Store time of last modified in cache dir

[afbjorklund]

Was looking more at caching, and thought it would be a good idea to store the last modified time too.

$ more ~/.cache/lima/download/by-url-sha256/2385cd7cc68589b1dab56df12415b9151d977ba9b9b668e913f02ebd35804ef7/url 
https://github.com/lima-vm/alpine-lima/releases/download/v0.2.38/alpine-lima-std-3.19.0-x86_64.iso
$ more ~/.cache/lima/download/by-url-sha256/2385cd7cc68589b1dab56df12415b9151d977ba9b9b668e913f02ebd35804ef7/time 
Fri, 26 Apr 2024 05:04:22 GMT
$ more ~/.cache/lima/download/by-url-sha256/2385cd7cc68589b1dab56df12415b9151d977ba9b9b668e913f02ebd35804ef7/type 
application/octet-stream

Then you can do things such as If-Modified-Since and helps when looking at implementing a simple proxy.

• PoC: Implement optional host proxy for lima cache #2367

When caching more things than OS images, it is also useful to record the content type (deb, tar, whatever)

WARN[0000] reference for unknown type: application/octet-stream

---

The information is available in the Result, when retreiving cached files (but the string can be empty)

It would be possible to add the content instead, and use empty string for older cache without the files.

 type Result struct {
        Status          Status
        CachePath       string // "/Users/foo/Library/Caches/lima/download/by-url-sha256/<SHA256_OF_URL>/data"
+       LastModified    time.Time
+       ContentType     string
        ValidatedDigest bool
 }

[afbjorklund]

Now the user (proxy) will need to use time.IsZero and time.Format(time.RFC1123), for LastModified

[afbjorklund]

Added some unit tests.

[AkihiroSuda]

Thanks