Ensure extracted tarball files are readable

Tarballs may contain files with restrictive permissions that prevent
reading after extraction. Fix permissions on unreadable files.

Author: ericmj

lib/diff/hex/hex.ex

@@ -57,10 +57,29 @@ defmodule Diff.Hex do
   def unpack_tarball(tarball_path, output_path) do
     with {:ok, _} <-
            :hex_tarball.unpack({:file, to_charlist(tarball_path)}, to_charlist(output_path)) do
+      ensure_readable(output_path)
       :ok
     end
   end
 
+  defp ensure_readable(dir) do
+    dir
+    |> Path.join("**")
+    |> Path.wildcard(match_dot: true)
+    |> Enum.each(fn path ->
+      case File.stat(path) do
+        {:ok, %{type: :directory, access: access}} when access in [:none, :write] ->
+          File.chmod(path, 0o755)
+
+        {:ok, %{type: :regular, access: access}} when access in [:none, :write] ->
+          File.chmod(path, 0o644)
+
+        _ ->
+          :ok
+      end
+    end)
+  end
+
   def get_checksums(package, versions) do
     with {:ok, {200, _, releases}} <- :hex_repo.get_package(config(), package) do
       checksums =