Errors when compiling ejabberd in Phoenix project

Environment

• ejabberd version: 20.3.0
• Erlang version: Erlang (SMP,ASYNC_THREADS,HIPE) (BEAM) emulator version 10.7.1
• OS: Linux (Fedora 32)
• Installed from: Hex
• Phoenix: 1.5.1

Bug description

I am embedding ejabberd in an Elixir Phoenix project. Running mix do deps.get, compile throws an error towards the end of compiling:

==> phoenix_live_reload
Compiling 4 files (.ex)
Generated phoenix_live_reload app
==> phoenix_ecto
Compiling 7 files (.ex)
Generated phoenix_ecto app

19:07:52.085 [info]  Application phoenix exited: exited in: Phoenix.start(:normal, [])
    ** (EXIT) exited in: :gen_server.call(:telemetry_handler_table, {:insert, {Phoenix.Logger, [:phoenix, :channel_handled_in]}, [[:phoenix, :channel_handled_in]], #Function<8.100890372/4 in Phoenix.Logger.install/0>, :ok})
        ** (EXIT) no process: the process is not alive or there's no process currently associated with the given name, possibly because its application isn't started

19:07:52.086 [info]  Application eex exited: :stopped
** (MatchError) no match of right hand side value: {:error, {:phoenix, {:bad_return, {{Phoenix, :start, [:normal, []]}, {:EXIT, {:noproc, {:gen_server, :call, [:telemetry_handler_table, {:insert, {Phoenix.Logger, [:phoenix, :channel_handled_in]}, [[:phoenix, :channel_handled_in]], #Function<8.100890372/4 in Phoenix.Logger.install/0>, :ok}]}}}}}}}
    (phoenix) lib/mix/tasks/compile.phoenix.ex:11: Mix.Tasks.Compile.Phoenix.run/1
    (mix) lib/mix/task.ex:331: Mix.Task.run_task/3
    (mix) lib/mix/tasks/compile.all.ex:73: Mix.Tasks.Compile.All.run_compiler/2
    (mix) lib/mix/tasks/compile.all.ex:53: Mix.Tasks.Compile.All.do_compile/4
    (mix) lib/mix/tasks/compile.all.ex:24: anonymous fn/1 in Mix.Tasks.Compile.All.run/1
    (mix) lib/mix/tasks/compile.all.ex:40: Mix.Tasks.Compile.All.with_logger_app/1
    (mix) lib/mix/task.ex:331: Mix.Task.run_task/3
    (mix) lib/mix/tasks/compile.ex:96: Mix.Tasks.Compile.run/1
    (mix) lib/mix/task.ex:331: Mix.Task.run_task/3
    (elixir) lib/enum.ex:783: Enum."-each/2-lists^foreach/1-0-"/2
    (elixir) lib/enum.ex:783: Enum.each/2
    (mix) lib/mix/task.ex:331: Mix.Task.run_task/3
    (mix) lib/mix/cli.ex:79: Mix.CLI.run_task/2
    (elixir) lib/code.ex:813: Code.require_file/2

After this, commands like mix ecto.setup or mix phx.start throw the same error.

There is a workaround:

• clean out the _build and deps folders
• comment out the ejabberd dependency in mix.exs
• run mix do deps.get, compile
• uncomment the ejabberd dependency
• run mix do deps.get, compile again

Then everything works as expected! So for some reason, compiling the ejabberd at the same time as the rest of the project breaks it. I have no idea how to debug this further, but perhaps someone here can point me in the right direction?

(This same project was raised back in September 2019 on the Elixir forums, but no-one opened a Github issue).

Start by reproducing mix do deps.get, compile with the ejabberd dependency enabled, then compare it with the clean-build workaround described in the issue. Inspect mix.exs and the reported failure path in lib/mix/tasks/compile.phoenix.ex; done means the dependencies compile together and mix ecto.setup or mix phx.start no longer raises the telemetry process error.