os: add func UserCacheDir() string

[bradfitz]

I've seen a dozen or so implementations of looking up the user's cache directory.

The latest is from @rsc in:

https://go-review.googlesource.com/c/go/+/68116/3/src/cmd/go/internal/cache/default.go

Which says:

// TODO(rsc): This code belongs somewhere else,
// like maybe ioutil.CacheDir or os.CacheDir.

Some examples of other implementations:
https://godoc.org/go4.org/xdgdir
https://godoc.org/camlistore.org/pkg/osutil#CacheDir
https://github.com/golang/crypto/blob/bd6f299fb381e4c3393d1c4b1f0b94f5e77650c8/acme/autocert/listener.go#L142

Consolidate into std?

[rsc]

Yes, I think we should do this. The windows docs I found said to use %localappdata% for caches, which none of those do. And I know of other copies in the dashboard scraper that (incorrectly) use $HOME/.cache on macOS, and so on. This definitely seems worth putting into the standard library.

Reading the actual definitions of ioutil.TempDir and os.TempDir, this looks closer to os.TempDir (return $TMPDIR) than ioutil.TempDir (make and return a fresh dir under $TMPDIR), so it should probably be os.CacheDir.

Probably should leave for Go 1.11 at this point.

[mvdan]

What about a config/data variant? It's similarly tricky to get right.

[bradfitz]

The os/user package might be a good place for this, since it os/user would likely be a dependency anyway, to find the current user and their $HOME.

[rsc]

If we put it in os/user then it's harder for cmd/go to use it (it cannot depend on cgo for bootstrap). I don't believe os/user is necessary for XDG anyway - the spec very clearly says $HOME not "the user's home directory".

[mvdan]

Perhaps a silly question, but wouldn't this be a bit limited then? If a Go program is run outside a shell, $HOME would likely not be set.

[rsc]

@mvdan that's not very specific. What execution environment do you have in mind that does not set $HOME?

[mvdan]

I was thinking anything that doesn't get run under a shell. I was hoping to demonstrate this with an empty docker container just running a Go program printing $HOME, but it seems like docker has a fallback of HOME=/ in that case.

Systemd also came to mind, but it seems to also take care of the variable on its own. And, on Linux, login programs apparently set it too, so opening a terminal without a shell still has $HOME present.

I thought that relying on this variable was more fragile than it actually is - my bad. Still curious if any reasonable execution environment wouldn't have it set.

[rsc]

It sounds like we should do this, as os.UserCacheDir. Per discussion with @bradfitz, let's run with $HOME (not using os/user) until we know a compelling reason to need the fallback.