Feature: Add :auto_port and :ignore_startup_errors configs (#950)

* Add :auto_port and :ignore_startup_errors configs

* Refactor

* CR suggestions

Author: hhubert6

docs/config.md

@@ -81,6 +81,38 @@ config :live_debugger,
   external_url: "http://your_external_url"
 ```
 
+## Port Conflict Handling
+
+LiveDebugger provides two options for dealing with port conflicts (e.g. when running multiple application instances).
+
+### Auto-select next available port
+
+Set `auto_port: true` to make LiveDebugger automatically find the next free port if the configured port is already in use:
+
+```elixir
+# config/dev.exs
+
+config :live_debugger,
+  port: 4007,
+  auto_port: true
+```
+
+LiveDebugger will try up to 3 consecutive ports starting from the configured one, logging a warning for each port that is skipped.
+
+Note: `auto_port` is ignored when using a Unix socket (`ip: {:local, path}`).
+
+### Ignore startup errors
+
+Set `ignore_startup_errors: true` to allow the host application to continue running even if LiveDebugger fails to start (e.g. due to a port conflict). LiveDebugger will be unavailable, but your application will not crash:
+
+```elixir
+# config/dev.exs
+
+config :live_debugger, :ignore_startup_errors, true
+```
+
+An error will be logged when startup fails. Both options can be combined: `auto_port` is attempted first, and if the endpoint still fails to start, `ignore_startup_errors` prevents the crash.
+
 ## Other Settings
 
 ```elixir

lib/live_debugger.ex

@@ -27,9 +27,13 @@ defmodule LiveDebugger do
   end
 
   def update_live_debugger_tags() do
-    @app_name
-    |> Application.get_all_env()
-    |> put_live_debugger_tags()
+    config = Application.get_all_env(@app_name)
+    endpoint_config = Keyword.get(config, LiveDebugger.App.Web.Endpoint, [])
+
+    resolved_port =
+      get_in(endpoint_config, [:http, :port]) || Keyword.get(config, :port, @default_port)
+
+    put_live_debugger_tags(config, resolved_port)
   end
 
   defp get_children() do
@@ -41,8 +45,9 @@ defmodule LiveDebugger do
       LiveDebugger.API.StatesStorage.init()
 
       config = Application.get_all_env(@app_name)
-      put_endpoint_config(config)
-      put_live_debugger_tags(config)
+      resolved_port = resolve_port(config)
+      put_endpoint_config(config, resolved_port)
+      put_live_debugger_tags(config, resolved_port)
 
       []
       |> LiveDebugger.App.append_app_children()
@@ -58,12 +63,22 @@ defmodule LiveDebugger do
     end
   end
 
-  defp put_endpoint_config(config) do
+  defp resolve_port(config) do
+    ip = Keyword.get(config, :ip, @default_ip)
+    port = Keyword.get(config, :port, @default_port)
+    auto_port? = Keyword.get(config, :auto_port, false)
+
+    LiveDebugger.PortResolver.resolve(ip, port, auto_port?)
+  end
+
+  defp put_endpoint_config(config, resolved_port) do
+    ip = Keyword.get(config, :ip, @default_ip)
+
     endpoint_config =
       [
         http: [
-          ip: Keyword.get(config, :ip, @default_ip),
-          port: Keyword.get(config, :port, @default_port)
+          ip: ip,
+          port: resolved_port
         ],
         secret_key_base: Keyword.get(config, :secret_key_base, @default_secret_key_base),
         live_view: [signing_salt: Keyword.get(config, :signing_salt, @default_signing_salt)],
@@ -84,13 +99,11 @@ defmodule LiveDebugger do
     Application.put_env(@app_name, LiveDebugger.App.Web.Endpoint, endpoint_config)
   end
 
-  defp put_live_debugger_tags(config) do
-    port = Keyword.get(config, :port, @default_port)
-
+  defp put_live_debugger_tags(config, resolved_port) do
     default_url =
       case Keyword.get(config, :ip, @default_ip) do
         {:local, _path} -> nil
-        ip_tuple -> "http://#{ip_tuple |> :inet.ntoa() |> List.to_string()}:#{port}"
+        ip_tuple -> "http://#{ip_tuple |> :inet.ntoa() |> List.to_string()}:#{resolved_port}"
       end
 
     live_debugger_url = Keyword.get(config, :external_url, default_url)

lib/live_debugger/app.ex

@@ -13,7 +13,7 @@ defmodule LiveDebugger.App do
     children ++
       [
         pubsub,
-        {LiveDebugger.App.Web.Endpoint,
+        {LiveDebugger.App.Web.EndpointStarter,
          [
            check_origin: false,
            pubsub_server: @pubsub_name

lib/live_debugger/app/web/endpoint_starter.ex

@@ -0,0 +1,37 @@
+defmodule LiveDebugger.App.Web.EndpointStarter do
+  @moduledoc """
+  Wrapper around `LiveDebugger.App.Web.Endpoint` that handles startup failures gracefully.
+
+  When `config :live_debugger, ignore_startup_errors: true` is set, a port conflict or
+  other startup error will log an error and allow the host application to continue running
+  without LiveDebugger, instead of crashing the whole application.
+  """
+
+  require Logger
+
+  def child_spec(opts) do
+    %{LiveDebugger.App.Web.Endpoint.child_spec(opts) | start: {__MODULE__, :start_link, [opts]}}
+  end
+
+  def start_link(opts) do
+    case LiveDebugger.App.Web.Endpoint.start_link(opts) do
+      {:ok, pid} ->
+        {:ok, pid}
+
+      {:error, reason} ->
+        if Application.get_env(:live_debugger, :ignore_startup_errors, false) do
+          Logger.error(
+            "LiveDebugger failed to start: #{inspect(reason)}. " <>
+              "LiveDebugger will be unavailable. " <>
+              "To disable LiveDebugger entirely, set `config :live_debugger, disabled?: true`."
+          )
+
+          Application.put_env(:live_debugger, :live_debugger_tags, [])
+
+          :ignore
+        else
+          {:error, reason}
+        end
+    end
+  end
+end

