When a top-level list-of-object contains an attribute that is also
list-of-object we need to do the fixup again inside the nested body (using
our synthetic attributes-only schema) so that the attr-as-blocks mechanism
can apply within the nested blocks too.
In order to preserve pre-v0.12 idiom for list-of-object attributes, we'll
prefer to use block syntax for them except for the special situation where
the user explicitly assigns an empty list, where attribute syntax is
required in order to allow existing provider logic to differentiate from
an implicit lack of blocks.
Because we handle FixUpBlockAttrs after dynamic block expansion, when
resolving variables we unfortunately need to consider the possibility of
both dynamic block expansion _and_ the block attrs fixup.
To accommodate this we have a variant of dynblock.VariablesHCLDec that
instead walks using the configschema.Block representation of the schema
and applies the same opportunistic schema rewrite used by FixUpBlockAttrs
at each body encountered during the walk.
This preprocessing step allows users to use nested block syntax to specify
elements of an attribute that is defined as being a list or set of an
object type.
This restores part of the unintended flexibility permitted in Terraform
v0.11 so that we can work around a few tricky edges where provider
implementations were relying on Terraform's failure to validate this in
earlier versions.
For any body that is pre-processed using this new helper, we will
recognize when a configuration author uses nested block syntax with a
name that is specified in the schema as an attribute of a suitable type
and tweak the schema just in time before decoding to expect that usage
and then fix up the result on the way out to conform to the original
schema.
Achieving this requires an abstraction inversion because only Terraform's
high-level schema has enough information to decide how to rewrite the
incoming low-level schema. We must therefore here implement HCL's
lowest-level API interface in terms of the higher-level abstractions of
hcldec and Terraform's configschema.
Because of the abstraction inversion this fixup mechanism cannot be used
generally for arbitrary HCL bodies but we can use it carefully inside the
lang package where its own API can guarantee the necessary invariants for
this to work.
Martin Atkins