(TK-487) Allow friendly init/start fail fast via ::exit throw

[rbrw]

Allow init and start methods to throw a request-shutdown style ex-info
map to short circuit the startup process and exit with a specified
message and status, rather than a backtrace.

This just provides a short circuiting (immediate) counterpart to the
existing, deferred shutdown requests provided by request-shutdown.

[rbrw]

This isn't quite ready (e.g. at least needs tests), but I wanted to see if it seemed plausible before proceeding further. The current motivation is wanting to be able to shut down in a friendly way when a service detects a problem with the configuration it retrieves via (get-config), without having to carefully coordinate with all other services in order to safely rely on a deferred request-shtudown.

[puppetcla]

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[mwaggett]

I'm a confused why there is a not here..

[rbrw]

Oh, right, because I believe check returns nil if it matches, info about the mismatch, otherwise.

[mwaggett]

Why select-keys instead of including all the exception data?
Also, based on your documentation, it looks like :puppetlabs.trapperkeeper.core/exit is not a top-level key, it's the value for the :kind key. so not sure what the intention was here.

[rbrw]

If I recall correctly, it's just to be sure that the exception only contributes the key that it's supposed to, e.g. can't clobber other top-level keys, etc.

And I think the idea is that :kind is a non-namespaced key that tells you what type of ex-data map you have. Everything else in the map can be type-specific. The ':kind` value is namespaced to avoid collisions across namespaces and even across different projects.

We could namespace :kind, as say :puppetlabs/kind, but I believe there's some precedence at puppet (and I vaguely recall maybe elsewhere) to use :kind globally. TK itself is already using :kind for some exit-related code.