Fix purge_deleted() method in memory backend (#3520)

* Reproduce memory backend issue in a test

* Fix backend implementation

* Do not include timestamp in deleted tombstones count

* fix test

Author: leplatrem

kinto/core/storage/memory.py

@@ -288,13 +288,26 @@ def purge_deleted(
         modified_field=DEFAULT_MODIFIED_FIELD,
     ):
         parent_id_match = re.compile(parent_id.replace("*", ".*"))
-        by_parent_id = {
+
+        timestamps_by_parent_id = {
             pid: resources
-            for pid, resources in self._cemetery.items()
+            for pid, resources in self._timestamps.items()
             if parent_id_match.match(pid)
         }
+        if resource_name is not None:
+            for pid, resources in timestamps_by_parent_id.items():
+                del self._timestamps[pid][resource_name]
+        else:
+            for pid, resources in timestamps_by_parent_id.items():
+                del self._timestamps[pid]
+
         num_deleted = 0
-        for pid, resources in by_parent_id.items():
+        tombstones_by_parent_id = {
+            pid: resources
+            for pid, resources in self._cemetery.items()
+            if parent_id_match.match(pid)
+        }
+        for pid, resources in tombstones_by_parent_id.items():
             if resource_name is not None:
                 resources = {resource_name: resources[resource_name]}
             for resource, resource_objects in resources.items():

kinto/core/storage/testing.py

@@ -1296,6 +1296,9 @@ def test_purge_deleted_removes_timestamps_by_parent_id(self):
         self.create_object(parent_id="/abc/a", resource_name="c")
         self.create_object(parent_id="/efg", resource_name="c")
 
+        all_timestamps = self.storage.all_resources_timestamps(resource_name="c")
+        self.assertEqual(set(all_timestamps.keys()), {"/abc/a", "/efg"})
+
         before1 = self.storage.resource_timestamp(parent_id="/abc/a", resource_name="c")
         # Different parent_id with object.
         before2 = self.storage.resource_timestamp(parent_id="/efg", resource_name="c")
@@ -1305,11 +1308,15 @@ def test_purge_deleted_removes_timestamps_by_parent_id(self):
         self.storage.delete_all(parent_id="/abc/*", resource_name=None, with_deleted=False)
         self.storage.purge_deleted(parent_id="/abc/*", resource_name=None)
 
+        all_timestamps = self.storage.all_resources_timestamps(resource_name="c")
+        self.assertEqual(set(all_timestamps.keys()), {"/efg", "/ijk"})
+
+        time.sleep(0.002)  # make sure we don't recreate timestamps at same msec.
         after1 = self.storage.resource_timestamp(parent_id="/abc/a", resource_name="c")
         after2 = self.storage.resource_timestamp(parent_id="/efg", resource_name="c")
         after3 = self.storage.resource_timestamp(parent_id="/ijk", resource_name="c")
 
-        self.assertNotEqual(before1, after1)
+        self.assertNotEqual(before1, after1)  # timestamp was removed, it will differ.
         self.assertEqual(before2, after2)
         self.assertEqual(before3, after3)