Do not touch lock manifest if there is no change

Author: José Valim

lib/mix/lib/mix/deps/lock.ex

@@ -58,11 +58,13 @@ defmodule Mix.Deps.Lock do
   def write(dict) do
     sorted = lc { app, rev } inlist Enum.sort(dict), rev != nil, do: { app, rev }
 
-    lines  = Enum.map_join sorted, ",\n  ", fn { app, rev } ->
-      %b("#{app}": #{inspect rev, raw: true, limit: :infinity})
-    end
+    unless sorted == read do
+      lines  = Enum.map_join sorted, ",\n  ", fn { app, rev } ->
+        %b("#{app}": #{inspect rev, raw: true, limit: :infinity})
+      end
 
-    File.write! lockfile, "[ " <> lines <> " ]\n"
-    touch
+      File.write! lockfile, "[ " <> lines <> " ]\n"
+      touch
+    end
   end
 end

lib/mix/test/mix/deps/lock_test.exs

@@ -0,0 +1,23 @@
+Code.require_file "../../test_helper.exs", __DIR__
+
+defmodule Mix.Deps.LockTest do
+  use MixTest.Case
+
+  test "creates new lock and manifest files" do
+    in_fixture "no_mixfile", fn ->
+      Mix.Deps.Lock.write [foo: :bar]
+      assert File.regular? "mix.lock"
+      assert File.regular? "ebin/.compile.lock"
+    end
+  end
+
+  test "does not touch manifest file there is no change" do
+    in_fixture "no_mixfile", fn ->
+      Mix.Deps.Lock.write [foo: :bar, bar: :bat]
+      File.rm! "ebin/.compile.lock"
+
+      Mix.Deps.Lock.write [bar: :bat, foo: :bar]
+      refute File.regular? "ebin/.compile.lock"
+    end
+  end
+end