File.mkdir_p: Fix TOCTOU issue & handle errors from parent mkdir

lib/elixir/lib/file.ex

@@ -310,24 +310,23 @@ defmodule File do
   end
 
   defp do_mkdir_p(path) do
-    if dir?(path) do
+    if path == "/" or path == "." do
       :ok
     else
       parent = Path.dirname(path)
 
-      if parent == path do
-        # Protect against infinite loop
-        {:error, :einval}
-      else
-        _ = do_mkdir_p(parent)
-
+      with {_, false} <- {:loop, parent == path},
+           {_, :ok} <- {:parent, do_mkdir_p(parent)} do
         case :file.make_dir(path) do
-          {:error, :eexist} = error ->
-            if dir?(path), do: :ok, else: error
+          {:error, :eexist} ->
+            if dir?(path), do: :ok, else: {:error, :enotdir}
 
           other ->
             other
         end
+      else
+        {:loop, _} -> {:error, :einval}
+        {:parent, e} -> e
       end
     end
   end