Catch pruner exception when no images are cached

This handles a situation where the pruner would raise an exception when
the image_cache_max_size was set lower than the size of the last image.

Fixes bug 1039854

Change-Id: I4a6c164e9f821f1250314974f829f058fbd02863

glance/image_cache/drivers/sqlite.py

@@ -272,7 +272,11 @@ class Driver(base.Driver):
         with self.get_db() as db:
             cur = db.execute("""SELECT image_id FROM cached_images
                              ORDER BY last_accessed LIMIT 1""")
-            image_id = cur.fetchone()[0]
+            try:
+                image_id = cur.fetchone()[0]
+            except TypeError:
+                # There are no more cached images
+                return None
 
         path = self.get_image_filepath(image_id)
         file_info = os.stat(path)

glance/tests/unit/test_image_cache.py

@@ -26,6 +26,8 @@ import stubout
 
 from glance.common import utils
 from glance import image_cache
+#NOTE(bcwaldon): This is imported to load the registry config options
+import glance.registry
 from glance.tests import utils as test_utils
 from glance.tests.utils import skip_if_disabled, xattr_writes_supported
 
@@ -182,6 +184,31 @@ class ImageCacheTestCase(object):
             self.assertTrue(self.cache.is_cached(x),
                             "Image %s was not cached!" % x)
 
+    @skip_if_disabled
+    def test_prune_to_zero(self):
+        """Test that an image_cache_max_size of 0 doesn't kill the pruner
+
+        This is a test specifically for LP #1039854
+        """
+        self.assertEqual(0, self.cache.get_cache_size())
+
+        FIXTURE_FILE = StringIO.StringIO(FIXTURE_DATA)
+        self.assertTrue(self.cache.cache_image_file('xxx', FIXTURE_FILE))
+
+        self.assertEqual(1024, self.cache.get_cache_size())
+
+        # OK, hit the image that is now cached...
+        buff = StringIO.StringIO()
+        with self.cache.open_for_read('xxx') as cache_file:
+            for chunk in cache_file:
+                buff.write(chunk)
+
+        self.config(image_cache_max_size=0)
+        self.cache.prune()
+
+        self.assertEqual(0, self.cache.get_cache_size())
+        self.assertFalse(self.cache.is_cached('xxx'))
+
     @skip_if_disabled
     def test_queue(self):
         """