Function to get file size

[shyim]

Is your feature request related to a problem? Please describe.

Currently, we are able to hash a file and read a file, but we can't get the file size

Describe the solution you'd like
I would like to have a function to get the file size of the given path

Describe alternatives you've considered

lib.toInt (lib.fileContents (pkgs.runCommand "update.zip" { } ''
              du -b ${./update.zip} | cut -f1 > $out;
            ''))

Additional context
Add any other context or screenshots about the feature request here.

Priorities

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[Ericson2314]

CC @roberth @aakropotkin as we were talking about this in #7447

[infinisil]

@shyim What are the use cases for such a function?

[shyim]

I generate a JSON for Wiremock (API mocking) in Nix and to mock an update service I need the sha1, sha256 of that file and the fileSize. Sha is already possible and didn't found a proper way to get the fileSize so I came to this issue :)

[infinisil]

Hmm, this doesn't sound very convincing, it's not really a use case Nix is meant for, and I feel like there's many ways to achieve the same result without Nix.

[aakropotkin]

If it were possible to lock inputs with fileSize I would agree with this being useful; but because filesize says nothing about the contents you can't lock, and if you can't lock what's the point?

If you really want filesize as a built in it's a great opportunity to write a small plugin though.

[shyim]

with plugin you mean a PR here? 🤔

[roberth]

If it were possible to lock inputs with fileSize I would agree with this being useful

Adding the file size to lock files (or FOD hashes!) is nice for giving an indication of total download size. There's a case to be made for that, but locking is not implemented in the Nix language, so I don't see that as a use case for a file size builtin.

Of course fileSize is derived info that is merely convenient to store in a lock file, and not essential.

I would like to have a function to get the file size of the given path

This sounds very hypothetical

I generate a JSON for Wiremock (API mocking) in Nix

Now we're talking.
This mock service doesn't run in Nix, so presumably this information can be computed in the derivation that writes the mocking config? That will also perform slightly better. The less work we do in the Nix language, the faster evaluation will be, and derivations are by nature cached.