caddytls: Allow disabling storage cleaning, avoids writing two files

[francislavoie]

Some users want to run Caddy in a totally read-only mode, but currently storage cleaning implicitly creates two files instance.uuid and last_clean.json which are undesirable in that situation. We can simply skip starting the storage cleaning goroutine via a config option.

I noticed storage_check off wasn't configurable in the Caddyfile, so I added that too, while I was at it.

[mholt]

Thanks for working on this! A couple questions/thoughts:

• What is the harm of trying to write these two files?
• Maybe we should just have a read_only mode in the global options (if it is actually needed). I don't know if I love this granularity since you need to turn several knobs for one thing.

[francislavoie]

Forgot to link the related forum thread https://caddy.community/t/disabling-file-storage/25772, those 2 files get written which is undesirable in a setup with the goal of being read-only.

The way I see it, doesn't hurt to have this option, solves an edgecase for someone. Very simple implementation.

Maybe we should just have a read_only mode in the global options (if it is actually needed). I don't know if I love this granularity since you need to turn several knobs for one thing.

I dunno, an option that does too much is not better.

[jleedev]

I often want to use read-only mode when I'm running an ephemeral Caddy instance purely for testing, e.g. to verify a server's behavior behind a reverse proxy. Something trivial like caddy respond hello should not be complaining about "unable to get instance ID; storage clean stamps will be incomplete" and "could not clean default/global storage", and moreover this does not give me confidence that I can run the program purely as an isolated, single-use shell tool (which, to be clear, Caddy does not claim to be; compare this to how it wants to install a local certificate authority).

If the main (only?) use of the "storage" module relates to certificate maintence, it should be reasonable to avoid running this when TLS is disabled. If I run caddy file-server without giving a --domain flag, it should have no reason to be writing these.