These changes restore a more-correct version of the behavior that was
disabled with #3014. The original implementation of this behavior was
done in the SDKs, which do not have access to the complete inputs for a
resource (in particular, default values filled in by the provider during
`Check` are not exposed to the SDK). This lack of information meant that
the resolved output values could disagree with the typings present in
a provider SDK. Exacerbating this problem was the fact that unknown
values were dropped entirely, causing `undefined` values to appear in
unexpected places.
By doing this in the engine and allowing unknown values to be
represented in a first-class manner in the SDK, we can attack both of
these issues.
Although this behavior is not _strictly_ consistent with respect to the
resource model--in an update, a resource's output properties will come
from its provider and may differ from its input properties--this
behavior was present in the product for a fairly long time without
significant issues. In the future, we may be able to improve the
accuracy of resource outputs during a preview by allowing the provider
to dry-run CRUD operations and return partially-known values where
possible.
These changes also introduce new APIs in the Node and Python SDKs
that work with unknown values in a first-class fashion:
- A new parameter to the `apply` function that indicates that the
callback should be run even if the result of the apply contains
unknown values
- `containsUnknowns` and `isUnknown`, which return true if a value
either contains nested unknown values or is exactly an unknown value
- The `Unknown` type, which represents unknown values
The primary use case for these APIs is to allow nested, properties with
known values to be accessed via the lifted property accessor even when
the containing property is not fully know. A common example of this
pattern is the `metadata.name` property of a Kubernetes `Namespace`
object: while other properties of the `metadata` bag may be unknown,
`name` is often known. These APIs allow `ns.metadata.name` to return a
known value in this case.
In order to avoid exposing downlevel SDKs to unknown values--a change
which could break user code by exposing it to unexpected values--a
language SDK must indicate whether or not it supports first-class
unknown values as part of each `RegisterResourceRequest`.
These changes also allow us to avoid breaking user code with the new
behavior introduced by the prior commit.
Fixes#3190.
137fd54f1c