feat: Implement hermit caching strategy

[wsutina]

✍️ Changes

The Hermit strategy accepts requests at /hermit/{host}/{path...} and acts as a smart router:

1. GitHub releases (github.com + /releases/download/) are redirected to the existing github-releases strategy using an internal HTTP call with NoOpCache to avoid double caching
2. All other sources (golang.org, npm, Maven, HashiCorp, etc.) are handled directly using the handler pattern with standard caching

🧪 Testing

GitHub releases (hermit install mk-6.0.1)

Cache miss

Cache hit

All other sources (hermit install go-1.17.3)

Cache Miss

Cache Hit

[js-murph]

Looks great @wsutina!

[alecthomas]

Suggested change

	BaseURL string `hcl:"base-url" help:"Base URL for internal redirects to github-releases strategy (e.g., http://localhost:8080)."`
	BaseURL string `hcl:"base-url" help:"Base URL for internal redirects to github-releases strategy (e.g., http://localhost:8080)." default:"${CACHEW_URL}"`

[alecthomas]

If we set this default, 99% of the time it won't need to be explicitly configured.

[alecthomas]

I wonder if this should have /github as the suffix too... I suspect that would be more flexible. ie. "${CACHEW_URL}/github"

[wsutina]

Is this what you mean?

My understanding is github-base-url will control where we redirect:

• If it sets to ${CACHEW_URL}/github.com -- it will go to github-release strategy
• If it sets to github.com -- it will go to github.com directly

[alecthomas]

Exactly!

[alecthomas]

Aaah, nice solution!

[alecthomas]

What is this for? The config package already expands envars in the HCL.

[wsutina]

I think config in envar expansion works only hcl file but not for default in struct. For example

	BaseURL string `hcl:"base-url" help:"Base URL for internal redirects to github-releases strategy" default:"$${CACHEW_URL}"`

results in

INF Hermit strategy initialized base_url=$${CACHEW_URL}

[alecthomas]

The HCL library supports defaults, eg. here

[alecthomas]

This already works, you can see it used with github-release:

github-releases {
  token = "${GITHUB_TOKEN}"

[wsutina]

The var expansion works but if we want the default to be set in the config only:

cachew.hcl

hermit { } <-- no value

hermit.go

type HermitConfig struct {
	GitHubBaseURL string `hcl:"github-base-url,optional" help:"Base URL for GitHub release redirects" default:"${CACHEW_URL}/github.com"`
}

and hcl is empty, the var expansion doesn't get called.

[wsutina]

Closing the loop. To addressed this, a change is made to alecthomas/hcl#46 to accept default transformer + fa8bc7c