don't evaluate destroy instances

Orphaned instances that are create_before_destroy will still be in the
state when their references are evaluated. We need to skip instances
that are planned to be destroyed altogether, as they can't be part of an
evaluation.

terraform/context_apply_test.go

@@ -11357,3 +11357,85 @@ output "myoutput" {
 		t.Fatal("expected empty state, got:", state)
 	}
 }
+
+func TestContext2Apply_scaleInCBD(t *testing.T) {
+	m := testModuleInline(t, map[string]string{
+		"main.tf": `
+resource "test_instance" "a" {
+  count = 1
+  lifecycle {
+	create_before_destroy = true
+  }
+}
+
+resource "test_instance" "b" {
+  foo = join(".", test_instance.a[*].id)
+}
+
+output "out" {
+  value = join(".", test_instance.a[*].id)
+}
+`})
+
+	state := states.NewState()
+	root := state.EnsureModule(addrs.RootModuleInstance)
+	root.SetResourceInstanceCurrent(
+		mustResourceInstanceAddr("test_instance.a[0]").Resource,
+		&states.ResourceInstanceObjectSrc{
+			Status:       states.ObjectReady,
+			AttrsJSON:    []byte(`{"id":"a0"}`),
+			Dependencies: []addrs.ConfigResource{mustResourceAddr("module.child.aws_instance.child")},
+		},
+		mustProviderConfig(`provider["registry.terraform.io/hashicorp/test"]`),
+	)
+	root.SetResourceInstanceCurrent(
+		mustResourceInstanceAddr("test_instance.a[1]").Resource,
+		&states.ResourceInstanceObjectSrc{
+			Status:       states.ObjectReady,
+			AttrsJSON:    []byte(`{"id":"a1"}`),
+			Dependencies: []addrs.ConfigResource{mustResourceAddr("module.child.aws_instance.child")},
+		},
+		mustProviderConfig(`provider["registry.terraform.io/hashicorp/test"]`),
+	)
+	root.SetResourceInstanceCurrent(
+		mustResourceInstanceAddr("test_instance.b").Resource,
+		&states.ResourceInstanceObjectSrc{
+			Status:       states.ObjectReady,
+			AttrsJSON:    []byte(`{"id":"b", "foo":"old.old"}`),
+			Dependencies: []addrs.ConfigResource{mustResourceAddr("test_instance.a")},
+		},
+		mustProviderConfig(`provider["registry.terraform.io/hashicorp/test"]`),
+	)
+
+	p := testProvider("test")
+	p.ApplyFn = func(info *InstanceInfo, s *InstanceState, d *InstanceDiff) (*InstanceState, error) {
+		return testApplyFn(info, s, d)
+	}
+
+	p.DiffFn = testDiffFn
+	ctx := testContext2(t, &ContextOpts{
+		Config: m,
+		Providers: map[addrs.Provider]providers.Factory{
+			addrs.NewDefaultProvider("test"): testProviderFuncFixed(p),
+		},
+		State: state,
+	})
+
+	_, diags := ctx.Plan()
+	if diags.HasErrors() {
+		t.Fatal(diags.ErrWithWarnings())
+	}
+
+	// if resource b isn't going to apply correctly, we will get an error about
+	// an invalid plan value
+	state, diags = ctx.Apply()
+	if diags.HasErrors() {
+		t.Fatal(diags.ErrWithWarnings())
+	}
+
+	// check the output, as those can't cause an error planning the value
+	out := state.RootModule().OutputValues["out"].Value.AsString()
+	if out != "a0" {
+		t.Fatalf(`expected output "new", got: %q`, out)
+	}
+}

terraform/evaluate.go

@@ -655,10 +655,20 @@ func (d *evaluationStateData) GetResource(addr addrs.Resource, rng tfdiags.Sourc
 
 		instAddr := addr.Instance(key).Absolute(d.ModulePath)
 
+		change := d.Evaluator.Changes.GetResourceInstanceChange(instAddr, states.CurrentGen)
+		if change != nil {
+			// Don't take any resources that are yet to be deleted into account.
+			// If the referenced resource is CreateBeforeDestroy, then orphaned
+			// instances will will be in the state, as they are not destroyed
+			// until after their dependants are updated.
+			if change.Action == plans.Delete {
+				continue
+			}
+		}
+
 		// Planned resources are temporarily stored in state with empty values,
 		// and need to be replaced bu the planned value here.
 		if is.Current.Status == states.ObjectPlanned {
-			change := d.Evaluator.Changes.GetResourceInstanceChange(instAddr, states.CurrentGen)
 			if change == nil {
 				// If the object is in planned status then we should not get
 				// here, since we should have found a pending value in the plan