Add UFO dir and mandatory .plist file read path existence checks and invalid path diagnostics

[chrissimpkins]

Current formatted error:

UFO load failed with error: PlistError(Error { inner: ErrorImpl { kind: Io(Os { code: 2, kind: NotFound, message: "No such file or directory" }), file_position: None } })

New formatted error with changes in this PR:

UFO load failed with error: invalid path: "bogus/path"

Tested in this executable with argument bogus/path:

fn main() {
    for arg in std::env::args().skip(1) {
        let mut ufo = match norad::Font::load(&arg) {
            Ok(u) => u,
            Err(e) => {
                eprintln!("UFO load failed with error: {}", e);
                std::process::exit(1);
            }
        };
    }
}

[madig]

Ah, yes, I'm interested in better error messages. I think this can be done more elegantly maybe, though. One could match on plist::from__file and if it returns an Io error, return that one instead of the generic PlistError. Not sure if one can implement a more user friendly user-facing error string on it though 🤔

Edit: there's also this method on plist's Error type: https://docs.rs/plist/1.1.0/plist/struct.Error.html#method.is_io, so it's easy to determine if it's actually an IO error.

[cmyr]

Thanks chris, improving diagnostics is always helpful!

[cmyr]

@madig do you have further thoughts on this? I'm happy to merge as-is, I think there are other possible approaches but this one feels reasonable. 🤷

[chrissimpkins]

@madig do you have further thoughts on this? I'm happy to merge as-is, I think there are other possible approaches but this one feels reasonable. 🤷

Let me know if you'd like me to pull d9219be out and address the mandatory file validations in another PR

[cmyr]

Thinking about this a bit more, what I would love is a new MissingFile error variant that specifically handles the case of a missing mandatory file; it can have a field where you pass a string representing the file in question ("metainfo.plist", "background/contents.plist"). So maybe a PR that adds that variant along with the check for metainfo.plist, using that variant, would be a good start?

[chrissimpkins]

Wrap the UFO dir check into that error handling approach and close this PR?

[chrissimpkins]

Thinking about this a bit more, what I would love is a new MissingFile error variant that specifically handles the case of a missing mandatory file; it can have a field where you pass a string representing the file in question ("metainfo.plist", "background/contents.plist"). So maybe a PR that adds that variant along with the check for metainfo.plist, using that variant, would be a good start?

How about this approach instead? #146