config: Add deprecated config value support.

This makes it possible to mark a key as deprecated, this implies it will be
ignored if present, and never included in the resulting config.

mopidy/config/schemas.py

@@ -74,6 +74,8 @@ class ConfigSchema(collections.OrderedDict):
             if key not in result and key not in errors:
                 result[key] = None
                 errors[key] = 'config key not found.'
+            if isinstance(result[key], types.DeprecatedValue):
+                del result[key]
 
         return result, errors
 

mopidy/config/types.py

@@ -31,6 +31,10 @@ class ExpandedPath(bytes):
         self.original = original
 
 
+class DeprecatedValue(object):
+    pass
+
+
 class ConfigValue(object):
     """Represents a config key's value and how to handle it.
 
@@ -59,6 +63,20 @@ class ConfigValue(object):
         return bytes(value)
 
 
+class Deprecated(ConfigValue):
+    """Deprecated value
+
+    Used for ignoring old config values that are no longer in use, but should
+    not cause the config parser to crash.
+    """
+
+    def deserialize(self, value):
+        return DeprecatedValue()
+
+    def serialize(self, value, display=False):
+        return DeprecatedValue()
+
+
 class String(ConfigValue):
     """String value.
 

tests/config/schemas_test.py

@@ -4,7 +4,7 @@ import logging
 import mock
 import unittest
 
-from mopidy.config import schemas
+from mopidy.config import schemas, types
 
 from tests import any_unicode
 
@@ -77,6 +77,14 @@ class ConfigSchemaTest(unittest.TestCase):
         self.assertIsNone(result['bar'])
         self.assertIsNone(result['baz'])
 
+    def test_deserialize_none_value(self):
+        self.schema['foo'].deserialize.return_value = types.DeprecatedValue()
+
+        result, errors = self.schema.deserialize(self.values)
+        print result, errors
+        self.assertItemsEqual(['bar', 'baz'], result.keys())
+        self.assertNotIn('foo', errors)
+
 
 class LogLevelConfigSchemaTest(unittest.TestCase):
     def test_conversion(self):

tests/config/types_test.py

@@ -33,6 +33,16 @@ class ConfigValueTest(unittest.TestCase):
         self.assertIsInstance(value.serialize(object(), display=True), bytes)
 
 
+class DeprecatedTest(unittest.TestCase):
+    def test_deserialize_returns_deprecated_value(self):
+        self.assertIsInstance(types.Deprecated().deserialize(b'foobar'),
+                              types.DeprecatedValue)
+
+    def test_serialize_returns_deprecated_value(self):
+        self.assertIsInstance(types.Deprecated().serialize('foobar'),
+                              types.DeprecatedValue)
+
+
 class StringTest(unittest.TestCase):
     def test_deserialize_conversion_success(self):
         value = types.String()