core: fix deadlock w/ CBD + modules

fixes #1947

Root cause was a bad edge being made by the CBD transform going from the
flattened destroy node to the unflattened create node, which was no
longer in the graph. The destroy node therefore had a dependency that
could never be satisfied, which locked up the walk.

terraform/context_test.go

@@ -139,6 +139,56 @@ func TestContext2Plan_moduleCycle(t *testing.T) {
 	}
 }
 
+func TestContext2Plan_moduleDeadlock(t *testing.T) {
+	m := testModule(t, "plan-module-deadlock")
+	p := testProvider("aws")
+	p.DiffFn = testDiffFn
+	timeout := make(chan bool, 1)
+	done := make(chan bool, 1)
+	go func() {
+		time.Sleep(3 * time.Second)
+		timeout <- true
+	}()
+	go func() {
+		ctx := testContext2(t, &ContextOpts{
+			Module: m,
+			Providers: map[string]ResourceProviderFactory{
+				"aws": testProviderFuncFixed(p),
+			},
+		})
+
+		plan, err := ctx.Plan()
+		done <- true
+		if err != nil {
+			t.Fatalf("err: %s", err)
+		}
+
+		actual := strings.TrimSpace(plan.String())
+		expected := strings.TrimSpace(`
+DIFF:
+
+module.child:
+  CREATE: aws_instance.foo.0
+  CREATE: aws_instance.foo.1
+  CREATE: aws_instance.foo.2
+
+STATE:
+
+<no state>
+		`)
+		if actual != expected {
+			t.Fatalf("expected:\n%sgot:\n%s", expected, actual)
+		}
+	}()
+
+	select {
+	case <-timeout:
+		t.Fatalf("timed out! probably deadlock")
+	case <-done:
+		// ok
+	}
+}
+
 func TestContext2Plan_moduleInput(t *testing.T) {
 	m := testModule(t, "plan-module-input")
 	p := testProvider("aws")

terraform/graph_config_node_resource.go

@@ -311,6 +311,7 @@ func (n *GraphNodeConfigResourceFlat) DestroyNode(mode GraphNodeDestroyMode) Gra
 	return &graphNodeResourceDestroyFlat{
 		graphNodeResourceDestroy: node,
 		PathValue:                n.PathValue,
+		FlatCreateNode:           n,
 	}
 }
 
@@ -318,6 +319,9 @@ type graphNodeResourceDestroyFlat struct {
 	*graphNodeResourceDestroy
 
 	PathValue []string
+
+	// Needs to be able to properly yield back a flattened create node to prevent
+	FlatCreateNode *GraphNodeConfigResourceFlat
 }
 
 func (n *graphNodeResourceDestroyFlat) Name() string {
@@ -329,6 +333,10 @@ func (n *graphNodeResourceDestroyFlat) Path() []string {
 	return n.PathValue
 }
 
+func (n *graphNodeResourceDestroyFlat) CreateNode() dag.Vertex {
+	return n.FlatCreateNode
+}
+
 // graphNodeResourceDestroy represents the logical destruction of a
 // resource. This node doesn't mean it will be destroyed for sure, but
 // instead that if a destroy were to happen, it must happen at this point.

terraform/test-fixtures/plan-module-deadlock/child/main.tf

@@ -0,0 +1,4 @@
+resource "aws_instance" "foo" {
+  count = "${length("abc")}"
+  lifecycle { create_before_destroy = true }
+}

terraform/test-fixtures/plan-module-deadlock/main.tf

@@ -0,0 +1,3 @@
+module "child" {
+  source = "./child"
+}