test for proper state persistence

The backend state tests weren't properly checking for persistence.
Update the test to persist states and fetch them again from the backend,
checking that lineage is preserved.

backend/testing.go

@@ -65,57 +65,109 @@ func testBackendStates(t *testing.T, b Backend) {
 	}
 
 	// Create a couple states
-	fooState, err := b.State("foo")
+	foo, err := b.State("foo")
 	if err != nil {
 		t.Fatalf("error: %s", err)
 	}
-	if err := fooState.RefreshState(); err != nil {
+	if err := foo.RefreshState(); err != nil {
 		t.Fatalf("bad: %s", err)
 	}
-	if v := fooState.State(); v.HasResources() {
+	if v := foo.State(); v.HasResources() {
 		t.Fatalf("should be empty: %s", v)
 	}
 
-	barState, err := b.State("bar")
+	bar, err := b.State("bar")
 	if err != nil {
 		t.Fatalf("error: %s", err)
 	}
-	if err := barState.RefreshState(); err != nil {
+	if err := bar.RefreshState(); err != nil {
 		t.Fatalf("bad: %s", err)
 	}
-	if v := barState.State(); v.HasResources() {
+	if v := bar.State(); v.HasResources() {
 		t.Fatalf("should be empty: %s", v)
 	}
 
-	// Verify they are distinct states
+	// Verify they are distinct states that can be read back from storage
 	{
-		s := barState.State()
-		if s == nil {
-			s = terraform.NewState()
+		// start with a fresh state, and record the lineage being
+		// written to "bar"
+		barState := terraform.NewState()
+		barLineage := barState.Lineage
+
+		// the foo lineage should be distinct from bar, and unchanged after
+		// modifying bar
+		fooState := terraform.NewState()
+		fooLineage := fooState.Lineage
+
+		// write a known state to foo
+		if err := foo.WriteState(fooState); err != nil {
+			t.Fatal("error writing foo state:", err)
+		}
+		if err := foo.PersistState(); err != nil {
+			t.Fatal("error persisting foo state:", err)
 		}
 
-		s.Lineage = "bar"
-		if err := barState.WriteState(s); err != nil {
+		// write a distinct known state to bar
+		if err := bar.WriteState(barState); err != nil {
 			t.Fatalf("bad: %s", err)
 		}
-		if err := barState.PersistState(); err != nil {
+		if err := bar.PersistState(); err != nil {
 			t.Fatalf("bad: %s", err)
 		}
 
-		if err := fooState.RefreshState(); err != nil {
-			t.Fatalf("bad: %s", err)
+		// verify that foo is unchanged with the existing state manager
+		if err := foo.RefreshState(); err != nil {
+			t.Fatal("error refreshing foo:", err)
 		}
-		if v := fooState.State(); v != nil && v.Lineage == "bar" {
-			t.Fatalf("bad: %#v", v)
+		fooState = foo.State()
+		switch {
+		case fooState == nil:
+			t.Fatal("nil state read from foo")
+		case fooState.Lineage == barLineage:
+			t.Fatalf("bar lineage read from foo: %#v", fooState)
+		case fooState.Lineage != fooLineage:
+			t.Fatal("foo lineage alterred")
+		}
+
+		// fetch foo again from the backend
+		foo, err = b.State("foo")
+		if err != nil {
+			t.Fatal("error re-fetching state:", err)
+		}
+		if err := foo.RefreshState(); err != nil {
+			t.Fatal("error refreshing foo:", err)
+		}
+		fooState = foo.State()
+		switch {
+		case fooState == nil:
+			t.Fatal("nil state read from foo")
+		case fooState.Lineage != fooLineage:
+			t.Fatal("incorrect state returned from backend")
+		}
+
+		// fetch the bar  again from the backend
+		bar, err = b.State("bar")
+		if err != nil {
+			t.Fatal("error re-fetching state:", err)
+		}
+		if err := bar.RefreshState(); err != nil {
+			t.Fatal("error refreshing bar:", err)
+		}
+		barState = bar.State()
+		switch {
+		case barState == nil:
+			t.Fatal("nil state read from bar")
+		case barState.Lineage != barLineage:
+			t.Fatal("incorrect state returned from backend")
 		}
 	}
 
 	// Verify we can now list them
 	{
+		// we determined that named stated are supported earlier
 		states, err := b.States()
-		if err == ErrNamedStatesNotSupported {
-			t.Logf("TestBackend: named states not supported in %T, skipping", b)
-			return
+		if err != nil {
+			t.Fatal(err)
 		}
 
 		sort.Strings(states)