Let me know what, if anything, you want in terms of integration tests. I wasn't sure whether you'd want to pull in a dep on, say, https://github.com/bradfitz/go-tool-cache and its cmd/gocached and client program.

Also, if you want this broken up into smaller chunks, I'm happy to do that.

e.g. I could send out just the Cache interface and renames to DiskCache first, without adding the GOCACHEPROG stuff.

But first I wanted to send it all (it's tolerably sized, IMO) and see if you had any high level thoughts before I revised it for review.

Some results from Tailscale deploying this change in our CI on March 9th...

@dominikh, can you take a look?

Applied as ba2c208. As for integration tests, that'll be my users (you) for now.

Thanks! Would you like integration tests? If so, how much are you okay with dependency-wise? Would you be okay depending on https://github.com/bradfitz/go-tool-cache for a test server, or prefer a copy?

I wouldn't mind some tests, and a dependency would be fine 👍