diff --git a/Library/Homebrew/extend/pathname.rb b/Library/Homebrew/extend/pathname.rb
index 8b690a2581..5534cd7528 100644
--- a/Library/Homebrew/extend/pathname.rb
+++ b/Library/Homebrew/extend/pathname.rb
@@ -238,6 +238,16 @@ class Pathname
   def / that
     join that.to_s
   end
+
+  def ensure_writable
+    saved_perms = unless writable?
+      chmod 0644
+      stat.mode
+    end
+    yield
+  ensure
+    chmod saved_perms if saved_perms
+  end
 end
 
 # sets $n and $d so you can observe creation of stuff