core: respect roots in CBD transform

terraform/context_test.go

@@ -40,6 +40,43 @@ func TestContext2Plan(t *testing.T) {
 	}
 }
 
+func TestContext2Plan_createBefore_maintainRoot(t *testing.T) {
+	m := testModule(t, "plan-cbd-maintain-root")
+	p := testProvider("aws")
+	p.DiffFn = testDiffFn
+	ctx := testContext2(t, &ContextOpts{
+		Module: m,
+		Providers: map[string]ResourceProviderFactory{
+			"aws": testProviderFuncFixed(p),
+		},
+		Variables: map[string]string{
+			"in": "a,b,c",
+		},
+	})
+
+	plan, err := ctx.Plan()
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	actual := strings.TrimSpace(plan.String())
+	expected := strings.TrimSpace(`
+DIFF:
+
+CREATE: aws_instance.bar.0
+CREATE: aws_instance.bar.1
+CREATE: aws_instance.foo.0
+CREATE: aws_instance.foo.1
+
+STATE:
+
+<no state>
+		`)
+	if actual != expected {
+		t.Fatalf("expected:\n%s, got:\n%s", expected, actual)
+	}
+}
+
 func TestContext2Plan_emptyDiff(t *testing.T) {
 	m := testModule(t, "plan-empty")
 	p := testProvider("aws")

terraform/test-fixtures/plan-cbd-maintain-root/main.tf

@@ -0,0 +1,13 @@
+resource "aws_instance" "foo" {
+  count = "2"
+  lifecycle { create_before_destroy = true }
+}
+
+resource "aws_instance" "bar" {
+  count = "2"
+  lifecycle { create_before_destroy = true }
+}
+
+output "out" {
+  value = "${aws_instance.foo.0.id}"
+}

terraform/transform_destroy.go

@@ -1,8 +1,6 @@
 package terraform
 
-import (
-	"github.com/hashicorp/terraform/dag"
-)
+import "github.com/hashicorp/terraform/dag"
 
 type GraphNodeDestroyMode byte
 
@@ -193,6 +191,14 @@ func (t *CreateBeforeDestroyTransformer) Transform(g *Graph) error {
 		// This ensures that.
 		for _, sourceRaw := range g.UpEdges(cn).List() {
 			source := sourceRaw.(dag.Vertex)
+
+			// If the graph has a "root" node (one added by a RootTransformer and not
+			// just a resource that happens to have no ancestors), we don't want to
+			// add any edges to it, because then it ceases to be a root.
+			if _, ok := source.(graphNodeRoot); ok {
+				continue
+			}
+
 			connect = append(connect, dag.BasicEdge(dn, source))
 		}