save null module outputs in state

internal/terraform/context_apply2_test.go

@@ -1832,3 +1832,58 @@ output "a" {
 	_, diags = ctx.Apply(plan, m)
 	assertNoErrors(t, diags)
 }
+
+func TestContext2Apply_destroyNullModuleOutput(t *testing.T) {
+	p := testProvider("test")
+	ctx := testContext2(t, &ContextOpts{
+		Providers: map[addrs.Provider]providers.Factory{
+			addrs.NewDefaultProvider("test"): testProviderFuncFixed(p),
+		},
+	})
+
+	m := testModuleInline(t, map[string]string{
+		"main.tf": `
+module "null_module" {
+  source = "./mod"
+}
+
+locals {
+  module_output = module.null_module.null_module_test
+}
+
+output "test_root" {
+  value = module.null_module.test_output
+}
+
+output "root_module" {
+  value = local.module_output #fails
+}
+`,
+
+		"mod/main.tf": `
+output "test_output" {
+  value = "test"
+}
+
+output "null_module_test" {
+  value = null
+}
+`,
+	})
+
+	// verify plan and apply
+	plan, diags := ctx.Plan(m, states.NewState(), &PlanOpts{
+		Mode: plans.NormalMode,
+	})
+	assertNoErrors(t, diags)
+	state, diags := ctx.Apply(plan, m)
+	assertNoErrors(t, diags)
+
+	// now destroy
+	plan, diags = ctx.Plan(m, state, &PlanOpts{
+		Mode: plans.DestroyMode,
+	})
+	assertNoErrors(t, diags)
+	_, diags = ctx.Apply(plan, m)
+	assertNoErrors(t, diags)
+}

internal/terraform/node_output.go

@@ -592,30 +592,31 @@ func (n *NodeApplyableOutput) setValue(state *states.SyncState, changes *plans.C
 		changes.RemoveOutputChange(n.Addr)
 	}
 
-	if val.IsKnown() && !val.IsNull() {
-		// The state itself doesn't represent unknown values, so we null them
-		// out here and then we'll save the real unknown value in the planned
-		// changeset below, if we have one on this graph walk.
-		log.Printf("[TRACE] setValue: Saving value for %s in state", n.Addr)
-
-		sensitive := n.Config.Sensitive
-		unmarkedVal, valueMarks := val.UnmarkDeep()
-
-		// If the evaluate value contains sensitive marks, the output has no
-		// choice but to declare itself as "sensitive".
-		for mark := range valueMarks {
-			if mark == marks.Sensitive {
-				sensitive = true
-				break
-			}
-		}
-
-		stateVal := cty.UnknownAsNull(unmarkedVal)
-		state.SetOutputValue(n.Addr, stateVal, sensitive)
-
-	} else {
+	// Null outputs must be saved for modules so that they can still be
+	// evaluated. Null root outputs are removed entirely, which is always fine
+	// because they can't be referenced by anything else in the configuration.
+	if n.Addr.Module.IsRoot() && val.IsNull() {
 		log.Printf("[TRACE] setValue: Removing %s from state (it is now null)", n.Addr)
 		state.RemoveOutputValue(n.Addr)
+		return
+	}
+
+	// The state itself doesn't represent unknown values, so we null them
+	// out here and then we'll save the real unknown value in the planned
+	// changeset, if we have one on this graph walk.
+	log.Printf("[TRACE] setValue: Saving value for %s in state", n.Addr)
+	sensitive := n.Config.Sensitive
+	unmarkedVal, valueMarks := val.UnmarkDeep()
+
+	// If the evaluated value contains sensitive marks, the output has no
+	// choice but to declare itself as "sensitive".
+	for mark := range valueMarks {
+		if mark == marks.Sensitive {
+			sensitive = true
+			break
+		}
 	}
 
+	stateVal := cty.UnknownAsNull(unmarkedVal)
+	state.SetOutputValue(n.Addr, stateVal, sensitive)
 }