How to debug a long error stack?

al2o3cr · March 7, 2022, 4:02pm

Start from the top - this is the root cause of the error - :erlang.iolist_size/1 is raising :badarg. The most common cause of this is passing something in that’s not the right shape.

The next two frames for :cow_ws.payload_length and :cow_ws.frame aren’t particularly interesting.

The frame for :cow_ws.websocket_send gives us another piece of information: we’re trying to send whatever the badly-shaped data is down a websocket

But the really important one is the frame after that: :cow_ws.handler_call. This is the “brains” of the websocket handler:

github.com/ninenines/cowboy

src/cowboy_websocket.erl

2a0825049


      
          handler_call(State=#state{handler=Handler}, HandlerState,
          		ParseState, Callback, Message, NextState) ->
          	try case Callback of
          		websocket_init -> Handler:websocket_init(HandlerState);
          		_ -> Handler:Callback(Message, HandlerState)
          	end of
          		{Commands, HandlerState2} when is_list(Commands) ->
          			handler_call_result(State,
          				HandlerState2, ParseState, NextState, Commands);
          		{Commands, HandlerState2, hibernate} when is_list(Commands) ->
          			handler_call_result(State#state{hibernate=true},
          				HandlerState2, ParseState, NextState, Commands);
          		%% The following call results are deprecated.
          		{ok, HandlerState2} ->
          			NextState(State, HandlerState2, ParseState);
          		{ok, HandlerState2, hibernate} ->
          			NextState(State#state{hibernate=true}, HandlerState2, ParseState);
          		{reply, Payload, HandlerState2} ->
          			case websocket_send(Payload, State) of

This is why the stack doesn’t have any application code in it - the application code is called in the first part of handler_call, and then the result of that is what’s being handled when the crash happens.

Check your code that’s sending data via websockets; has a stray {:ok, "data that should be sent"} snuck in someplace where you’re expecting just "data that should be sent"?