states/statefile: consistently sort resources across modules

states/statefile/version4.go

@@ -535,6 +535,8 @@ func (sr sortResourcesV4) Len() int      { return len(sr) }
 func (sr sortResourcesV4) Swap(i, j int) { sr[i], sr[j] = sr[j], sr[i] }
 func (sr sortResourcesV4) Less(i, j int) bool {
 	switch {
+	case sr[i].Module != sr[j].Module:
+		return sr[i].Module < sr[j].Module
 	case sr[i].Mode != sr[j].Mode:
 		return sr[i].Mode < sr[j].Mode
 	case sr[i].Type != sr[j].Type:

states/statefile/version4_test.go

@@ -0,0 +1,41 @@
+package statefile
+
+import (
+	"sort"
+	"testing"
+)
+
+// This test verifies that modules are sorted before resources:
+// https://github.com/hashicorp/terraform/issues/21552
+func TestVersion4_sort(t *testing.T) {
+	resources := sortResourcesV4{
+		{
+			Module: "module.child",
+			Type:   "test_instance",
+			Name:   "foo",
+		},
+		{
+			Type: "test_instance",
+			Name: "foo",
+		},
+		{
+			Module: "module.kinder",
+			Type:   "test_instance",
+			Name:   "foo",
+		},
+		{
+			Module: "module.child.grandchild",
+			Type:   "test_instance",
+			Name:   "foo",
+		},
+	}
+	sort.Stable(resources)
+
+	moduleOrder := []string{"", "module.child", "module.child.grandchild", "module.kinder"}
+
+	for i, resource := range resources {
+		if resource.Module != moduleOrder[i] {
+			t.Errorf("wrong sort order: expected %q, got %q\n", moduleOrder[i], resource.Module)
+		}
+	}
+}