Previously we were treating "dynamic" blocks in configuration the same as
any other block type when merging config bodies, so that dynamic blocks
in the override would override any dynamic blocks present in the base,
without considering the dynamic block type.
It's more useful and intuitive for us to treat dynamic blocks as if they
are instances of their given block type for the purpose of overriding.
That means a foo block can be overridden by a dynamic "foo" block and
vice-versa, and dynamic blocks of different types do not interact at all
during overriding.
This requires us to recognize dynamic blocks and treat them specially
during decoding of a merged body. We leave them unexpanded here because
this package is not responsible for dynamic block expansion (that happens
in the sibling "lang" package) but we do decode them enough to recognize
their labels so we can treat them as if they were blocks of the labelled
type.
Some of the fields in our config structs are either mandatory in primary
files or there is a default value that we apply if absent.
Unfortunately override files impose the additional constraint that we
be allowed to omit required fields (which have presumably already been
set in the primary files) and that we are able to distinguish between a
default value and omitting a value entirely.
Since most of our fields were already acceptable for override files, here
we just add some new fields to deal with the few cases where special
handling is required and a helper function to disable the "Required" flag
on attributes in a given schema.
This is a first pass of decoding of the main Terraform configuration file
format. It hasn't yet been tested with any real-world configurations, so
it will need to be revised further as we test it more thoroughly.
Martin Atkins