core: separate lifecycle for data resource "orphans"

The handling of data "orphans" is simpler than for managed resources
because the only thing we need to deal with is our own state, and the
validation pass guarantees that by the time we get to refresh or apply
the instance state is no longer needed by any other resources and so
we can safely drop it with no fanfare.

terraform/transform_orphan.go

@@ -203,8 +203,6 @@ func (n *graphNodeOrphanResource) ProvidedBy() []string {
 
 // GraphNodeEvalable impl.
 func (n *graphNodeOrphanResource) EvalTree() EvalNode {
-	var provider ResourceProvider
-	var state *InstanceState
 
 	seq := &EvalSequence{Nodes: make([]EvalNode, 0, 5)}
 
@@ -212,8 +210,33 @@ func (n *graphNodeOrphanResource) EvalTree() EvalNode {
 	info := &InstanceInfo{Id: n.ResourceKey.String(), Type: n.ResourceKey.Type}
 	seq.Nodes = append(seq.Nodes, &EvalInstanceInfo{Info: info})
 
+	// Each resource mode has its own lifecycle
+	switch n.ResourceKey.Mode {
+	case config.ManagedResourceMode:
+		seq.Nodes = append(
+			seq.Nodes,
+			n.managedResourceEvalNodes(info)...,
+		)
+	case config.DataResourceMode:
+		seq.Nodes = append(
+			seq.Nodes,
+			n.dataResourceEvalNodes(info)...,
+		)
+	default:
+		panic(fmt.Errorf("unsupported resource mode %s", n.ResourceKey.Mode))
+	}
+
+	return seq
+}
+
+func (n *graphNodeOrphanResource) managedResourceEvalNodes(info *InstanceInfo) []EvalNode {
+	var provider ResourceProvider
+	var state *InstanceState
+
+	nodes := make([]EvalNode, 0, 3)
+
 	// Refresh the resource
-	seq.Nodes = append(seq.Nodes, &EvalOpFilter{
+	nodes = append(nodes, &EvalOpFilter{
 		Ops: []walkOperation{walkRefresh},
 		Node: &EvalSequence{
 			Nodes: []EvalNode{
@@ -244,7 +267,7 @@ func (n *graphNodeOrphanResource) EvalTree() EvalNode {
 
 	// Diff the resource
 	var diff *InstanceDiff
-	seq.Nodes = append(seq.Nodes, &EvalOpFilter{
+	nodes = append(nodes, &EvalOpFilter{
 		Ops: []walkOperation{walkPlan, walkPlanDestroy},
 		Node: &EvalSequence{
 			Nodes: []EvalNode{
@@ -267,7 +290,7 @@ func (n *graphNodeOrphanResource) EvalTree() EvalNode {
 
 	// Apply
 	var err error
-	seq.Nodes = append(seq.Nodes, &EvalOpFilter{
+	nodes = append(nodes, &EvalOpFilter{
 		Ops: []walkOperation{walkApply, walkDestroy},
 		Node: &EvalSequence{
 			Nodes: []EvalNode{
@@ -308,7 +331,35 @@ func (n *graphNodeOrphanResource) EvalTree() EvalNode {
 		},
 	})
 
-	return seq
+	return nodes
+}
+
+func (n *graphNodeOrphanResource) dataResourceEvalNodes(info *InstanceInfo) []EvalNode {
+	nodes := make([]EvalNode, 0, 3)
+
+	// This will remain nil, since we don't retain states for orphaned
+	// data resources.
+	var state *InstanceState
+
+	// On both refresh and apply we just drop our state altogether,
+	// since the config resource validation pass will have proven that the
+	// resources remaining in the configuration don't need it.
+	nodes = append(nodes, &EvalOpFilter{
+		Ops: []walkOperation{walkRefresh, walkApply},
+		Node: &EvalSequence{
+			Nodes: []EvalNode{
+				&EvalWriteState{
+					Name:         n.ResourceKey.String(),
+					ResourceType: n.ResourceKey.Type,
+					Provider:     n.Provider,
+					Dependencies: n.DependentOn(),
+					State:        &state, // state is nil
+				},
+			},
+		},
+	})
+
+	return nodes
 }
 
 func (n *graphNodeOrphanResource) dependableName() string {