Add a failing unit test for testing the wal feature

msgbus_test.go

@@ -161,25 +161,39 @@ func TestMessageBusWAL(t *testing.T) {
 	mb, err := NewMessageBus(WithLogPath(testdir))
 	require.NoError(err)
 
-	msgs := mb.Subscribe("id1", "foo")
-
-	topic := mb.NewTopic("foo")
-	expected := mb.NewMessage(topic, []byte("foo"))
-	mb.Put(expected)
-
-	actual := <-msgs
-	assert.Equal(expected, actual)
-	assert.Equal(0, actual.ID)
-
-	mb.Unsubscribe("id1", "foo")
+	msgs := mb.Subscribe("id1", "hello")
 
+	topic := mb.NewTopic("hello")
+	mb.Put(mb.NewMessage(topic, []byte("foo"))) // ID == 0
 	mb.Put(mb.NewMessage(topic, []byte("bar"))) // ID == 1
 	mb.Put(mb.NewMessage(topic, []byte("baz"))) // ID == 2
 
-	msgs = mb.Subscribe("id1", "foo", WithIndex(1))
-
+	assert.Equal([]byte("foo"), (<-msgs).Payload)
 	assert.Equal([]byte("bar"), (<-msgs).Payload)
 	assert.Equal([]byte("baz"), (<-msgs).Payload)
+
+	assert.Equal(3, topic.Sequence)
+
+	mb.Unsubscribe("id1", "foo")
+
+	// Now ensure when we start back up we've re-filled the queues and retain the same
+	// message ids and topic sequence number
+
+	mb, err = NewMessageBus(WithLogPath(testdir))
+	require.NoError(err)
+
+	// we have to tell the bus we want to subscribe from the start
+	msgs = mb.Subscribe("id1", "hello", WithIndex(0))
+
+	topic = mb.NewTopic("hello")
+	assert.Equal(3, topic.Sequence)
+
+	assert.Equal([]byte("foo"), (<-msgs).Payload)
+	assert.Equal([]byte("bar"), (<-msgs).Payload)
+	assert.Equal([]byte("baz"), (<-msgs).Payload)
+
+	msg := mb.NewMessage(topic, []byte("foobar"))
+	assert.Equal(4, msg.ID)
 }
 
 func TestServeHTTPGETIndexEmpty(t *testing.T) {