[Spike] - See if vanity imports puts us in an infinite loop

[marwan-at-work]

1. Create a vanity import under your own domain name, marwan.io/mycoolpackage
2. Have the vanity import return a meta to an athens server (localhost since we don't have athens up yet) <meta name="go-import" content="marwan.io/mycoolpackage mod http://localhost:3000">
3. Run GOPROXY=http://localhost:3000 go get marwan.io/mycoolpackage
4. Proxy will internally call go mod download on marwan.io/mycoolpackage, which will then redirect go mod download to use a mod server, which is the Proxy itself and so we potentially end up in an infinite loop.

Whether we redirect to CDN or not matters, but that depends on the type of storage and whether it's Proxy/Registry.

[ghost]

If there is no other, rjl.li/license exists No longer, I re-purposed the domain.

[arschles]

note that as of #772, we're not going to try and build a registry for the time being. Does this still applies to the proxy @marwan-at-work ?

[marwan-at-work]

@arschles yes. This is not about a registry. This is about vanity imports. The steps above describe how you'd get stuck in an infinite loop

[arschles]

@marwan-at-work good point (and sorry for the delay). I see what you mean now. What do you think about recording each cache miss that we see and cancelling a go mod download call if we see the same cache miss within duration D?

I think this relies on #760 being built, but assuming it is, I can't think of any other reason that there would be multiple cache misses within a small duration for the same module@version