Make a distinction between touching and updating the lock manifest

Previously, deps.compile would update the parent project
lock manifest, which updated its Elixir version and made
it impossible to detect later on that we were upgrading
from a previous Elixir version.

This commit adds the ability to both touch and update
the manifest file and change deps.compile to only touch
it.

Author: José Valim

lib/mix/lib/mix/compilers/elixir.ex

@@ -1,7 +1,7 @@
 defmodule Mix.Compilers.Elixir do
   @moduledoc false
 
-  @manifest_vsn 1
+  @manifest_vsn :v1
 
   @doc """
   Compiles stale Elixir files.
@@ -199,7 +199,7 @@ defmodule Mix.Compilers.Elixir do
 
   defp read_manifest(manifest) do
     case :file.consult(manifest) do
-      {:ok, [{:version, @manifest_vsn}|t]} -> t
+      {:ok, [@manifest_vsn|t]} -> t
       _ -> []
     end
   end
@@ -223,7 +223,7 @@ defmodule Mix.Compilers.Elixir do
     File.mkdir_p!(Path.dirname(manifest))
 
     File.open!(manifest, [:write], fn device ->
-      :io.format(device, '~p.~n', [{:version, @manifest_vsn}])
+      :io.format(device, '~p.~n', [@manifest_vsn])
 
       Enum.map entries, fn {beam, _, _, _, _, _, binary} = entry ->
         if binary, do: File.write!(beam, binary)
@@ -236,6 +236,6 @@ defmodule Mix.Compilers.Elixir do
     # The Mix.Dep.Lock keeps all the project dependencies. Since Elixir
     # is a dependency itself, we need to touch the lock so the current
     # Elixir version, used to compile the files above, is properly stored.
-    Mix.Dep.Lock.touch
+    Mix.Dep.Lock.update_manifest
   end
 end

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

@@ -8,17 +8,20 @@ defmodule Mix.Dep.Lock do
 
   @doc """
   Returns the manifest file for dependencies.
+
+  The manifest is used to check if the lockfile
+  itself is up to date.
   """
   @spec manifest(Path.t) :: Path.t
   def manifest(manifest_path \\ Mix.Project.manifest_path) do
     Path.join(manifest_path, @manifest)
   end
 
   @doc """
-  Touches the manifest storing the current project info unless it is an umbrella application.
+  Updates the lock manifest with the latest metadata.
   """
-  @spec touch :: :ok
-  def touch do
+  @spec update_manifest :: :ok
+  def update_manifest do
     config = Mix.Project.config
     unless Mix.Project.umbrella?(config) do
       manifest_path = Mix.Project.manifest_path(config)
@@ -29,6 +32,19 @@ defmodule Mix.Dep.Lock do
     :ok
   end
 
+  @doc """
+  Touches the manifest file to force recompilation.
+  """
+  @spec touch_manifest :: :ok
+  def touch_manifest do
+    config = Mix.Project.config
+    unless Mix.Project.umbrella?(config) do
+      manifest = manifest()
+      File.exists?(manifest) && File.touch(manifest)
+    end
+    :ok
+  end
+
   @doc """
   Returns the manifest status with Elixir version and scm.
   """
@@ -71,7 +87,7 @@ defmodule Mix.Dep.Lock do
           ~s("#{app}": #{inspect rev, limit: :infinity})
         end
       File.write! lockfile, "%{" <> Enum.join(lines, ",\n  ") <> "}\n"
-      touch
+      update_manifest()
     end
     :ok
   end

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

@@ -72,7 +72,7 @@ defmodule Mix.Tasks.Deps.Compile do
         compiled
       end)
 
-    if Enum.any?(compiled), do: Mix.Dep.Lock.touch, else: :ok
+    if Enum.any?(compiled), do: Mix.Dep.Lock.touch_manifest, else: :ok
   end
 
   defp touch_fetchable(scm, path) do

lib/mix/lib/mix/tasks/loadpaths.ex

@@ -59,7 +59,10 @@ defmodule Mix.Tasks.Loadpaths do
     vsn = System.version
     scm = config[:build_scm]
 
-    # Force recompile if we have lock mismatch
+    # Erase the app build if we have lock mismatch.
+    # We do this to force full recompilation when
+    # any of SCM or Elixir version changes. Applies
+    # to dependencies and the main project alike.
     case Mix.Dep.Lock.status() do
       {:ok, old_vsn, _} when old_vsn != vsn -> rm_rf_app(config)
       {:ok, _, old_scm} when old_scm != scm -> rm_rf_app(config)

lib/mix/test/mix/dep/lock_test.exs

@@ -29,7 +29,9 @@ defmodule Mix.Dep.LockTest do
   test "stores status in manifest" do
     in_fixture "no_mixfile", fn ->
       assert Mix.Dep.Lock.status == :error
-      Mix.Dep.Lock.touch
+      Mix.Dep.Lock.touch_manifest
+      assert Mix.Dep.Lock.status == :error
+      Mix.Dep.Lock.update_manifest
       assert Mix.Dep.Lock.status == {:ok, System.version, nil}
     end
   end