Phoenix.LiveDashboard.TelemetryListener Enum.EmptyError

[Tim Gent]

Environment

Make sure you are using the latest LiveView and Dashboard versions before continuing.

• Elixir version (elixir -v): 1.13.1-otp-24
• Phoenix version (mix deps):
• Phoenix LiveView version (mix deps): 0.17.5
• Phoenix Dashboard version (mix deps): 0.6.2
• Operating system: Alpine
• Browsers you attempted to reproduce this bug on (the more the merrier): n/a

Actual behavior

We've got some errors from our application with this stacktrace. Unfortunately it's hard to track down exactly what led to this:

Handler {Phoenix.LiveDashboard.TelemetryListener, [:phoenix, :endpoint, :stop],
 #PID<0.6696.0>} has failed and has been detached. Class=:error
Reason=%Enum.EmptyError{message: "empty error"}
Stacktrace=[
  {Enum, :reduce, 2,
   [file: 'lib/enum.ex', line: 2342, error_info: %{module: Exception}]},
  {Phoenix.LiveDashboard.TelemetryListener, :tags_to_label, 2,
   [file: 'lib/phoenix/live_dashboard/telemetry_listener.ex', line: 66]},
  {Phoenix.LiveDashboard.TelemetryListener, :extract_datapoint_f

Looking at this code in live_dashboard it appears that we would get this error if:

• We receive non-empty tags
• The tag_values map doesn't contain any of the items in tags
• So the first reduce returns an empty list
• The second reduce would then fail

  defp tags_to_label(%{tags: []}, _metadata), do: nil

  defp tags_to_label(%{tags: tags, tag_values: tag_values}, metadata) do
    tag_values = tag_values.(metadata)

    tags
    |> Enum.reduce([], fn tag, acc ->
      case tag_values do
        %{^tag => value} -> [to_string(value) | acc]
        %{} -> acc
      end
    end)
    |> Enum.reduce(&[&1, " " | &2])
    |> IO.iodata_to_binary()
  end

I've not dug into what this code is doing and how it's consumed. It could simply be a case of adding a default accumulator (empty list) to the second reduce function. Thoughts appreciated!

Expected behavior

These errors to not be thrown

[Tim Gent]

Thanks for the insanely fast fix!