Revert the evaluation change from #29862.
While returning a dynamic value for all expanded resources during
validation is not optimal, trying to work around this using unknown maps
and lists is causing other undesirable behaviors during evaluation.
Allow `GetResource` to return correct types values during validation,
rather than relying on `cty.DynamicVal` as a placeholder. This allows
other dependent expressions to be more correctly evaluated.
The current behavior of module input variables is to allow users to
override a default by assigning `null`, which works contrary to the
behavior of resource attributes, and prevents explicitly accepting a
default when the input must be defined in the configuration.
Add a new variable attribute called `nullable` will allow explicitly
defining when a variable can be set to null or not. The current default
behavior is that of `nullable=true`.
Setting `nullable=false` in a variable block indicates that the variable
value can never be null. This either requires a non-null input value, or
a non-null default value. In the case of the latter, we also opt-in to
the new behavior of a `null` input value taking the default rather than
overriding it.
In a future language edition where we make `nullable=false` the default,
setting `nullable=true` will allow the legacy behavior of `null`
overriding a default value. The only future configuration in which this
would be required even if the legacy behavior were not desired is when
setting an optional+nullable value. In that case `default=null` would
also be needed and we could therefor imply `nullable=true` without
requiring it in the configuration.
In order to handle optional attributes, the Variable type needs to keep
track of the type constraint for decoding and conversion, as well as the
concrete type for creating values and type comparison.
Since the Type field is referenced throughout the codebase, and for
future refactoring if the handling of optional attributes changes
significantly, the constraint is now loaded into an entirely new field
called ConstraintType. This prevents types containing
ObjectWithOptionalAttrs from escaping the decode/conversion codepaths
into the rest of the codebase.
Previously our graph walker expected to recieve a data structure
containing schemas for all of the provider and provisioner plugins used in
the configuration and state. That made sense back when
terraform.NewContext was responsible for loading all of the schemas before
taking any other action, but it no longer has that responsiblity.
Instead, we'll now make sure that the "contextPlugins" object reaches all
of the locations where we need schema -- many of which already had access
to that object anyway -- and then load the needed schemas just in time.
The contextPlugins object memoizes schema lookups, so we can safely call
it many times with the same provider address or provisioner type name and
know that it'll still only load each distinct plugin once per Context
object.
As of this commit, the Context.Schemas method is now a public interface
only and not used by logic in the "terraform" package at all. However,
that does leave us in a rather tenuous situation of relying on the fact
that all practical users of terraform.Context end up calling "Schemas" at
some point in order to verify that we have all of the expected versions
of plugins. That's a non-obvious implicit dependency, and so in subsequent
commits we'll gradually move all responsibility for verifying plugin
versions into the caller of terraform.NewContext, which'll heal a
long-standing architectural wart whereby the caller is responsible for
installing and locating the plugin executables but not for verifying that
what's installed is conforming to the current configuration and dependency
lock file.
This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.
If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.
