /nix/store/jpxm4snhyx07i4gr7r2nzq4dfqm8rfy5-rhz512l9ahfz4228n11lwqqxx4a2z1hs-source#ls-hello
+ LS_HELLO='-r-xr-xr-x 2 nobody nogroup 55712 Jan  1  1970 /nix/store/r6lqcxqvm8q574rfba804d5gsxspnrrp-hello'
+ /nix/store/cydbsmqkxk30didm1rlz8ffk5wfa9gva-nix-2.18.2/bin/nix-store --query --referrers-closure /nix/store/r6lqcxqvm8q574rfba804d5gsxspnrrp-hello
+ xargs nix-store --delete
finding garbage collector roots...
deleting '/nix/store/m1q7wpyrbgpfv8qzfjyxnk3bgmncbr6l-ls-hello'
deleting '/nix/store/r6lqcxqvm8q574rfba804d5gsxspnrrp-hello'
deleting unused links...
note: currently hard linking saves 136659.87 MiB
2 store paths deleted, 0.00 MiB freed
+ echo '-r-xr-xr-x 2 nobody nogroup 55712 Jan  1  1970 /nix/store/r6lqcxqvm8q574rfba804d5gsxspnrrp-hello'
-r-xr-xr-x 2 nobody nogroup 55712 Jan  1  1970 /nix/store/r6lqcxqvm8q574rfba804d5gsxspnrrp-hello

The output is unexpected because when running ls -l /nix/store/r6lqcxqvm8q574rfba804d5gsxspnrrp-hello outside of a nix build sandbox, it is a symbolic link, while when building ls-hello, it is a regular file.

The behavior is not good. For example, the inconsistent file types of r6lqcxqvm8q574rfba804d5gsxspnrrp-hello between build time and run time makes the digest computed by nix2container at build time mismatch with the digest at run time, leading to nlewo/nix2container#127.

Expected behavior

/nix/store/r6lqcxqvm8q574rfba804d5gsxspnrrp-hello should be always a symbolic link even in a sandbox, not a regular file.

nix-env --version output

nix-env (Nix) 2.21.0

Additional context

When building ls-hello in nix-daemon 2.18.2, it sees a symbolic link as expected. Therefore, this issue is a regression.

The nix client version does not matter. I used nix client 2.18.2 with nix-daemon 2.21.0 to reproduce this bug.

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[thufschmitt]

That looks like a potential instance of #9579. Can you try with latest master to see whether the bug's still there?

[Atry]

Fixed in #10456