Add API for get current transaction ID

lib/tres/controller.ex

@@ -1,7 +1,10 @@
 defmodule Tres.Controller do
   def controller_helpers do
     quote do
-      import Tres.SwitchRegistry, only: [send_message: 2]
+      import Tres.SwitchRegistry, only: [
+        send_message: 2,
+        get_current_xid: 1
+      ]
 
       use Tres.Messages
       use Tres.MessageHelper

lib/tres/secure_channel.ex

@@ -251,6 +251,11 @@ defmodule Tres.SecureChannel do
     :keep_state_and_data
   end
 
+  defp handle_CONNECTED({:call, from}, :get_xid, state_data) do
+    xid = State.get_transaction_id(state_data.xid)
+    {:keep_state_and_data, [{:reply, from, {:ok, xid}}]}
+  end
+
   defp handle_CONNECTED(:cast, {:send_message, message} = action, state_data) do
     new_action_queue =
       if XACT_KV.is_empty(state_data.xact_kv_ref) do

lib/tres/switch_registry.ex

@@ -19,13 +19,22 @@ defmodule Tres.SwitchRegistry do
   end
 
   def send_message(message, {_dpid, _aux_id} = datapath_id) do
-    Registry.dispatch(__MODULE__, datapath_id, &dispatch(&1, message))
+    Registry.dispatch(__MODULE__, datapath_id, &do_send_message(&1, message))
   end
 
   def send_message(message, dpid) when is_binary(dpid) do
     send_message(message, {dpid, 0})
   end
 
+  def get_current_xid({_dpid, _aux_id} = datapath_id) do
+    [{pid, _}|_] = Registry.lookup(__MODULE__,  datapath_id)
+    :gen_statem.call(pid, :get_xid, 1000)
+  end
+
+  def get_current_xid(datapath_id) do
+    get_current_xid({datapath_id, 0})
+  end
+
   def monitor(datapath_id) do
     datapath_id
     |> lookup_pid
@@ -34,7 +43,7 @@ defmodule Tres.SwitchRegistry do
 
   # private function
 
-  defp dispatch(entries, message) do
+  defp do_send_message(entries, message) do
     for {pid, _} <- entries, do: :gen_statem.cast(pid, {:send_message, message})
   end
 end