states/statefile: consistently sort resources across modules (#25498)

If a statefile had resources with the same name in different modules,
the sort order could be inconsistent between state refreshes. This adds
the module to the Less() function used in sorting and a minimal test to
verify consistent ordering.

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)
+		}
+	}
+}