feat(state): add runtime state snapshot

internal/state/state_test.go

@@ -0,0 +1,65 @@
+// SPDX-License-Identifier: MIT
+
+package state
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestRoundTrip(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "run", "prx", "state.json")
+	s := &State{
+		Enabled:      true,
+		WANInterface: "wlan0",
+		Gateway4:     "192.168.1.1",
+		Mark:         0x100,
+		TableV4:      100,
+		TableV6:      101,
+		PriorityV4:   100,
+		PriorityV6:   100,
+	}
+	if err := Save(path, s); err != nil {
+		t.Fatalf("Save: %v", err)
+	}
+	got, err := Load(path)
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if !got.Enabled || got.WANInterface != "wlan0" || got.Mark != 0x100 {
+		t.Fatalf("round trip mismatch: %+v", got)
+	}
+	if err := Remove(path); err != nil {
+		t.Fatalf("Remove: %v", err)
+	}
+	empty, err := Load(path)
+	if err != nil {
+		t.Fatalf("Load after remove: %v", err)
+	}
+	if empty.Enabled {
+		t.Fatal("expected disabled state after removal")
+	}
+	if err := Remove(path); err != nil {
+		t.Fatalf("second Remove should be a no-op: %v", err)
+	}
+}
+
+// TestLoadCorruptNeverReturnsNil guards against the nil-pointer panic that a
+// caller ignoring the error would otherwise hit on a corrupt state file.
+func TestLoadCorruptNeverReturnsNil(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "state.json")
+	if err := os.WriteFile(path, []byte("{ not json"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	st, err := Load(path)
+	if err == nil {
+		t.Fatal("expected a parse error for a corrupt state file")
+	}
+	if st == nil {
+		t.Fatal("Load must never return a nil state alongside an error")
+	}
+	if st.Enabled {
+		t.Fatal("expected a zero-value state")
+	}
+}