Poison.EncodeError when incoming post request fails

Phoenix Forum Questions / Help
1

I’ve been seeing this error in the logs recently but I am not sure what it is being caused by:

[error] #PID<0.2222.0> running Service.Web.Endpoint terminated
Server: service.com:443 (https)
Request: POST /webhook
** (exit) an exception was raised:
  ** (Poison.EncodeError) unable to encode value: {:safe, ["" | "<p>500 — internal server error</p>\n"]}
    (poison) lib/poison/encoder.ex:354: Poison.Encoder.Any.encode/2
    (poison) lib/poison.ex:41: Poison.encode!/2
    (phoenix) lib/phoenix/controller.ex:642: Phoenix.Controller.do_render/4
    (phoenix) lib/phoenix/endpoint/render_errors.ex:58: Phoenix.Endpoint.RenderErrors.__catch__/5
    (plug) lib/plug/adapters/cowboy/handler.ex:15: Plug.Adapters.Cowboy.Handler.upgrade/4
    (cowboy) src/cowboy_protocol.erl:442: :cowboy_protocol.execute/4

I suppose I should somehow add “500.json” template or render clause to the ErrorView, but I am not sure. My current view looks like this

defmodule Service.Web.ErrorView do
  use Service.Web, :view

  def render("404.html", _assigns) do
    render "not_found.html", %{}
  end

  def render("500.html", _assigns) do
    render "internal.html", %{}
  end

  # In case no render clause matches or no
  # template is found, let's render it as 500
  def template_not_found(_template, assigns) do
    render "500.html", assigns
  end
end

Is this a symptom of a bigger problem in my code since it wants to render 500 page for some reason? Would a longer stack-trace help? What is the usual way of dealing with problems like this (when the cause of the problem is not readily apparent) when they appear on a “production” machine?

2

Since poison is involved as well as a :safe-tuple, it seems as if you render some *.html but then you try to encode that as JSON.

And poison has no clue how to do that, since there is no replacement for a tuple in JSON as well as you can’t convert improper lists to JSON.

Therefore I think, there is happening something weird in your corresponding action.

edit

I have just seen, that you are rendering “500.html” as the default fallback. Please try to filter on the requested format like this:

def template_not_found(template, assigns) do
  render "500#{Path.extname template}", assigns
end

Also make sure you do have a corresponding template for all fileexetensions that are somehow generated dynamically and thus could produce a 500y error.

2 Likes
3

Thanks. That and adding two additional render clauses for 404.json and 500.json solved the issue with Poison. And the problem that caused 500.html being rendered became apparent in the new stack-trace.