Add --preload-modules to mix app.start

It preloads all modules from all loaded applications after
all applications are started. This is useful to ensure code
loading does not affect code executed by further mix tasks,
such as mix test --slowest N.

Closes #6626.

lib/mix/lib/mix/tasks/app.start.ex

@@ -30,6 +30,7 @@ defmodule Mix.Tasks.App.Start do
     * `--force` - forces compilation regardless of compilation times
     * `--temporary` - starts the application as temporary
     * `--permanent` - starts the application as permanent
+    * `--preload-modules` - preloads all modules defined in applications
     * `--no-compile` - does not compile even if files require compilation
     * `--no-protocols` - does not load consolidated protocols
     * `--no-archives-check` - does not check archives
@@ -38,11 +39,18 @@ defmodule Mix.Tasks.App.Start do
     * `--no-start` - does not start applications after compilation
 
   """
+
+  @switches [
+    permanent: :boolean,
+    temporary: :boolean,
+    preload_modules: :boolean
+  ]
+
   def run(args) do
     Mix.Project.get!()
     config = Mix.Project.config()
 
-    {opts, _, _} = OptionParser.parse(args, switches: [permanent: :boolean, temporary: :boolean])
+    {opts, _, _} = OptionParser.parse(args, switches: @switches)
     Mix.Task.run("loadpaths", args)
 
     unless "--no-compile" in args do
@@ -76,6 +84,13 @@ defmodule Mix.Tasks.App.Start do
       end
     else
       start(Mix.Project.config(), opts)
+
+      # If there is a build path, we will let the application
+      # that owns the build path do the actual check
+      unless config[:build_path] do
+        loaded = loaded_applications(opts)
+        check_configured(loaded)
+      end
     end
 
     :ok
@@ -97,13 +112,6 @@ defmodule Mix.Tasks.App.Start do
 
     type = type(config, opts)
     Enum.each(apps, &ensure_all_started(&1, type))
-
-    # If there is a build path, we will let the application
-    # that owns the build path do the actual check
-    unless config[:build_path] do
-      check_configured()
-    end
-
     :ok
   end
 
@@ -146,25 +154,35 @@ defmodule Mix.Tasks.App.Start do
     end
   end
 
-  defp check_configured() do
+  defp loaded_applications(opts) do
+    preload_modules? = opts[:preload_modules]
+
+    for {app, _, _} <- Application.loaded_applications() do
+      if modules = preload_modules? && Application.spec(app, :modules) do
+        :code.ensure_modules_loaded(modules)
+      end
+
+      app
+    end
+  end
+
+  defp check_configured(loaded) do
     configured = Mix.ProjectStack.configured_applications()
-    loaded = for {app, _, _} <- Application.loaded_applications(), do: app
 
-    _ =
-      for app <- configured -- loaded, :code.lib_dir(app) == {:error, :bad_name} do
-        Mix.shell().error("""
-        You have configured application #{inspect(app)} in your configuration
-        file, but the application is not available.
+    for app <- configured -- loaded, :code.lib_dir(app) == {:error, :bad_name} do
+      Mix.shell().error("""
+      You have configured application #{inspect(app)} in your configuration file,
+      but the application is not available.
 
-        This usually means one of:
+      This usually means one of:
 
         1. You have not added the application as a dependency in a mix.exs file.
 
         2. You are configuring an application that does not really exist.
 
-        Please ensure #{inspect(app)} exists or remove the configuration.
-        """)
-      end
+      Please ensure #{inspect(app)} exists or remove the configuration.
+      """)
+    end
 
     :ok
   end

lib/mix/lib/mix/tasks/run.ex

@@ -40,6 +40,7 @@ defmodule Mix.Tasks.Run do
     * `--eval`, `-e` - evaluate the given code
     * `--require`, `-r` - requires pattern before running the command
     * `--parallel`, `-p` - makes all requires parallel
+    * `--preload-modules` - preloads all modules defined in applications
     * `--no-compile` - does not compile even if files require compilation
     * `--no-deps-check` - does not check dependencies
     * `--no-archives-check` - does not check archives
@@ -67,7 +68,8 @@ defmodule Mix.Tasks.Run do
           start: :boolean,
           archives_check: :boolean,
           elixir_version_check: :boolean,
-          parallel_require: :keep
+          parallel_require: :keep,
+          preload_modules: :boolean
         ]
       )
 

lib/mix/lib/mix/tasks/test.ex

@@ -67,11 +67,12 @@ defmodule Mix.Tasks.Test do
     * `--no-elixir-version-check` - does not check the Elixir version from mix.exs
     * `--no-start` - does not start applications after compilation
     * `--only` - runs only tests that match the filter
+    * `--preload-modules` - preloads all modules defined in applications
     * `--raise` - raises if the test suite failed
     * `--seed` - seeds the random number generator used to randomize tests order;
       `--seed 0` disables randomization
-    * `--slowest` - prints timing information for the N slowest tests; automatically
-      sets `--trace`
+    * `--slowest` - prints timing information for the N slowest tests
+      Automatically sets `--trace` and `--preload-modules`
     * `--stale` - runs only tests which reference modules that changed since the
       last `test --stale`. You can read more about this option in the "Stale" section below.
     * `--timeout` - sets the timeout for the tests
@@ -190,7 +191,8 @@ defmodule Mix.Tasks.Test do
     stale: :boolean,
     listen_on_stdin: :boolean,
     formatter: :keep,
-    slowest: :integer
+    slowest: :integer,
+    preload_modules: :boolean
   ]
 
   @cover [output: "cover", tool: Cover]
@@ -235,7 +237,8 @@ defmodule Mix.Tasks.Test do
     # available in test_helper.exs. Then configure exunit again so
     # that command line options override test_helper.exs
     Mix.shell().print_app
-    Mix.Task.run("app.start", args)
+    app_start_args = if opts[:slowest], do: ["--preload-modules" | args], else: args
+    Mix.Task.run("app.start", app_start_args)
 
     # Ensure ExUnit is loaded.
     case Application.load(:ex_unit) do

lib/mix/test/mix/tasks/app.start_test.exs

@@ -35,12 +35,18 @@ defmodule Mix.Tasks.App.StartTest do
       assert File.regular?("_build/dev/lib/app_start_sample/ebin/Elixir.A.beam")
       assert File.regular?("_build/dev/lib/app_start_sample/ebin/app_start_sample.app")
 
+      assert :code.is_loaded(A)
       refute List.keyfind(Application.started_applications(), :app_start_sample, 0)
       assert List.keyfind(Application.started_applications(), :logger, 0)
+      :code.delete(A)
 
       Mix.Tasks.App.Start.run([])
+      refute :code.is_loaded(A)
       assert List.keyfind(Application.started_applications(), :app_start_sample, 0)
       assert List.keyfind(Application.started_applications(), :logger, 0)
+
+      Mix.Tasks.App.Start.run(["--preload-modules"])
+      assert :code.is_loaded(A)
     end
   end