We previously attempted to make the special diff apply behavior for nested
sets of objects work with attribute mode by totally discarding attribute
mode for all shims.
In practice, that is too broad a solution: there are lots of other shimming
behaviors that we _don't_ want when attribute mode is enabled. In
particular, we need to make sure that the difference between null and
empty can be seen in configuration.
As a compromise then, we will give all of the shims access to the real
ConfigMode and then do a more specialized fixup within the diff-apply
logic: we'll construct a synthetic nested block schema and then use that
to run our existing logic to deal with nested sets of objects, while
using the previous behavior in all other cases.
In effect, this means that the special new behavior only applies when the
provider uses the opt-in ConfigMode setting on a particular attribute,
and thus this change has much less risk of causing broad, unintended
regressions elsewhere.
The previous commit added this flag but did not implement it. Here we
implement it by adjusting the shape of schema we return to Terraform Core
to mark the attribute as untyped and then ensure that gets handled
correctly on the SDK side.
This makes some slight adjustments to the shape of the schema we
present to Terraform Core without affecting how it is consumed by the
SDK and thus the provider. This mechanism is designed specifically to
avoid changing how the schema is interpreted by the SDK itself or by the
provider, so that prior behavior can be preserved in Terraform v0.11 mode.
This also includes a new rule that Computed-only (i.e. not also Optional)
schemas _always_ map to attributes, because that is a better mapping of
the intent: they are object values to be used in expressions. Nested
blocks conceptually represent nested objects that are in some sense
independent of what they are embedded in, and so they cannot themselves be
computed.
Martin Atkins