In study of existing providers we've found a pattern we werent previously
accounting for of using a nested block type to represent a group of
arguments that relate to a particular feature that is always enabled but
where it improves configuration readability to group all of its settings
together in a nested block.
The existing NestingSingle was not a good fit for this because it is
designed under the assumption that the presence or absence of the block
has some significance in enabling or disabling the relevant feature, and
so for these always-active cases we'd generate a misleading plan where
the settings for the feature appear totally absent, rather than showing
the default values that will be selected.
NestingGroup is, therefore, a slight variation of NestingSingle where
presence vs. absence of the block is not distinguishable (it's never null)
and instead its contents are treated as unset when the block is absent.
This then in turn causes any default values associated with the nested
arguments to be honored and displayed in the plan whenever the block is
not explicitly configured.
The current SDK cannot activate this mode, but that's okay because its
"legacy type system" opt-out flag allows it to force a block to be
processed in this way anyway. We're adding this now so that we can
introduce the feature in a future SDK without causing a breaking change
to the protocol, since the set of possible block nesting modes is not
extensible.
Our usual "ground rules" for mapping configschema to cty call for the
collection values representing nested block types to always be known and
non-null, using an empty collection to represent the absense of any blocks
of that type so that users can always safely use length(...) etc on them
without worrying about them sometimes being null.
However, due to some different behaviors in the legacy SDK we've allowed
it an exception to this rule which means that we can see unknown and null
collections in these positions in object values returned from provider
operations like PlanResourceChange and ApplyResourceChange when the legacy
SDK opt-out is activated.
As a consequence of this, we need to be mindful in our safety check
functions, like AssertObjectCompatible here, of tolerating these non-ideal
situations to allow the safety checks to complete. We run these checks
even when the provider requests an opt-out, because we want to note any
inconsistencies as WARNING level log lines to aid in debugging.
Since these error messages get printed in Terraform's output and we
encourage users to share them as part of bug reports, we should avoid
including sensitive information in them to reduce the risk of accidental
exposure.
Previously we were just asserting that the number of elements didn't grow
between planned and actual. We still can't precisely correlate elements in
sets with unknown values, but here we adapt some logic we added earlier
to config/hcl2shim to ensure that we can find a plausible correlation for
each element in each set to at least one element in the other set, and
thus catch more cases where set elements might vanish or appear between
plan and apply, for improved safety.
This will still generate false negatives in some cases where unknown
values are present due to having to assume correlation is intended
wherever it is possible, but we'll catch situations where the actual value
is obviously contrary to what was planned.
We need to check for the known-ness of the prior value before we check for
the null-ness of actual, because it's valid for an unknown value to become
a null.
We need to make the collection itself be a tuple or object rather than
list or map in this case, since otherwise all of the elements of the
collection are constrained to be of the same type and that isn't the
intent of a provider indicating that it accepts any type.
This function's goal is to ensure that the "final" plan value produced
by a provider during the apply step is always consistent with the known
parts of the planned value produced during the plan step.
Any error produced here indicates a bug in the provider.
Martin Atkins