core: treat refs to unknown set resource attrs as unknown during plan

References to computed list-ish attributes (set, list, map) were being
improperly resolved as an empty list `[]` during the plan phase (when
the value of the reference is not yet known) instead of as an
UnknownValue.

A "diffs didn't match" failure in an AWS DirectoryServices test led to
this discovery (and this commit fixes the failing test):

https://travis-ci.org/hashicorp/terraform/jobs/104812951

Refs #2157 which has the original work to support computed list
attributes at all. This is just a simple tweak to that work.

/cc @radeksimko

terraform/interpolate.go

@@ -519,6 +519,10 @@ func (i *Interpolater) interpolateListAttribute(
  log.Printf("[DEBUG] Interpolating computed list attribute %s (%s)",
  resourceID, attr)
 
+ if attr == config.UnknownVariableValue {
+ return attr, nil
+ }
+
  var members []string
  numberedListMember := regexp.MustCompile("^" + resourceID + "\\.[0-9]+$")
  for id, value := range attributes {

terraform/interpolate_test.go

@@ -348,6 +348,42 @@ func TestInterpolator_resourceMultiAttributesWithResourceCount(t *testing.T) {
  })
 }
 
+func TestInterpolator_resourceMultiAttributesComputed(t *testing.T) {
+ lock := new(sync.RWMutex)
+ state := &State{
+ Modules: []*ModuleState{
+ &ModuleState{
+ Path: rootModulePath,
+ Resources: map[string]*ResourceState{
+ "aws_route53_zone.yada": &ResourceState{
+ Type: "aws_route53_zone",
+ Primary: &InstanceState{
+ ID: "AAABBBCCCDDDEEE",
+ Attributes: map[string]string{
+ "name_servers.#": config.UnknownVariableValue,
+ },
+ },
+ },
+ },
+ },
+ },
+ }
+ i := &Interpolater{
+ Module: testModule(t, "interpolate-multi-vars"),
+ StateLock: lock,
+ State: state,
+ }
+
+ scope := &InterpolationScope{
+ Path: rootModulePath,
+ }
+
+ testInterpolate(t, i, scope, "aws_route53_zone.yada.name_servers", ast.Variable{
+ Value: config.UnknownVariableValue,
+ Type: ast.TypeString,
+ })
+}
+
 func getInterpolaterFixture(t *testing.T) *Interpolater {
  lock := new(sync.RWMutex)
  state := &State{