Safely convert payloads to strings (#110)

For non-error tuples `Exception.normalize/3` returns the original
payload, which may be anything. Some payloads such as tuples don't
implement the `String.Chars` protocol. So calling `to_string/1` will
fail. In such cases we fall back to `inspect/1`.

I made a few updates to the `live.exs` script to reproduce this problem.
You can run `elixir live.exs` and click the button called «GenServer
timeout». This will cause an error that should be recorded by the error
tracker.

If you revert the changes that I made and try this same button you will
se that the error payload cannot be converted to string and detaches the
ErrorTracker telemetry handler.

Closes #109

lib/error_tracker.ex

@@ -280,14 +280,18 @@ defmodule ErrorTracker do
 
   defp normalize_exception({kind, ex}, stacktrace) do
     case Exception.normalize(kind, ex, stacktrace) do
-      %struct{} = ex ->
-        {to_string(struct), Exception.message(ex)}
-
-      other ->
-        {to_string(kind), to_string(other)}
+      %struct{} = ex -> {to_string(struct), Exception.message(ex)}
+      payload -> {to_string(kind), safe_to_string(payload)}
     end
   end
 
+  defp safe_to_string(term) do
+    to_string(term)
+  rescue
+    Protocol.UndefinedError ->
+      inspect(term)
+  end
+
   defp exception_breadcrumbs(exception) do
     case exception do
       {_kind, exception} -> exception_breadcrumbs(exception)

live.exs

@@ -1,7 +1,6 @@
 Mix.install([
-  {:phoenix_playground,
-   github: "phoenix-playground/phoenix_playground",
-   ref: "ee6da0fc3b141f78b9f967ce71a4fb015c6764a6"},
+  {:phoenix_playground, "~> 0.1.7"},
+  {:postgrex, "~> 0.19.3"},
   {:error_tracker, path: "."}
 ])
 
@@ -13,6 +12,7 @@ end
 Application.put_env(:error_tracker, :repo, ErrorTrackerDev.Repo)
 Application.put_env(:error_tracker, :application, :error_tracker_dev)
 Application.put_env(:error_tracker, :prefix, "private")
+Application.put_env(:error_tracker, :otp_app, :error_tracker_dev)
 
 Application.put_env(:error_tracker, ErrorTrackerDev.Repo,
   url: "ecto://postgres:[email protected]/error_tracker_dev"
@@ -26,6 +26,29 @@ defmodule Migration0 do
   def down, do: ErrorTracker.Migration.down(prefix: "private")
 end
 
+defmodule ErrorTrackerDev.TimeoutGenServer do
+  use GenServer
+
+  # Client
+
+  def start_link(_) do
+    GenServer.start_link(__MODULE__, %{})
+  end
+
+  # Server (callbacks)
+
+  @impl true
+  def init(initial_state) do
+    {:ok, initial_state}
+  end
+
+  @impl true
+  def handle_call(:timeout, _from, state) do
+    :timer.sleep(5000)
+    {:reply, state, state}
+  end
+end
+
 defmodule DemoLive do
   use Phoenix.LiveView
 
@@ -47,6 +70,7 @@ defmodule DemoLive do
     <button phx-click="inc">+</button>
     <button phx-click="dec">-</button>
     <button phx-click="error">Crash on handle_event</button>
+    <button phx-click="genserver-timeout">GenServer timeout</button>
 
     <.link href="/?crash=mount">Crash on mount</.link>
     <.link patch="/?crash=handle_params">Crash on handle_params</.link>
@@ -69,6 +93,11 @@ defmodule DemoLive do
     raise "Crash on handle_event"
   end
 
+  def handle_event("genserver-timeout", _params, socket) do
+    GenServer.call(TimeoutGenServer, :timeout, 2000)
+    {:noreply, socket}
+  end
+
   def handle_params(params, _uri, socket) do
     if params["crash"] == "handle_params" do
       raise "Crash on handle_params"
@@ -78,7 +107,42 @@ defmodule DemoLive do
   end
 end
 
-PhoenixPlayground.start(live: DemoLive, child_specs: [ErrorTrackerDev.Repo])
+defmodule DemoRouter do
+  use Phoenix.Router
+  use ErrorTracker.Web, :router
+
+  import Phoenix.LiveView.Router
+
+  pipeline :browser do
+    plug :put_root_layout, html: {PhoenixPlayground.Layout, :root}
+  end
+
+  scope "/" do
+    pipe_through :browser
+    live "/", DemoLive
+    error_tracker_dashboard "/errors"
+  end
+end
+
+defmodule DemoEndpoint do
+  use Phoenix.Endpoint, otp_app: :phoenix_playground
+  plug Plug.Logger
+  socket "/live", Phoenix.LiveView.Socket
+  plug Plug.Static, from: {:phoenix, "priv/static"}, at: "/assets/phoenix"
+  plug Plug.Static, from: {:phoenix_live_view, "priv/static"}, at: "/assets/phoenix_live_view"
+  socket "/phoenix/live_reload/socket", Phoenix.LiveReloader.Socket
+  plug Phoenix.LiveReloader
+  plug Phoenix.CodeReloader, reloader: &PhoenixPlayground.CodeReloader.reload/2
+  plug DemoRouter
+end
+
+PhoenixPlayground.start(
+  endpoint: DemoEndpoint,
+  child_specs: [
+    {ErrorTrackerDev.Repo, []},
+    {ErrorTrackerDev.TimeoutGenServer, [name: TimeoutGenServer]}
+  ]
+)
 
 # Create the database if it does not exist and run migrations if needed
 _ = Ecto.Adapters.Postgres.storage_up(ErrorTrackerDev.Repo.config())