Add a version filtering strategy

The current patch proposes the addition of a new data source strategy
for filtering the strategies by a given list of versions.

Each data source exports a new API, called ``version()``, which should
return the underlying version of the said data source. This implies that
each data source module can export multiple versioned data sources and it's
the duty of an external strategy to select the appropiate data source with
regard to a particular set of versions.

Change-Id: I6ef2fdd6520a53fc600232247f23bd22b848461e

cloudinit/sources/base.py

@@ -140,6 +140,10 @@ class BaseDataSource(object):
         This should return an instance of :class:`APIResponse`.
         """
 
+    @abc.abstractmethod
+    def version(self):
+        """Get the version of the current data source."""
+
     def instance_id(self):
         """Get this instance's id."""
 

cloudinit/sources/openstack/base.py

@@ -50,6 +50,10 @@ class BaseOpenStackSource(base.BaseDataSource):
     def _path_join(self, path, *addons):
         """Join one or more components together."""
 
+    def version(self):
+        """Get the underlying data source version."""
+        return self._version
+
     def _working_version(self):
         versions = self._available_versions()
         # OS_VERSIONS is stored in chronological order, so

cloudinit/sources/strategy.py

@@ -77,3 +77,22 @@ class SerialSearchStrategy(BaseSearchStrategy):
         for data_source in data_sources:
             if self.is_datasource_available(data_source):
                 yield data_source
+
+
+class FilterVersionStrategy(BaseSearchStrategy):
+    """A strategy for filtering data sources by their version
+
+    :param versions:
+        A list of strings, where each strings is a possible
+        version that a data source can have.
+    """
+
+    def __init__(self, versions=None):
+        if versions is None:
+            versions = []
+        self._versions = versions
+        super(FilterVersionStrategy, self).__init__()
+
+    def search_data_sources(self, data_sources):
+        return (source for source in data_sources
+                if source.version() in self._versions)

cloudinit/tests/sources/test_strategy.py

@@ -63,3 +63,38 @@ class TestStrategy(tests.TestCase):
             valid_sources = list(instance.search_data_sources(sources))
 
         self.assertEqual(available_sources, valid_sources)
+
+    def test_filter_version_strategy(self):
+        class SourceV1(object):
+            def version(self):
+                return 'first'
+
+        class SourceV2(SourceV1):
+            def version(self):
+                return 'second'
+
+        class SourceV3(object):
+            def version(self):
+                return 'third'
+
+        sources = [SourceV1(), SourceV2(), SourceV3()]
+        instance = strategy.FilterVersionStrategy(['third', 'first'])
+
+        filtered_sources = sorted(
+            source.version()
+            for source in instance.search_data_sources(sources))
+
+        self.assertEqual(len(filtered_sources), 2)
+        self.assertEqual(filtered_sources, ['first', 'third'])
+
+    def test_filter_version_strategy_no_versions_given(self):
+        class SourceV1(object):
+            def version(self):
+                return 'first'
+
+        sources = [SourceV1()]
+        instance = strategy.FilterVersionStrategy()
+
+        filtered_sources = list(instance.search_data_sources(sources))
+
+        self.assertEqual(len(filtered_sources), 0)