Add a helper to make a deep copy of a ConfigParser

This will help avoid unintentional side-effects

src/allmydata/test/test_configutil.py

@@ -280,3 +280,16 @@ enabled = false
             ),
             None,
         )
+
+    @given(arbitrary_config_dicts())
+    def test_copy_config(self, cfgdict):
+        """
+        ``copy_config`` creates a new ``ConfigParser`` object containing the same
+        values as its input.
+        """
+        cfg = to_configparser(cfgdict)
+        copied = configutil.copy_config(cfg)
+        # Should be equal
+        self.assertEqual(cfg, copied)
+        # But not because they're the same object.
+        self.assertIsNot(cfg, copied)

src/allmydata/util/configutil.py

@@ -173,6 +173,23 @@ class ValidConfiguration(object):
         )
 
 
+def copy_config(old):
+    """
+    Return a brand new ``ConfigParser`` containing the same values as
+    the given object.
+
+    :param ConfigParser old: The configuration to copy.
+
+    :return ConfigParser: The new object containing the same configuration.
+    """
+    new = ConfigParser()
+    for section_name in old.sections():
+        new.add_section(section_name)
+        for k, v in old.items(section_name):
+            new.set(section_name, k, v.replace("%", "%%"))
+    return new
+
+
 def _either(f, g):
     """
     :return: A function which returns True if either f or g returns True.