make and use set_config instead of internals

src/allmydata/node.py

@@ -321,6 +321,21 @@ class _Config(object):
                 )
             return default
 
+    def set_config(self, section, option, value):
+        """
+        Set a config options in a section and re-write the tahoe.cfg file
+        """
+        if option.endswith(".furl") and self._contains_unescaped_hash(value):
+            raise UnescapedHashError(section, option, item)
+
+        try:
+            self.config.add_section(section)
+        except ConfigParser.DuplicateSectionError:
+            pass
+        self.config.set(section, option, value)
+        with open(self._config_fname, "w") as f:
+            self.config.write(f)
+
     def get_config_from_file(self, name, required=False):
         """Get the (string) contents of a config file, or None if the file
         did not exist. If required=True, raise an exception rather than

src/allmydata/scripts/admin.py

@@ -124,12 +124,10 @@ def add_grid_manager_cert(options):
         print("Already have file '{}'".format(cert_path), file=options.parent.parent.stderr)
         return 1
 
-    cfg = config.config  # why aren't methods we call on cfg in _Config itself?
-
     gm_certs = config.get_config("storage", "grid_manager_certificate_files", "").split()
     if cert_fname not in gm_certs:
         gm_certs.append(cert_fname)
-    cfg.set("storage", "grid_manager_certificate_files", " ".join(gm_certs))
+    config.set_config("storage", "grid_manager_certificate_files", " ".join(gm_certs))
 
     # print("grid_manager_certificate_files in {}: {}".format(config_path, len(gm_certs)))
 

src/allmydata/test/test_node.py

@@ -316,6 +316,12 @@ class TestCase(testutil.SignalMixin, unittest.TestCase):
         yield client.create_client(basedir)
         self.failUnless(ns.called)
 
+    def test_set_config_new_section(self):
+        basedir = "test_node/test_set_config_new_section"
+        config = config_from_string(basedir, "", "")
+        config.set_config("foo", "bar", "value1")
+        config.set_config("foo", "bar", "value2")
+
 
 class TestMissingPorts(unittest.TestCase):
     """