Pleroma/lib/pleroma/gun/connection_pool.ex

defmodule Pleroma.Gun.ConnectionPool do
  @registry __MODULE__

  alias Pleroma.Gun.ConnectionPool.WorkerSupervisor

  def children do
    [
      {Registry, keys: :unique, name: @registry},
      Pleroma.Gun.ConnectionPool.WorkerSupervisor
    ]
  end

  def get_conn(uri, opts) do
    key = "#{uri.scheme}:#{uri.host}:#{uri.port}"

    case Registry.lookup(@registry, key) do
      # The key has already been registered, but connection is not up yet
      [{worker_pid, nil}] ->
        get_gun_pid_from_worker(worker_pid)

      [{worker_pid, {gun_pid, _used_by, _crf, _last_reference}}] ->
        GenServer.cast(worker_pid, {:add_client, self(), false})
        {:ok, gun_pid}

      [] ->
        # :gun.set_owner fails in :connected state for whatevever reason,
        # so we open the connection in the process directly and send it's pid back
        # We trust gun to handle timeouts by itself
        case WorkerSupervisor.start_worker([key, uri, opts, self()]) do
          {:ok, _worker_pid} ->
            receive do
              {:conn_pid, pid} -> {:ok, pid}
            end

          {:error, {:already_started, worker_pid}} ->
            get_gun_pid_from_worker(worker_pid)

          err ->
            err
        end
    end
  end

  defp get_gun_pid_from_worker(worker_pid) do
    # GenServer.call will block the process for timeout length if
    # the server crashes on startup (which will happen if gun fails to connect)
    # so instead we use cast + monitor

    ref = Process.monitor(worker_pid)
    GenServer.cast(worker_pid, {:add_client, self(), true})

    receive do
      {:conn_pid, pid} -> {:ok, pid}
      {:DOWN, ^ref, :process, ^worker_pid, reason} -> reason
    end
  end

  def release_conn(conn_pid) do
    # :ets.fun2ms(fn {_, {worker_pid, {gun_pid, _, _, _}}} when gun_pid == conn_pid ->
    #    worker_pid end)
    query_result =
      Registry.select(@registry, [
        {{:_, :"$1", {:"$2", :_, :_, :_}}, [{:==, :"$2", conn_pid}], [:"$1"]}
      ])

    case query_result do
      [worker_pid] ->
        GenServer.cast(worker_pid, {:remove_client, self()})

      [] ->
        :ok
    end
  end
end