Ensure changes in deps propagate to all umbrella children

This fix a long standing issue where users would have to
delete _build after updating a dependency to recompile.
This is done by a currently private and recursive task
called will_recompile.

Author: José Valim

lib/mix/lib/mix/dep/lock.ex

@@ -5,27 +5,6 @@
 defmodule Mix.Dep.Lock do
   @moduledoc false
 
-  @manifest "compile.lock"
-
-  @doc """
-  Returns the manifest file for dependencies.
-
-  The manifest is used to check if the lockfile
-  itself is up to date.
-  """
-  def manifest(path \\ Mix.Project.manifest_path()) do
-    Path.join(path, @manifest)
-  end
-
-  @doc """
-  Touches the manifest file to force recompilation.
-  """
-  def touch_manifest do
-    path = Mix.Project.manifest_path()
-    File.mkdir_p!(path)
-    File.touch!(manifest(path))
-  end
-
   @doc """
   Reads the lockfile, returns a map containing
   each app name and its current lock information.
@@ -57,7 +36,7 @@ defmodule Mix.Dep.Lock do
         end
 
       File.write!(lockfile(), ["%{\n", lines, "}\n"])
-      touch_manifest()
+      Mix.Task.run("will_recompile")
     end
 
     :ok

lib/mix/lib/mix/project.ex

@@ -207,7 +207,7 @@ defmodule Mix.Project do
   """
   @spec config_files() :: [Path.t()]
   def config_files do
-    [Mix.Dep.Lock.manifest() | Mix.ProjectStack.config_files()]
+    [Mix.Tasks.WillRecompile.manifest() | Mix.ProjectStack.config_files()]
   end
 
   @doc """
@@ -220,7 +220,7 @@ defmodule Mix.Project do
   @doc since: "1.7.0"
   @spec config_mtime() :: posix_mtime when posix_mtime: integer()
   def config_mtime do
-    Mix.Dep.Lock.manifest()
+    Mix.Tasks.WillRecompile.manifest()
     |> Mix.Utils.last_modified()
     |> max(Mix.ProjectStack.config_mtime())
   end

lib/mix/lib/mix/tasks/deps.compile.ex

@@ -101,7 +101,7 @@ defmodule Mix.Tasks.Deps.Compile do
         compiled? and fetchable?
       end)
 
-    if true in compiled, do: Mix.Dep.Lock.touch_manifest(), else: :ok
+    if true in compiled, do: Mix.Task.run("will_recompile"), else: :ok
   end
 
   defp maybe_clean(dep, opts) do

lib/mix/lib/mix/tasks/will_recompile.ex

@@ -0,0 +1,23 @@
+defmodule Mix.Tasks.WillRecompile do
+  use Mix.Task
+
+  @moduledoc false
+  @recursive true
+  @manifest "compile.lock"
+
+  @doc """
+  Returns the will_recompile manifest for the project.
+  """
+  def manifest(path \\ Mix.Project.manifest_path()) do
+    Path.join(path, @manifest)
+  end
+
+  @doc """
+  Annotates the current project will recompile.
+  """
+  def run(_) do
+    path = Mix.Project.manifest_path()
+    File.mkdir_p!(path)
+    File.touch!(manifest(path))
+  end
+end

lib/mix/test/mix/tasks/will_recompile_test.exs

@@ -0,0 +1,27 @@
+Code.require_file("../../test_helper.exs", __DIR__)
+
+defmodule Mix.Tasks.WillRecompileTest do
+  use MixTest.Case
+
+  test "marks current project to recompile" do
+    in_fixture("deps_status/deps/ok", fn ->
+      Mix.Project.in_project(:ok, ".", fn _ ->
+        refute File.exists?("_build/dev/lib/ok/.mix/compile.lock")
+        Mix.Task.run("will_recompile")
+        assert File.exists?("_build/dev/lib/ok/.mix/compile.lock")
+      end)
+    end)
+  end
+
+  test "marks all projects in umbrella to recompile" do
+    in_fixture("umbrella_dep/deps/umbrella", fn ->
+      Mix.Project.in_project(:umbrella, ".", fn _ ->
+        refute File.exists?("_build/dev/lib/foo/.mix/compile.lock")
+        refute File.exists?("_build/dev/lib/bar/.mix/compile.lock")
+        Mix.Task.run("will_recompile")
+        assert File.exists?("_build/dev/lib/foo/.mix/compile.lock")
+        assert File.exists?("_build/dev/lib/bar/.mix/compile.lock")
+      end)
+    end)
+  end
+end