Merge pull request AppScale#3179 from cdonati/cleared-kinds

Check if a kind has at least one entity

AppDB/appscale/datastore/fdb/indexes.py

@@ -360,8 +360,9 @@ def next_page(self):
 
 
 class KindIterator(object):
-  def __init__(self, tr, project_dir, namespace):
+  def __init__(self, tr, tornado_fdb, project_dir, namespace):
     self._tr = tr
+    self._tornado_fdb = tornado_fdb
     self._project_dir = project_dir
     self._namespace = namespace
     self._done = False
@@ -375,13 +376,39 @@ def next_page(self):
     ns_dir = self._project_dir.open(
       self._tr, (KindIndex.DIR_NAME, self._namespace))
     kinds = ns_dir.list(self._tr)
+    populated_kinds = [
+      kind for kind, populated in zip(
+        kinds, (yield [self._populated(ns_dir, kind) for kind in kinds]))
+      if populated]
+
     results = [IndexEntry(self._project_dir.get_path()[-1], self._namespace,
                           (u'__kind__', kind), None, None)
-               for kind in kinds]
+               for kind in populated_kinds]
 
     self._done = True
     raise gen.Return((results, False))
 
+  @gen.coroutine
+  def _populated(self, ns_dir, kind):
+    """ Checks if at least one entity exists for a given kind. """
+    kind_dir = ns_dir.open(self._tr, (kind,))
+    kind_index = KindIndex(kind_dir)
+    # TODO: Check if the presence of stat entities should mark a kind as being
+    #  populated.
+    index_slice = kind_index.get_slice(())
+    # This query is reversed to increase the likelihood of getting a relevant
+    # (not marked for GC) entry.
+    iterator = IndexIterator(self._tr, self._tornado_fdb, kind_index,
+                             index_slice, fetch_limit=1, reverse=True,
+                             snapshot=True)
+    while True:
+      results, more_results = yield iterator.next_page()
+      if results:
+        raise gen.Return(True)
+
+      if not more_results:
+        raise gen.Return(False)
+
 
 class MergeJoinIterator(object):
   """
@@ -1142,7 +1169,8 @@ def get_iterator(self, tr, query, read_versionstamp=None):
       raise gen.Return(NamespaceIterator(tr, project_dir))
     elif query.has_kind() and query.kind() == u'__kind__':
       project_dir = yield self._directory_cache.get(tr, (project_id,))
-      raise gen.Return(KindIterator(tr, project_dir, namespace))
+      raise gen.Return(KindIterator(tr, self._tornado_fdb, project_dir,
+                                    namespace))
 
     index = yield self._get_perfect_index(tr, query)
     reverse = get_scan_direction(query, index) == Query_Order.DESCENDING