Save cwd fd instead of path

[cole-h]

This reverts commit 56009b2.

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Submitted as a draft at the behest of @Ericson2314.

Note that this does not work, for some reason. See some of the discussion in #6348 for what went wrong (specifically, it appears to strip /home/vin from /home/vin/workspace/vcs/nix for some unknown reason).

[edolstra]

I thought the FD method didn't work? I didn't follow the entire discussion though.

[cole-h]

Sorry I didn't make this part clear -- it didn't, but @Ericson2314 requested I resubmit this part as a draft so that someone in the future can try to take this further / investigate the exact issue behind it.

[Ericson2314]

Yeah because of things like #2759 I am very keen on doing as much as possible with file descriptors. So I figured I could assign this draft PR to myself to hopefully debug someday.

[Ericson2314]

@cole-h Would you know how to make a test here demonstrating the failure?

[cole-h]

Currently? No. But I'll give it a shot tomorrow and see if I can't make one. I'll probably start with just a simple reproducer using the magic of flakes and work my way to a more comprehensive test.

[Ericson2314]

That would be wonderful, thanks in advance, @cole-h!

[cole-h]

(Rebased)

OK, so here's a simple reproducer:

1. nix build github:cole-h/nix/7aebe9181068d7313a4b10f56df00adad2b0e924
2. sudo result/bin/nix shell nixpkgs#coreutils -c pwd
3. See the error: pwd: couldn't find directory entry in ‘../../../..’ with matching i-node

However, if you do sudo result/bin/nix shell nixpkgs#coreutils -c sh -c pwd, it works as expected. I don't know what's going on there, but hopefully this is enough info for someone to investigate further if they want to.

I had a hard time trying to write a NixOS test for it, so this is the best I can do.