builtins.fetchTarball with a single-file tarball produces an empty store path

[Silic0nS0ldier]

Describe the bug

Using a .tar.gz file which only contains a file with builtins.fetchTarball results in a derivation that is an empty directory.

This behaviour is inconsistent with nix-prefetch-url --unpack which is often cited as the recommended way to obtain sha256 for these fixed output derivations.

Steps To Reproduce

let
  dotslashPath = builtins.fetchTarball {
    url = "https://github.com/facebook/dotslash/releases/download/v0.4.1/dotslash-macos.v0.4.1.tar.gz";
  };
in
  dotslashPath

❯ nix-instantiate --eval ./testing.nix                  
"/nix/store/0ccnxa25whszw7mgbgyzdm4nqc0zwnm8-source"
❯ file /nix/store/0ccnxa25whszw7mgbgyzdm4nqc0zwnm8-source
/nix/store/0ccnxa25whszw7mgbgyzdm4nqc0zwnm8-source: directory
❯ ls -l /nix/store/0ccnxa25whszw7mgbgyzdm4nqc0zwnm8-source
total 0

Expected behavior

The documentation states that the top-level directory will be removed, but does not specify what happens when there is no top-level directory.

#4785 suggests that prior to a certain Nix release the file would have been extracted successfully. Looking back at a past release may help to inform what should happen.

My personal preference is that fetching of a single-file tarball would work, and that the filename be perserved (this does not happen with nix-prefetch-url --unpack, the file ends up being placed at the store path itself). If that is not possible, a clear error.

nix-env --version output

nix-env (Nix) 2.23.0

Additional context

Similar expected behaviour ambiguity to #4785

Priorities

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[nixos-discourse]

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

https://discourse.nixos.org/t/2024-07-03-nix-team-meeting-minutes-158/49097/1

[roberth]

Triaged by the Nix team 2024-07-03 (apologies for the delay)
Relevant notes:

Somewhat related to the code tested here https://github.com/DeterminateSystems/nix-src/pull/1/files

Assigned to Eelco

[roberth]

Somewhat related to the code tested here https://github.com/DeterminateSystems/nix-src/pull/1/files

Something went wrong there.

The relevant test is https://github.com/NixOS/nix/pull/11077/files#diff-7b407c6754b02bc382cb4344e00c566919584fd7d75ab2624de5d4f6b32bab46

It shows the behavior of the file system object sink which should not describe tarball-specific behavior, but currently does.
This code is probably related to the problem, and I think we should change the GitFileSystemObjectSink to have more straightforward behavior, to make room for the tarball fetching code to handle the "complexity" of handling various layouts of tarball contents.