lib/live_debugger/port_resolver.ex

@@ -0,0 +1,52 @@
+defmodule LiveDebugger.PortResolver do
+  @moduledoc """
+  Resolves the port for the LiveDebugger endpoint.
+
+  When `auto_port: true` is configured, scans upward from the configured port
+  to find an available one (up to `@max_attempts` tries). Skipped for non-TCP
+  IP configurations (e.g. Unix sockets).
+  """
+
+  require Logger
+
+  @max_attempts 3
+
+  @spec resolve(ip :: term(), port :: term(), auto_port? :: boolean()) :: term()
+  def resolve(ip, port, auto_port?) do
+    if auto_port? and tcp_ip?(ip) and is_integer(port) and port > 0 do
+      find_available_port(ip, port, @max_attempts)
+    else
+      port
+    end
+  end
+
+  defp tcp_ip?({_, _, _, _}), do: true
+  defp tcp_ip?({_, _, _, _, _, _, _, _}), do: true
+  defp tcp_ip?(_), do: false
+
+  defp find_available_port(_ip, port, 0) do
+    Logger.warning(
+      "LiveDebugger: could not find an available port after #{@max_attempts} attempts, " <>
+        "using port #{port}"
+    )
+
+    port
+  end
+
+  defp find_available_port(ip, port, attempts_left) do
+    inet_family = if tuple_size(ip) == 4, do: :inet, else: :inet6
+
+    case :gen_tcp.listen(port, [inet_family, {:ip, ip}]) do
+      {:ok, socket} ->
+        :gen_tcp.close(socket)
+        port
+
+      {:error, :eaddrinuse} ->
+        Logger.warning("LiveDebugger: port #{port} is already in use, trying #{port + 1}")
+        find_available_port(ip, port + 1, attempts_left - 1)
+
+      {:error, _} ->
+        port
+    end
+  end
+end

test/live_debugger/port_resolver_test.exs

@@ -0,0 +1,50 @@
+defmodule LiveDebugger.PortResolverTest do
+  use ExUnit.Case, async: false
+
+  alias LiveDebugger.PortResolver
+
+  @ip {127, 0, 0, 1}
+
+  describe "resolve/3" do
+    test "returns configured port when auto_port is false" do
+      assert PortResolver.resolve(@ip, 4007, false) == 4007
+    end
+
+    test "returns same port when auto_port is true and port is free" do
+      assert PortResolver.resolve(@ip, 39_871, true) == 39_871
+    end
+
+    test "finds next available port when configured port is occupied" do
+      {:ok, socket} = :gen_tcp.listen(39_872, [:inet, {:ip, @ip}, {:reuseaddr, true}])
+
+      resolved = PortResolver.resolve(@ip, 39_872, true)
+
+      :gen_tcp.close(socket)
+
+      assert resolved == 39_873
+    end
+
+    test "stops after max attempts and returns the next port" do
+      sockets =
+        for port <- 39_874..39_876 do
+          {:ok, socket} = :gen_tcp.listen(port, [:inet, {:ip, @ip}, {:reuseaddr, true}])
+          socket
+        end
+
+      resolved = PortResolver.resolve(@ip, 39_874, true)
+
+      Enum.each(sockets, &:gen_tcp.close/1)
+
+      # After 3 failed attempts (39874, 39875, 39876), returns 39877
+      assert resolved == 39_877
+    end
+
+    test "skips auto_port for Unix socket IP" do
+      assert PortResolver.resolve({:local, "/tmp/test.sock"}, 4007, true) == 4007
+    end
+
+    test "skips auto_port when port is not a positive integer" do
+      assert PortResolver.resolve(@ip, 0, true) == 0
+    end
+  end
+end

test/live_debugger_test.exs

@@ -1,6 +1,4 @@
 defmodule LiveDebuggerTest do
-  @moduledoc false
-
   use ExUnit.Case, async: false
 
   import Mox
@@ -41,6 +39,24 @@ defmodule LiveDebuggerTest do
       assert tags == []
     end
 
+    test "uses resolved port from endpoint config" do
+      Application.put_env(:live_debugger, :ip, {127, 0, 0, 1})
+      Application.put_env(:live_debugger, :port, 4007)
+      Application.put_env(:live_debugger, LiveDebugger.App.Web.Endpoint, http: [port: 4009])
+
+      LiveDebugger.MockAPISettingsStorage
+      |> expect(:get, fn :debug_button -> true end)
+
+      LiveDebugger.update_live_debugger_tags()
+
+      tags = Application.get_env(:live_debugger, :live_debugger_tags)
+      rendered = tags |> Phoenix.HTML.Safe.to_iodata() |> IO.iodata_to_binary()
+      assert rendered =~ "http://127.0.0.1:4009"
+      refute rendered =~ "http://127.0.0.1:4007"
+
+      Application.delete_env(:live_debugger, LiveDebugger.App.Web.Endpoint)
+    end
+
     test "generates tags when Unix socket IP is used with external_url" do
       Application.put_env(:live_debugger, :ip, {:local, "/tmp/live_debugger.sock"})
       Application.put_env(:live_debugger, :port, 0)