Since we focus on safety, it would be nice to reduce the chance of any
runtime errors if we made a typo for a resource parameter. With this
patch, each resource can export constants into the global namespace so
that typos would cause a compile error.
Of course in the future if we had a more advanced type system, then we
could support precise types for each individual resource param, but in
an attempt to keep things simple, we'll leave that for another day. It
would add complexity too if we ever wanted to store a parameter
externally.
Lastly, we might consider adding "special case" parsing so that directly
specified fields would parse intelligently. For example, we could allow:
file "/tmp/hello" {
state => exists, # magic sugar!
}
This isn't supported for now, but if it works after all the other parser
changes have been made, it might be something to consider.
This adds the first reversible resource (file) and the necessary engine
API hooks to make it all work. This allows a special "reversed" resource
to be added to the subsequent graph in the stream when an earlier
version "disappears". This disappearance can happen if it was previously
in an if statement that then becomes false.
It might be wise to combine the use of this meta parameter with the use
of the `realize` meta parameter to ensure that your reversed resource
actually runs at least once, if there's a chance that it might be gone
for a while.
This patch also adds a new test harness for testing resources. It
doesn't test the "live" aspect of resources, as it doesn't run Watch,
but it was designed to ensure CheckApply works as intended, and it runs
very quickly with a simplified timeline of happenings.
