terraform: destroy edge must include resources through outputs

This fixes: `TestContext2Apply_moduleDestroyOrder`

The new destroy graph wasn't properly creating edges that happened
_through_ an output, it was only created the edges for _direct_
dependents.

To fix this, the DestroyEdgeTransformer now creates the full transitive
list of destroy edges by walking all ancestors. This will create more
edges than are necessary but also will no longer miss resources through
an output.

terraform/test-fixtures/transform-destroy-edge-module/child/main.tf

@@ -0,0 +1,5 @@
+resource "aws_instance" "b" {
+    value = "foo"
+}
+
+output "output" { value = "${aws_instance.b.value}" }

terraform/test-fixtures/transform-destroy-edge-module/main.tf

@@ -0,0 +1,7 @@
+resource "aws_instance" "a" {
+    value = "${module.child.output}"
+}
+
+module "child" {
+    source = "./child"
+}

terraform/transform_destroy_edge.go

@@ -116,8 +116,10 @@ func (t *DestroyEdgeTransformer) Transform(g *Graph) error {
 	// in the graph. BUT if resource A is just pure destroy, then only
 	// destroy A is in the graph, and create A is not.
 	steps := []GraphTransformer{
+		&OutputTransformer{Module: t.Module},
 		&AttachResourceConfigTransformer{Module: t.Module},
 		&AttachStateTransformer{State: t.State},
+		&ReferenceTransformer{},
 	}
 
 	// Go through all the nodes being destroyed and create a graph.
@@ -127,6 +129,7 @@ func (t *DestroyEdgeTransformer) Transform(g *Graph) error {
 	//   A, B (with no edges)
 	//
 	var tempG Graph
+	var tempDestroyed []dag.Vertex
 	for d, _ := range destroyers {
 		// d is what is being destroyed. We parse the resource address
 		// which it came from it is a panic if this fails.
@@ -140,6 +143,7 @@ func (t *DestroyEdgeTransformer) Transform(g *Graph) error {
 		// attach config and state transformers then ask for references.
 		node := &NodeAbstractResource{Addr: addr}
 		tempG.Add(node)
+		tempDestroyed = append(tempDestroyed, node)
 	}
 
 	// Run the graph transforms so we have the information we need to
@@ -150,14 +154,22 @@ func (t *DestroyEdgeTransformer) Transform(g *Graph) error {
 		}
 	}
 
-	// Create a reference map for easy lookup
+	log.Printf("[TRACE] DestroyEdgeTransformer: reference graph: %s", tempG.String())
-	refMap := NewReferenceMap(tempG.Vertices())
 
 	// Go through all the nodes in the graph and determine what they
 	// depend on.
-	for _, v := range tempG.Vertices() {
+	for _, v := range tempDestroyed {
-		// Find all the references
+		// Find all descendents of this to determine the edges we'll depend on
-		refs, _ := refMap.References(v)
+		vs, err := tempG.Ancestors(v)
+		if err != nil {
+			return err
+		}
+
+		refs := make([]dag.Vertex, 0, vs.Len())
+		for _, raw := range vs.List() {
+			refs = append(refs, raw.(dag.Vertex))
+		}
+
 		log.Printf(
 			"[TRACE] DestroyEdgeTransformer: creation node %q references %v",
 			dag.VertexName(v), refs)

terraform/transform_destroy_edge_test.go

@@ -78,6 +78,24 @@ func TestDestroyEdgeTransformer_selfRef(t *testing.T) {
 	}
 }
 
+func TestDestroyEdgeTransformer_module(t *testing.T) {
+	g := Graph{Path: RootModulePath}
+	g.Add(&graphNodeDestroyerTest{AddrString: "module.child.aws_instance.b"})
+	g.Add(&graphNodeDestroyerTest{AddrString: "aws_instance.a"})
+	tf := &DestroyEdgeTransformer{
+		Module: testModule(t, "transform-destroy-edge-module"),
+	}
+	if err := tf.Transform(&g); err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	actual := strings.TrimSpace(g.String())
+	expected := strings.TrimSpace(testTransformDestroyEdgeModuleStr)
+	if actual != expected {
+		t.Fatalf("bad:\n\n%s", actual)
+	}
+}
+
 type graphNodeCreatorTest struct {
 	AddrString string
 }
@@ -125,6 +143,7 @@ test.B (destroy)
 const testTransformDestroyEdgeMultiStr = `
 test.A (destroy)
   test.B (destroy)
+  test.C (destroy)
 test.B (destroy)
   test.C (destroy)
 test.C (destroy)
@@ -133,3 +152,9 @@ test.C (destroy)
 const testTransformDestroyEdgeSelfRefStr = `
 test.A (destroy)
 `
+
+const testTransformDestroyEdgeModuleStr = `
+aws_instance.a (destroy)
+module.child.aws_instance.b (destroy)
+  aws_instance.a (destroy)
+`