Okay, here is what I ended up with:
Instead of adding MyApp.Endpoint directly to my supervision tree I am adding this LsppWeb.PhoenixPortSupervisor that uses DynamicSupervisor to start up my Phoenix Endpoint, incrementing the port if it fails to startup initially.
defmodule LsppWeb.PhoenixPortSupervisor do
@moduledoc """
Starts up Phoenix, but controls the port with application configuration,
increments the port until Phoenix starts up successfully.
"""
use GenServer
def start_link(_, name \\ __MODULE__) do
GenServer.start_link(__MODULE__, nil, name: name)
end
def init(_) do
initial_port = initial_port()
start_phoenix(initial_port, initial_port)
end
defp start_phoenix(initial_port, port) when port - initial_port < 15 do
set_port(port)
DynamicSupervisor.start_child(LsppWeb.DynamicSupervisor, LsppWebWeb.Endpoint)
|> interpret_results()
|> case do
{:ok, _} -> {:ok, []}
{:error, :eaddrinuse} -> start_phoenix(initial_port, port + 1)
end
end
defp start_phoenix(_, _) do
{:error, :ports_exhausted}
end
def get_port, do: Application.get_env(:lspp_web, :phoenix_port)
defp set_port(port) do
Application.put_env(:lspp_web, :phoenix_port, port)
end
defp interpret_results({:ok, pid}), do: {:ok, pid}
defp interpret_results({:error, error}) do
case error do
{:shutdown,
{:failed_to_start_child, {:ranch_listener_sup, LsppWebWeb.Endpoint.HTTP},
{:shutdown,
{:failed_to_start_child, :ranch_acceptors_sup,
{:listen_error, LsppWebWeb.Endpoint.HTTP, :eaddrinuse}}}}} ->
{:error, :eaddrinuse}
end
end
defp initial_port do
cond do
port = System.get_env("PORT") ->
String.to_integer(port)
config = Application.get_env(:lspp_web, LsppWebWeb.Endpoint) ->
get_in(config, [:http, :port])
true ->
raise "Port not set"
end
end
end
Then in LsppWeb.Endpoint I define an init/2 callback:
def init(supervisor, config) do
port = LsppWeb.PhoenixPortSupervisor.get_port()
config = put_in(config[:http], [:inet6, {:port, port}])
{:ok, config}
end
Any feedback or critique is welcome. The ugliest part of the code right now is how I shuttle the new port through the application environment. I suppose since LsppWeb.PhoenixPortSupervisor is a GenServer maybe that could be handled with a GenServer.call, but then I would need another GenServer/process since currently LsppWeb.PhoenixPortSupervisor doesn’t finish it’s startup until my LsppWeb.Endpoint has finished.
