Fix marker checking when value is None

There are cases where users sort a table using compound-values sort_key
and one of the key has nullable set to True. For example, sorting a
table using ['id', 'updated_at'] where 'updated_at' can be None.

When marker_value is None, we cannot do value comparison using '<' or
'>' operators. This patch adds a check if the value from the marker
corresponding to the nullable-key has None value. If that is the case,
we skip the comparison.

Back to the example above, instead of always getting the following
criteria (which doesn't work):

(id > MARKER_ID) or (id == MARKER_ID && updated_at > None) <-- failure

we will get the following criteria when 'updated_at' is None:

(id > MARKER_ID)

This is not hurting in any way to existing / legal use cases where
callers are expected to include a unique key in sort keys. If there are
such cases, this patch is not making things worse because the sorting
is already unpredictable.

Closes-Bug: #1615938
Change-Id: Iea2cd0bb2556b0b15a0baaa76ef522a3097f9928
(cherry picked from commit b3869d04cf)

oslo_db/sqlalchemy/utils.py

@@ -146,7 +146,8 @@ def paginate_query(query, model, limit, sort_keys, marker=None,
     the lexicographical ordering:
     (k1 > X1) or (k1 == X1 && k2 > X2) or (k1 == X1 && k2 == X2 && k3 > X3)
 
-    We also have to cope with different sort_directions.
+    We also have to cope with different sort_directions and cases where k2,
+    k3, ... are nullable.
 
     Typically, the id of the last row is used as the client-facing pagination
     marker, then the actual marker object must be fetched from the db and
@@ -223,18 +224,24 @@ def paginate_query(query, model, limit, sort_keys, marker=None,
         criteria_list = []
         for i in range(len(sort_keys)):
             crit_attrs = []
-            for j in range(i):
-                model_attr = getattr(model, sort_keys[j])
-                crit_attrs.append((model_attr == marker_values[j]))
+            # NOTE: We skip the marker value comparison if marker_values[i] is
+            #       None, for two reasons: 1) the comparison operators below
+            #       ('<', '>') are not applicable on None value; 2) this is
+            #       safe because we can assume the primary key is included in
+            #       sort_key, thus checked as (one of) marker values.
+            if marker_values[i] is not None:
+                for j in range(i):
+                    model_attr = getattr(model, sort_keys[j])
+                    crit_attrs.append((model_attr == marker_values[j]))
 
-            model_attr = getattr(model, sort_keys[i])
-            if sort_dirs[i].startswith('desc'):
-                crit_attrs.append((model_attr < marker_values[i]))
-            else:
-                crit_attrs.append((model_attr > marker_values[i]))
+                model_attr = getattr(model, sort_keys[i])
+                if sort_dirs[i].startswith('desc'):
+                    crit_attrs.append((model_attr < marker_values[i]))
+                else:
+                    crit_attrs.append((model_attr > marker_values[i]))
 
-            criteria = sqlalchemy.sql.and_(*crit_attrs)
-            criteria_list.append(criteria)
+                criteria = sqlalchemy.sql.and_(*crit_attrs)
+                criteria_list.append(criteria)
 
         f = sqlalchemy.sql.or_(*criteria_list)
         query = query.filter(f)

oslo_db/tests/sqlalchemy/test_utils.py

@@ -77,6 +77,7 @@ class FakeTable(Base):
     user_id = Column(String(50), primary_key=True)
     project_id = Column(String(50))
     snapshot_id = Column(String(50))
+    updated_at = Column(DateTime, nullable=True)
 
     # mox is comparing in some awkward way that
     # in this case requires the same identity of object
@@ -171,7 +172,8 @@ class TestPaginateQuery(test_base.BaseTestCase):
         self.mox.StubOutWithMock(sqlalchemy, 'desc')
         self.marker = FakeTable(user_id='user',
                                 project_id='p',
-                                snapshot_id='s')
+                                snapshot_id='s',
+                                updated_at=None)
         self.model = FakeTable
 
     def test_paginate_query_no_pagination_no_sort_dirs(self):
@@ -301,6 +303,35 @@ class TestPaginateQuery(test_base.BaseTestCase):
                              marker=self.marker,
                              sort_dirs=['asc-nullslast', 'desc-nullsfirst'])
 
+    def test_paginate_query_marker_null(self):
+        self.mox.StubOutWithMock(self.model.user_id, 'isnot')
+        self.model.user_id.isnot(None).AndReturn('asc_null_1')
+        sqlalchemy.desc('asc_null_1').AndReturn('asc_null_2')
+        self.query.order_by('asc_null_2').AndReturn(self.query)
+
+        sqlalchemy.asc(self.model.user_id).AndReturn('asc_1')
+        self.query.order_by('asc_1').AndReturn(self.query)
+
+        self.mox.StubOutWithMock(self.model.updated_at, 'is_')
+        self.model.updated_at.is_(None).AndReturn('desc_null_1')
+        sqlalchemy.desc('desc_null_1').AndReturn('desc_null_2')
+        self.query.order_by('desc_null_2').AndReturn(self.query)
+
+        sqlalchemy.desc(self.model.updated_at).AndReturn('desc_1')
+        self.query.order_by('desc_1').AndReturn(self.query)
+
+        self.mox.StubOutWithMock(sqlalchemy.sql, 'and_')
+        sqlalchemy.sql.and_(mock.ANY).AndReturn('some_crit')
+        self.mox.StubOutWithMock(sqlalchemy.sql, 'or_')
+        sqlalchemy.sql.or_('some_crit').AndReturn('some_f')
+        self.query.filter('some_f').AndReturn(self.query)
+        self.query.limit(5).AndReturn(self.query)
+        self.mox.ReplayAll()
+        utils.paginate_query(self.query, self.model, 5,
+                             ['user_id', 'updated_at'],
+                             marker=self.marker,
+                             sort_dirs=['asc-nullslast', 'desc-nullsfirst'])
+
     def test_paginate_query_value_error(self):
         sqlalchemy.asc(self.model.user_id).AndReturn('asc_1')
         self.query.order_by('asc_1').AndReturn(self.query)