Fix nested module "unknown variable" during dest (#7496)

* Fix nested module "unknown variable" during dstry

During a destroy with nested modules, accessing a variable between them
causes an "unknown variable accessed" during destroy.

terraform/context_apply_test.go

@@ -4704,3 +4704,83 @@ aws_instance.foo:
 		t.Fatalf("bad: \n%s", actual)
 	}
 }
+
+// https://github.com/hashicorp/terraform/issues/7378
+func TestContext2Apply_destroyNestedModuleWithAttrsReferencingResource(t *testing.T) {
+	m := testModule(t, "apply-destroy-nested-module-with-attrs")
+	p := testProvider("null")
+	p.ApplyFn = testApplyFn
+	p.DiffFn = testDiffFn
+
+	var state *State
+	var err error
+	{
+		ctx := testContext2(t, &ContextOpts{
+			Module: m,
+			Providers: map[string]ResourceProviderFactory{
+				"null": testProviderFuncFixed(p),
+			},
+		})
+
+		// First plan and apply a create operation
+		if _, err := ctx.Plan(); err != nil {
+			t.Fatalf("plan err: %s", err)
+		}
+
+		state, err = ctx.Apply()
+		if err != nil {
+			t.Fatalf("apply err: %s", err)
+		}
+	}
+
+	{
+		ctx := testContext2(t, &ContextOpts{
+			Destroy: true,
+			Module:  m,
+			State:   state,
+			Providers: map[string]ResourceProviderFactory{
+				"null": testProviderFuncFixed(p),
+			},
+		})
+
+		plan, err := ctx.Plan()
+		if err != nil {
+			t.Fatalf("destroy plan err: %s", err)
+		}
+
+		var buf bytes.Buffer
+		if err := WritePlan(plan, &buf); err != nil {
+			t.Fatalf("plan write err: %s", err)
+		}
+
+		planFromFile, err := ReadPlan(&buf)
+		if err != nil {
+			t.Fatalf("plan read err: %s", err)
+		}
+
+		ctx, err = planFromFile.Context(&ContextOpts{
+			Providers: map[string]ResourceProviderFactory{
+				"null": testProviderFuncFixed(p),
+			},
+		})
+		if err != nil {
+			t.Fatalf("err: %s", err)
+		}
+
+		state, err = ctx.Apply()
+		if err != nil {
+			t.Fatalf("destroy apply err: %s", err)
+		}
+	}
+
+	//Test that things were destroyed
+	actual := strings.TrimSpace(state.String())
+	expected := strings.TrimSpace(`
+<no state>
+module.middle:
+  <no state>
+		`)
+	if actual != expected {
+		t.Fatalf("expected: \n%s\n\nbad: \n%s", expected, actual)
+	}
+}

terraform/graph_config_node_variable.go

@@ -130,6 +130,7 @@ func (n *GraphNodeConfigVariable) hasDestroyEdgeInPath(opts *NoopOpts, vertex da
 	if vertex == nil {
 		vertex = opts.Vertex
 	}
+
 	log.Printf("[DEBUG] hasDestroyEdgeInPath: Looking for destroy edge: %s - %T", dag.VertexName(vertex), vertex)
 	for _, v := range opts.Graph.UpEdges(vertex).List() {
 		if len(opts.Graph.UpEdges(v).List()) > 1 {
@@ -137,10 +138,12 @@ func (n *GraphNodeConfigVariable) hasDestroyEdgeInPath(opts *NoopOpts, vertex da
 				return true
 			}
 		}
+
 		// Here we borrow the implementation of DestroyEdgeInclude, whose logic
-		// and semantics are exactly what we want here.
+		// and semantics are exactly what we want here. We add a check for the
+		// the root node, since we have to always depend on its existance.
 		if cv, ok := vertex.(*GraphNodeConfigVariableFlat); ok {
-			if cv.DestroyEdgeInclude(v) {
+			if dag.VertexName(v) == rootNodeName || cv.DestroyEdgeInclude(v) {
 				return true
 			}
 		}

terraform/test-fixtures/apply-destroy-nested-module-with-attrs/middle/bottom/bottom.tf

@@ -0,0 +1 @@
+variable "bottom_param" {}

terraform/test-fixtures/apply-destroy-nested-module-with-attrs/middle/middle.tf

@@ -0,0 +1,8 @@
+variable "param" {}
+
+resource "null_resource" "n" {}
+
+module "bottom" {
+  source       = "./bottom"
+  bottom_param = "${var.param}"
+}

terraform/test-fixtures/apply-destroy-nested-module-with-attrs/top.tf

@@ -0,0 +1,4 @@
+module "middle" {
+  source = "./middle"
+  param  = "foo"
+}