Error when self-invoking send_frame/2

- The error suggested returning a `:reply` tuple instead.

- Closes Azolo#34

CHANGELOG.md

@@ -9,6 +9,7 @@
   a callback. (e.g. an exception was raised)
 - Properly handle unexpected SSL socket termination.
   - This _seems_ pretty important, but I don't know...
+- Return a descriptive error when trying to use `send_frame/2` in a callback.
 
 ## 0.4.0
 ### Breaking Changes

lib/websockex.ex

@@ -338,10 +338,15 @@ defmodule WebSockex do
   error tuple with a `WebSockex.ConnError` exception struct as the second
   element.
   """
-  @spec send_frame(client, frame) :: :ok | {:error, %WebSockex.FrameEncodeError{}
-                                                  | %WebSockex.ConnError{}
-                                                  | %WebSockex.NotConnectedError{}
-                                                  | %WebSockex.InvalidFrameError{}}
+  @spec send_frame(client, frame) :: :ok |
+    {:error, %WebSockex.FrameEncodeError{}
+      | %WebSockex.ConnError{}
+      | %WebSockex.NotConnectedError{}
+      | %WebSockex.InvalidFrameError{}} |
+    none
+  def send_frame(client, _) when client == self() do
+    raise %WebSockex.CallingSelfError{function: :send_frame}
+  end
   def send_frame(client, frame) do
     try do
       {:ok, res} = :gen.call(client, :"$websockex_send", frame)

lib/websockex/errors.ex

@@ -118,3 +118,18 @@ defmodule WebSockex.NotConnectedError do
     "Not Connected: Currently Opening the Connection."
   end
 end
+
+defmodule WebSockex.CallingSelfError do
+  defexception [:function]
+
+  def message(%__MODULE__{function: :send_frame}) do
+    """
+    Process attempted to call itself.
+
+    The function send_frame/2 cannot be used inside of a callback. Instead try returning {:reply, frame, state} from the callback.
+    """
+  end
+  def message(%__MODULE__{}) do
+    "Process attempted to call itself."
+  end
+end

test/websockex_test.exs

@@ -94,6 +94,9 @@ defmodule WebSockexTest do
     def handle_cast({:send, frame}, state), do: {:reply, frame, state}
     def handle_cast(:close, state), do: {:close, state}
     def handle_cast({:close, code, reason}, state), do: {:close, {code, reason}, state}
+    def handle_cast(:self_send, _) do
+      WebSockex.send_frame(self(), :ping)
+    end
     def handle_cast(:delayed_close, state) do
       receive do
         {:tcp_closed, socket} ->
@@ -582,6 +585,15 @@ defmodule WebSockexTest do
         {:error, %WebSockex.NotConnectedError{connection_state: :opening}}
     end
 
+    test "returns a descriptive error message for trying to send a frame from self", context do
+      pid = context.pid
+      Process.flag(:trap_exit, true)
+      WebSockex.cast(pid, :self_send)
+
+      assert_receive {:EXIT, ^pid, {%WebSockex.CallingSelfError{} = error, _}}
+      assert Exception.message(error) =~ "try returning {:reply, frame, state} from the callback"
+    end
+
     test "returns an error while closing", %{pid: pid} = context do
       Process.flag(:trap_exit, true)
       send(context.server_pid, :stall)