Asset directory mounts do not work on Windows

[itowlson]

The was reported by freaky.ashes on Discord - thanks to them for flagging it.

To reproduce:

• Create a Spin app with an asset directory mount (e.g. files = [{ source = "assets", destination = "/" }])
• Run it on Windows
• You get a "couldn't resolve \assets<component>" error for the component asset directory, because the assets directory didn't get created

This happens because of a conflict between canonicalisation and the glob crate: when a glob is prefixed with a canonical (UNC) path, it does not match any files. (And asset directories get created only when files exist to be copied into them.) To reproduce this, create this Rust app:

use std::path::PathBuf;

fn main() {
    println!("Using plain dir");
    let bp = PathBuf::from(env!("CARGO_MANIFEST_DIR"));
    let paths = glob::glob(bp.join("src/**/*").to_str().unwrap()).unwrap();
    for path in paths {
        println!("{path:?}");
    }

    println!("Using canonicalised dir");
    let bp = PathBuf::from(env!("CARGO_MANIFEST_DIR")).canonicalize().unwrap();
    let paths = glob::glob(bp.join("src/**/*").to_str().unwrap()).unwrap();
    for path in paths {
        println!("{path:?}");
    }
}

It prints something like

Using plain dir
Ok("D:\\Code Experiments\\globtest\\src\\main.rs")
Using canonicalised dir

This breaks directory mounts on Spin 2 because we canonicalise the source path, and use <source-path>/**/* to implement directory copying. I assume it also breaks glob mounts too but have not tested that case.

This is a known bug in the upstream glob crate: rust-lang/glob#132. The issue suggests a workaround which, although unsafe for general use, should be fine for us. Alternatively it might suffice to absolutise rather than canonicalise. The issue is 6 months old and there's no movement on it so we should not expect a fix from upstream.

[itowlson]

Also, this implies we're not testing asset loading on Windows. It would be good to cover that gap - I thought those tests ran across all platforms.

[itowlson]

This also affects wildcard globs, but not single-file patterns. If you have a mix that contains a single-file pattern, the app runs but the files from wildcards and directory mounts are not there, only the single files.