Cc @peti

I mentioned in #108246 (comment), but I don't think this is the correct solution. We explicitly switched to using cabal2nix-unwrapped in #81125.

However, I don't know why @bgamari would be seeing this error.

I agree with @cdepillabout that it's surprising that cabal2nix calls nix-prefetch-url, and that this solution is not great, since we loose all the benefits of cabal2nix-unwrapped.

I'm wondering what the nix-prefetch-url call is exactly, if it is legitimate, and if so, if it is possible to make another wrapper that has just nix-prefetch-url without the many other prefetch scripts.

It's unfortunately quite hard to know why cabal2nix falls back to using nix-prefetch-url. However, I can say with pretty good confidence that the relevant logic is https://github.com/NixOS/cabal2nix/blob/88b8edb59ad0076f140e9948f233a43d6dfb842b/src/Distribution/Nixpkgs/Fetch.hs#L103.

That is, the process call at the end of fetchLocal fails for some reason, causing us to fall through to localArchive, which calls nix-prefetch-url.

@bgamari Thanks for looking into this.

If anyone is willing to debug the process call in fetchLocal, I'd be really interested in what is exactly failing here.

It's unfortunately quite hard to know why cabal2nix falls back to using nix-prefetch-url.

cabal2nix uses that external command to download the tarball for some Hackage package in case it needs to determine the SHA512 hash of that tarball to put into the expression. These days, that should no longer be necessary because the cabal-install database contains all that information, so if you've run cabal update at some point, then the necessary hash is known locally already.

There might be other occasions where this command is called, but if I remember correctly then all of those occasions have to do with people trying to generate build instructions for remote git and/or darcs repositories. The code that implements those features is quite complicated, unfortunately, so no-one knows for sure what is going on. :-/

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

https://discourse.nixos.org/t/hakyll-nix-starter-template/9111/3

I marked this as stale due to inactivity. → More info

@sternenseemann If I remember correctly, we were thinking that this error was originally caused by cabal2nix not correctly showing errors. For instance, not being able to find a .cabal file.

I think recently we got a PR to cabal2nix that made it show more errors (instead of falling back to looking for nix-prefetch-url).

This can probably be closed?

This issue should be fixed with cabal2nix 2.18.0 indeed (NixOS/cabal2nix#489). To be precise it was a mistake in the fetching logic where the file “fetcher” would signal it was not applicable to the given URL/path instead of failing because it was able to access the path, but missing the cabal file.

Now the failure looks like this:

nix-repl> :b haskellPackages.haskellSrc2nix { name = "foo"; src = ./.; }
builder for '/nix/store/nlld1psymr14dwldlv2dib8fc7ds1qri-cabal2nix-foo.drv' failed with exit code 1; last 2 log lines:
  installing
  cabal2nix: user error (*** Found neither a .cabal file nor package.yaml. Exiting.)

Okay great, I'll go ahead and close this PR then.