Allow bypassing app router with FARSIDE_NO_ROUTER

Setting the aforementioned env var skips creation of the app router,
which is useful for running update.exs when the main app is already
running (otherwise there's a port conflict).

README.md

@@ -73,3 +73,9 @@ request per second per IP.
 - Run Farside: `mix run --no-halt`
   - Uses localhost:4001
 
+### Environment Variables
+
+| Name | Purpose |
+| -- | -- |
+| FARSIDE_TEST | If enabled, skips the instance availability check in `update.exs`. |
+| FARSIDE_NO_ROUTER | If enabled, skips creation of the router. Useful for running `update.exs` with `mix run` when the app is already running. | 

config/config.exs

@@ -1,6 +1,7 @@
 import Config
 
 config :farside,
+  port: 4001,
   redis_conn: "redis://localhost:6379",
   update_file: ".update-results",
   service_prefix: "service-",

lib/farside/application.ex

@@ -1,17 +1,28 @@
 defmodule Farside.Application do
+  @farside_port Application.fetch_env!(:farside, :port)
   @redis_conn Application.fetch_env!(:farside, :redis_conn)
   @moduledoc false
 
   use Application
 
   @impl true
   def start(_type, _args) do
-    children = [
-      Plug.Cowboy.child_spec(scheme: :http, plug: Farside.Router, options: [port: 4001]),
-      {Redix, {@redis_conn, [name: :redix]}},
+    plug_children = [
+      Plug.Cowboy.child_spec(
+        scheme: :http,
+        plug: Farside.Router,
+        options: [
+          port: @farside_port
+        ]
+      ),
       {PlugAttack.Storage.Ets, name: Farside.Throttle.Storage, clean_period: 60_000}
     ]
 
+    children = [
+      {Redix, {@redis_conn, [name: :redix]}} |
+      System.get_env("FARSIDE_NO_ROUTER") && [] || plug_children
+    ]
+
     opts = [strategy: :one_for_one, name: Farside.Supervisor]
     Supervisor.start_link(children, opts)
   end

update.sh

@@ -3,4 +3,4 @@
 SCRIPT_DIR="$(builtin cd "$(dirname "${BASH_SOURCE[0]}")" && pwd -P)"
 
 cd "$SCRIPT_DIR"
-mix run update.exs
+FARSIDE_NO_ROUTER=1 mix run update.exs