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

Author: lanodan

lib/elixir/lib/file.ex

@@ -309,25 +309,28 @@ defmodule File do
     :ok
   end
 
-  defp do_mkdir_p(path) do
-    if dir?(path) do
-      :ok
-    else
-      parent = Path.dirname(path)
+  defp do_mkdir_p(".") do
+    :ok
+  end
 
-      if parent == path do
-        # Protect against infinite loop
-        {:error, :einval}
-      else
-        _ = do_mkdir_p(parent)
+  defp do_mkdir_p(path) do
+    parent = Path.dirname(path)
 
-        case :file.make_dir(path) do
-          {:error, :eexist} = error ->
-            if dir?(path), do: :ok, else: error
+    if parent == path do
+      {:error, :einval}
+    else
+      case do_mkdir_p(parent) do
+        :ok ->
+          case :file.make_dir(path) do
+            {:error, :eexist} ->
+              if dir?(path), do: :ok, else: {:error, :enotdir}
+
+            other ->
+              other
+          end
 
-          other ->
-            other
-        end
+        e ->
+          e
       end
     end
   end