nix-prefetch-url --unpack segfaults with zero sized file as input

[bmellstrom]

Describe the bug

nix-prefetch-url --unpack crashes if given a zero size file as input using the file protocol.

Steps To Reproduce

$ touch foo
$ nix-prefetch-url --unpack file://$PWD/foo
Segmentation fault         (core dumped) nix-prefetch-url --unpack file://$PWD/foo

Expected behavior

Segmentation fault should never occur regardless of input values.

Metadata

nix-env (Nix) 2.31.2

Platform: aarch64

nixpkgs commit: d351d0653aeb7877273920cd3e823994e7579b0b

Additional context

Checklist

• checked latest Nix manual (source)
• checked open bug issues and pull requests for possible duplicates

---

Add 👍 to issues you find important.

[xokdvium]

Yeah not great. prefetchFile dies when it gets an empty directory as output. Seems to be a thing starting since ~2.24

[amaanq]

I'm wondering what the intended behavior for this should be for older versions of nix, which create an empty file. In my mind, this is an unintended side effect, and isn't a behavior that should be relied upon.

[xokdvium]

I'm fine with making this an error.