caddytest.Tester should ensure that AppDataDir exists

[tgeoghegan]

Caddy (including Caddies run as caddytest.Tester) will create instance.uuid if it doesn't already exist. However, this will fail if caddy.AppDataDir() doesn't already exist, which appears to be the case when caddytest.Tester is used on Linux (though not on Windows or macOS, which I don't understand).

This makes it difficult to use caddytest.Tester to test modules that need caddy.InstanceID() to work. See for example my struggles in mholt/caddy-ratelimit#34, where I resorted to doing os.MkdirAll(caddy.AppDataDir()), which works well enough in CI, but doesn't feel like a great solution.

It'd be nice if caddytest.Tester could consistently guarantee that AppDataDir will exist, or perhaps it could avoid writing the instance ID to disk altogether since a caddytest.Tester instance is not likely to need persistence across restarts.

[mholt]

Good point. Redoing our caddytest package was on my list for 2.8 (then the baby came early, then life got busy, and I got behind on my backlog, so testing has been deferred another cycle again, sorry).

But in the meantime we could probably work in a fix for this before then.

[armadi1809]

I can take a look at this!

[mholt]

Sounds good, let us know if you have questions @armadi1809

[armadi1809]

@mholt I guess this can be resolved in two ways:

1- Inside the InstanceID function, check if the path returned by AppDataDir() exists and create it if it does not.
2- get rid the part of the InstanceID function that writes the UUID to disk.

Which one is preferred? After a quick search through the code base, it doesn't look like the UUID file is being used somewhere else other than where it's being created (i.e. inside the InstanceID function) but I might be wrong.

[mholt]

@armadi1809 I think 1 is best. A call to MkdirAll() should do the trick.

@tgeoghegan AppDataDir() honors XDG env vars, so maybe tests could set XDG_DATA_HOME to a temporary directory.