Ensure priv/include are copied on Windows even if source did not change

Closes #2908

Signed-off-by: José Valim <jose.valim@plataformatec.com.br>

Author: José Valim

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

@@ -64,7 +64,10 @@ defmodule Mix.Tasks.Deps.Compile do
               "(pass :compile as an option to customize compilation, set it to false to do nothing)"
         end
 
-        unless mix?(dep), do: build_structure(dep, config)
+        # Always build the structure even for Mix projects
+        # because if nothing was compiled, we need to re-copy
+        # priv/include when symlinks are not available.
+        build_structure(dep, config)
         touch_fetchable(scm, opts[:build])
         compiled
       end)
@@ -170,15 +173,15 @@ defmodule Mix.Tasks.Deps.Compile do
     build_path = Path.dirname(opts[:build])
     Enum.each Mix.Dep.source_paths(dep), fn source ->
       app = Path.join(build_path, Path.basename(source))
-      build_structure(source, app, config)
+      build_structure(dep, source, app, config)
       Code.prepend_path(Path.join(app, "ebin"))
     end
   end
 
-  defp build_structure(dest, build, config) do
+  defp build_structure(dep, dest, build, config) do
     File.cd! dest, fn ->
       config = Keyword.put(config, :app_path, build)
-      Mix.Project.build_structure(config, symlink_ebin: true)
+      Mix.Project.build_structure(config, symlink_ebin: not mix?(dep))
     end
   end
 

lib/mix/lib/mix/utils.ex

@@ -300,33 +300,31 @@ defmodule Mix.Utils do
   """
   def symlink_or_copy(source, target) do
     if File.exists?(source) do
-      source_list = String.to_char_list(source)
+      # Relative symbolic links on windows are broken
+      link = case :os.type do
+        {:win32, _} -> source
+        _           -> make_relative_path(source, target)
+      end |> String.to_char_list
+
       case :file.read_link(target) do
-        {:ok, ^source_list} ->
+        {:ok, ^link} ->
           :ok
         {:ok, _} ->
           File.rm!(target)
-          do_symlink_or_copy(source, target)
+          do_symlink_or_copy(source, target, link)
         {:error, :enoent} ->
-          do_symlink_or_copy(source, target)
+          do_symlink_or_copy(source, target, link)
         {:error, _} ->
           _ = File.rm_rf!(target)
-          do_symlink_or_copy(source, target)
+          do_symlink_or_copy(source, target, link)
       end
     else
       {:error, :enoent}
     end
   end
 
-  defp do_symlink_or_copy(source, target) do
-
-    # relative symbolic links on windows are broken
-    source_path = case :os.type do
-      {:win32, _} -> source
-      _ -> make_relative_path(source, target)
-    end
-
-    case :file.make_symlink(source_path, target) do
+  defp do_symlink_or_copy(source, target, link) do
+    case :file.make_symlink(link, target) do
       :ok -> :ok
       {:error, _} -> {:ok, File.cp_r!(source, target)}
     end