nix-prefetch-git: fix checkout_ref() for "dumb http" repositories

[logan12358]

The checkout_ref() function was written against Git 1.6.5, just before "smart http" repositories were implemented in Git 1.6.6. In 1.6.5, the --depth parameter is ignored when fetching from a dumb http repository. From 1.6.6, this scenario causes a failure. This change updates the comment and falls back to fetching without --depth.

Motivation for this change

This change causes checkout_ref to succeed in cases where I believe it is intended. I think that previously fetches from a dumb http repository would usually succeed through the checkout_hash fallback. As of #104714 checkout_hash doesn't seem to support tags (perhaps this should be changed as well?), so now nix-prefetch-git noticeably fails for tag revs on dumb http repositories.

I'm not 100% convinced about the chained ||, but I can't find a way to instruct Git to "attempt" --depth=1 or find a way to robustly fall back only if the first fetch fails due to a "dumb http" repository.

This seems to resolve #115145.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[r-rmcgibbo]

Result of nixpkgs-review pr 116289 at 15364cb run on aarch64-linux 1

2 packages skipped due to time constraints:

• haskellPackages.update-nix-fetchgit
• update-nix-fetchgit

9 packages built successfully:

• bundix
• cabal2nix
• crate2nix
• dep2nix
• go2nix
• nix-prefetch-git
• nix-prefetch-scripts
• nix-update-source
• vgo2nix

---

Result of nixpkgs-review pr 116289 at 15364cb run on x86_64-linux 1

12 packages built successfully:

• bundix
• cabal2nix
• crate2nix
• crystal2nix
• dep2nix
• go2nix
• haskellPackages.update-nix-fetchgit
• nix-prefetch-git
• nix-prefetch-scripts
• nix-update-source
• update-nix-fetchgit
• vgo2nix

[nixos-discourse]

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

https://discourse.nixos.org/t/cant-build-solvespace-from-scratch-fetching-git-source-fails/46536/2

[ShamrockLee]

Hi @logan12358 , could you help check if PR #283663 addressed the issue?

I refactored the code a bit in PR #465497 to

• use if-else instead of and-or
• avoids fetching all tags whenever backward compatible (when not leaving .git)
• only clone again with the deep-clone-and-fetch-all-tags flags when the forst clone is not like that.

Nevertheless, it would be better if we could check if a remote support the smart protocol or not instead of retrying blindly, but I don't know how to implement such check.