Previously we tried to short-circuit this if the schema version hadn't
changed and we were already using JSON serialization. However, if we
instead call into UpgradeResourceState every time we can let the provider
or the SDK do some general, systematic normalization and cleanup steps
without always requiring a schema version increase.
What exactly would be fixed up this way is for the SDK to decide, but for
example the SDK might choose to automatically delete from the state
anything that is no longer present in the schema, avoiding the need to
write explicit state migration functions for that common case where the
remedy is always the same.
The actual update logic is delegated to the provider/SDK intentionally so
that it can evolve over time and potentially differ depending on how
each SDK thinks about schema.
When a resource type schema includes dynamically-typed attributes we can't
do any automatic conversion from flatmap to JSON because we don't know
how to interpret the keys that start with the dynamically-typed
attribute's prefix.
To work around that, we'll instead just ask the SDK to do a no-op upgrade
(current and target versions are the same) which will convert from flatmap
to JSON using the SDK's own logic as a side-effect.
This situation should rarely arise in real-world use, but it ends up being
very important for the helper/resource provider test harness because it
is forced to lower the state back to flatmap repeatedly after every step
in order to run legacy checking and processing code.
If an instance object in state has an earlier schema version number then
it is likely that the schema we're holding won't be able to decode the
raw data that is stored. Instead, we must ask the provider to upgrade it
for us first, which might also include translating it from flatmap form
if it was last updated with a Terraform version earlier than v0.12.
This ends up being a "seam" between our use of int64 for schema versions
in the providers package and uint64 everywhere else. We intend to
standardize on int64 everywhere eventually, but for now this remains
consistent with existing usage in each layer to keep the type conversion
noise contained here and avoid mass-updates to other Terraform components
at this time.
This also includes a minor change to the test helpers for the
backend/local package, which were inexplicably setting a SchemaVersion of
1 on the basic test state but setting the mock schema version to zero,
creating an invalid situation where the state would need to be downgraded.
