Prevent error when duplicate mapping is created

The mapping has unique constraint and creating a mapping with the same
local entity will fail. Creation of mapping with the same local entity
might happen because of race condition, that would happen if 2 processes
start mapping of the same local entity at the same time.

Return the existing public_id if creation failed due to duplicate.

Change-Id: Ic6e6ec9614e16a49c791dc6c06a4464d18be09be
Related-Bug: 1582585

keystone/identity/mapping_backends/sql.py

@@ -65,12 +65,17 @@ class Mapping(base.MappingDriverV8):
 
     def create_id_mapping(self, local_entity, public_id=None):
         entity = local_entity.copy()
-        with sql.session_for_write() as session:
-            if public_id is None:
-                public_id = self.id_generator_api.generate_public_ID(entity)
-            entity['public_id'] = public_id
-            mapping_ref = IDMapping.from_dict(entity)
-            session.add(mapping_ref)
+        try:
+            with sql.session_for_write() as session:
+                if public_id is None:
+                    public_id = self.id_generator_api.generate_public_ID(
+                        entity)
+                entity['public_id'] = public_id
+                mapping_ref = IDMapping.from_dict(entity)
+                session.add(mapping_ref)
+        except sql.DBDuplicateEntry:
+            # something else created the mapping already. We can use it.
+            public_id = self.get_public_id(local_entity)
         return public_id
 
     def delete_id_mapping(self, public_id):

keystone/tests/unit/test_backend_id_mapping_sql.py

@@ -196,3 +196,21 @@ class SqlIDMapping(test_backend_sql.SqlTests):
         self.id_mapping_api.purge_mappings({})
         self.assertThat(mapping_sql.list_id_mappings(),
                         matchers.HasLength(initial_mappings))
+
+    def test_create_duplicate_mapping(self):
+        local_entity = {
+            'domain_id': self.domainA['id'],
+            'local_id': uuid.uuid4().hex,
+            'entity_type': mapping.EntityType.USER}
+        public_id1 = self.id_mapping_api.create_id_mapping(local_entity)
+
+        # second call should be successful and return the same
+        # public_id as above
+        public_id2 = self.id_mapping_api.create_id_mapping(local_entity)
+        self.assertEqual(public_id1, public_id2)
+
+        # even if public_id was specified, it should not be used,
+        # and still the same public_id should be returned
+        public_id3 = self.id_mapping_api.create_id_mapping(
+            local_entity, public_id=uuid.uuid4().hex)
+        self.assertEqual(public_id1, public_id3)