core: if InstanceState has empty id after apply, instance is deleted

Previously we would just retain an empty InstanceState in this case, but
now that we must enumerate all of the available instances during
expression evaluation it's important that we be able to recognize
instances that have been deleted.

terraform/eval_apply.go

@@ -80,6 +80,14 @@ func (n *EvalApply) Eval(ctx EvalContext) (interface{}, error) {
 		}
 	}
 
+	// If the provider produced an InstanceState with an empty id then
+	// that really means that there's no state at all.
+	// FIXME: Change the provider protocol so that the provider itself returns
+	// a null in this case, and stop treating the ID as special.
+	if state.ID == "" {
+		state = nil
+	}
+
 	// Write the final state
 	if n.Output != nil {
 		*n.Output = state

terraform/eval_state.go

@@ -141,6 +141,11 @@ type EvalWriteState struct {
 func (n *EvalWriteState) Eval(ctx EvalContext) (interface{}, error) {
 	return writeInstanceToState(ctx, n.Name, n.ResourceType, n.Provider.String(), n.Dependencies,
 		func(rs *ResourceState) error {
+			if *n.State != nil {
+				log.Printf("[TRACE] EvalWriteState: %s has non-nil state", n.Name)
+			} else {
+				log.Printf("[TRACE] EvalWriteState: %s has nil state", n.Name)
+			}
 			rs.Primary = *n.State
 			return nil
 		},