Allow watchers to be anonymous functions

lib/phoenix/endpoint/supervisor.ex

@@ -203,19 +203,12 @@ defmodule Phoenix.Endpoint.Supervisor do
 
   defp watcher_children(_mod, conf, server?) do
     if server? do
-      Enum.map(conf[:watchers], fn {cmd, args} ->
-        {Phoenix.Endpoint.Watcher, watcher_args(cmd, args)}
-      end)
+      Enum.map(conf[:watchers], &{Phoenix.Endpoint.Watcher, &1})
     else
       []
     end
   end
 
-  defp watcher_args(cmd, cmd_args) do
-    {args, opts} = Enum.split_while(cmd_args, &is_binary(&1))
-    {cmd, args, opts}
-  end
-
   @doc """
   The endpoint configuration used at compile time.
   """

lib/phoenix/endpoint/watcher.ex

@@ -10,25 +10,44 @@ defmodule Phoenix.Endpoint.Watcher do
     }
   end
 
-  def start_link({cmd, args, opts}) do
-    Task.start_link(__MODULE__, :watch, [to_string(cmd), args, opts])
+  def start_link({cmd, args}) do
+    Task.start_link(__MODULE__, :watch, [to_string(cmd), args])
   end
 
-  def watch(cmd, args, opts) do
-    merged_opts = Keyword.merge(
-      [into: IO.stream(:stdio, :line), stderr_to_stdout: true], opts)
-    :ok = validate(cmd, args, merged_opts)
+  def watch(_cmd, args) when is_function(args, 0) do
+    try do
+      args.()
+    catch
+      kind, reason ->
+        # The function returned a non-zero exit code.
+        # Sleep for a couple seconds before exiting to
+        # ensure this doesn't hit the supervisor's
+        # max_restarts/max_seconds limit.
+        Process.sleep(2000)
+        :erlang.raise(kind, reason, __STACKTRACE__)
+    end
+  end
+
+  def watch(cmd, args) when is_list(args) do
+    {args, opts} = Enum.split_while(args, &is_binary(&1))
+    opts = Keyword.merge([into: IO.stream(:stdio, :line), stderr_to_stdout: true], opts)
+    :ok = validate(cmd, args, opts)
 
     try do
-      System.cmd(cmd, args, merged_opts)
+      System.cmd(cmd, args, opts)
     catch
       :error, :enoent ->
         relative = Path.relative_to_cwd(cmd)
-        Logger.error "Could not start watcher #{inspect relative} from #{inspect cd(merged_opts)}, executable does not exist"
+
+        Logger.error(
+          "Could not start watcher #{inspect(relative)} from #{inspect(cd(opts))}, executable does not exist"
+        )
+
         exit(:shutdown)
     else
       {_, 0} ->
         :ok
+
       {_, _} ->
         # System.cmd returned a non-zero exit code
         # sleep for a couple seconds before exiting to ensure this doesn't
@@ -40,23 +59,30 @@ defmodule Phoenix.Endpoint.Watcher do
 
   # We specially handle Node.js to make sure we
   # provide a good getting started experience.
-  defp validate("node", [script|_], merged_opts) do
+  defp validate("node", [script | _], merged_opts) do
     script_path = Path.expand(script, cd(merged_opts))
 
     cond do
       !System.find_executable("node") ->
-        Logger.error "Could not start watcher because \"node\" is not available. Your Phoenix " <>
-                     "application is still running, however assets won't be compiled. " <>
-                     "You may fix this by installing \"node\" and then running \"npm install\" inside the \"assets\" directory."
+        Logger.error(
+          "Could not start watcher because \"node\" is not available. Your Phoenix " <>
+            "application is still running, however assets won't be compiled. " <>
+            "You may fix this by installing \"node\" and then running \"npm install\" inside the \"assets\" directory."
+        )
+
         exit(:shutdown)
 
       not File.exists?(script_path) ->
-        Logger.error "Could not start Node.js watcher because script #{inspect script_path} does not " <>
-                     "exist. Your Phoenix application is still running, however assets " <>
-                     "won't be compiled. You may fix this by running \"npm install\" inside the \"assets\" directory."
+        Logger.error(
+          "Could not start Node.js watcher because script #{inspect(script_path)} does not " <>
+            "exist. Your Phoenix application is still running, however assets " <>
+            "won't be compiled. You may fix this by running \"npm install\" inside the \"assets\" directory."
+        )
+
         exit(:shutdown)
 
-      true -> :ok
+      true ->
+        :ok
     end
   end
 

test/phoenix/endpoint/watcher_test.exs

@@ -4,9 +4,17 @@ defmodule Phoenix.Endpoint.WatcherTest do
   alias Phoenix.Endpoint.Watcher
   import ExUnit.CaptureIO
 
-  test "starts watching and writes to stdio" do
+  test "starts watching and writes to stdio with args" do
     assert capture_io(fn ->
-      {:ok, pid} = Watcher.start_link({"echo", ["hello"], cd: File.cwd!()})
+      {:ok, pid} = Watcher.start_link({"echo", ["hello", cd: File.cwd!()]})
+      ref = Process.monitor(pid)
+      assert_receive {:DOWN, ^ref, :process, ^pid, :normal}, 1000
+    end) == "hello\n"
+  end
+
+  test "starts watching and writes to stdio with fun" do
+    assert capture_io(fn ->
+      {:ok, pid} = Watcher.start_link({"echo", fn -> IO.puts("hello") end})
       ref = Process.monitor(pid)
       assert_receive {:DOWN, ^ref, :process, ^pid, :normal}, 1000
     end) == "hello\n"