Don't let users drop or generally break tables in system_distributed

patch by Aleksey Yeschenko; reviewed by Sylvain Lebresne for
CASSANDRA-13813
diff --git a/system_keyspaces_test.py b/system_keyspaces_test.py
new file mode 100644
index 0000000..90eb583
--- /dev/null
+++ b/system_keyspaces_test.py
@@ -0,0 +1,118 @@
+from cassandra import Unauthorized
+from dtest import Tester
+from tools.assertions import assert_all, assert_exception, assert_none
+from tools.decorators import since
+
+class TestSystemKeyspaces(Tester):
+
+    @since('3.0')
+    def test_local_system_keyspaces(self):
+        cluster = self.cluster
+        cluster.populate(1).start()
+
+        node = cluster.nodelist()[0]
+        session = self.patient_cql_connection(node)
+
+        # ALTER KEYSPACE should fail for system and system_schema
+        stmt = """
+            ALTER KEYSPACE system
+            WITH replication = {'class': 'NetworkTopologyStrategy', 'datacenter1' : '1'};"""
+        assert_exception(session, stmt, expected=Unauthorized)
+
+        stmt = """
+            ALTER KEYSPACE system_schema
+            WITH replication = {'class': 'NetworkTopologyStrategy', 'datacenter1' : '1'};"""
+        assert_exception(session, stmt, expected=Unauthorized)
+
+        # DROP KEYSPACE should fail for system and system_schema
+        assert_exception(session, 'DROP KEYSPACE system;',        expected=Unauthorized)
+        assert_exception(session, 'DROP KEYSPACE system_schema;', expected=Unauthorized)
+
+        # CREATE TABLE should fail in system and system_schema
+        assert_exception(session,
+                         'CREATE TABLE system.new_table (id int PRIMARY KEY);',
+                         expected=Unauthorized)
+
+        assert_exception(session,
+                         'CREATE TABLE system_schema.new_table (id int PRIMARY KEY);',
+                         expected=Unauthorized)
+
+        # ALTER TABLE should fail in system and system_schema
+        assert_exception(session,
+                         "ALTER TABLE system.local WITH comment = '';",
+                         expected=Unauthorized)
+
+        assert_exception(session,
+                         "ALTER TABLE system_schema.tables WITH comment = '';",
+                         expected=Unauthorized)
+
+        # DROP TABLE should fail in system and system_schema
+        assert_exception(session, 'DROP TABLE system.local;',         expected=Unauthorized)
+        assert_exception(session, 'DROP TABLE system_schema.tables;', expected=Unauthorized)
+
+    @since('3.0')
+    def test_replicated_system_keyspaces(self):
+        cluster = self.cluster
+        cluster.populate(1).start()
+
+        node = cluster.nodelist()[0]
+        session = self.patient_cql_connection(node)
+
+        # ALTER KEYSPACE should work for system_auth, system_distributed, and system_traces
+        stmt = """
+            ALTER KEYSPACE system_auth
+            WITH replication = {'class': 'NetworkTopologyStrategy', 'datacenter1' : '1'};"""
+        assert_none(session, stmt)
+
+        stmt = """
+            ALTER KEYSPACE system_distributed
+            WITH replication = {'class': 'NetworkTopologyStrategy', 'datacenter1' : '1'};"""
+        assert_none(session, stmt)
+
+        stmt = """
+            ALTER KEYSPACE system_traces
+            WITH replication = {'class': 'NetworkTopologyStrategy', 'datacenter1' : '1'};"""
+        assert_none(session, stmt)
+
+        stmt = """
+            SELECT replication
+            FROM system_schema.keyspaces
+            WHERE keyspace_name IN ('system_auth', 'system_distributed', 'system_traces');"""
+        replication = {'class': 'org.apache.cassandra.locator.NetworkTopologyStrategy', 'datacenter1': '1'}
+        assert_all(session, stmt, [[replication], [replication], [replication]])
+
+        # DROP KEYSPACE should fail for system_auth, system_distributed, and system_traces
+        assert_exception(session, 'DROP KEYSPACE system_auth;', expected=Unauthorized)
+        assert_exception(session, 'DROP KEYSPACE system_distributed;', expected=Unauthorized)
+        assert_exception(session, 'DROP KEYSPACE system_traces;', expected=Unauthorized)
+
+        # CREATE TABLE should fail in system_auth, system_distributed, and system_traces
+        assert_exception(session,
+                         'CREATE TABLE system_auth.new_table (id int PRIMARY KEY);',
+                         expected=Unauthorized)
+
+        assert_exception(session,
+                         'CREATE TABLE system_distributed.new_table (id int PRIMARY KEY);',
+                         expected=Unauthorized)
+
+        assert_exception(session,
+                         'CREATE TABLE system_traces.new_table (id int PRIMARY KEY);',
+                         expected=Unauthorized)
+
+        # ALTER TABLE should fail in system_auth, system_distributed, and system_traces
+        assert_exception(session,
+                         "ALTER TABLE system_auth.roles WITH comment = '';",
+                         expected=Unauthorized)
+
+        assert_exception(session,
+                         "ALTER TABLE system_distributed.repair_history WITH comment = '';",
+                         expected=Unauthorized)
+
+        assert_exception(session,
+                         "ALTER TABLE system_traces.sessions WITH comment = '';",
+                         expected=Unauthorized)
+
+        # DROP TABLE should fail in system_auth, system_distributed, and system_traces
+        assert_exception(session, 'DROP TABLE system_auth.roles;', expected=Unauthorized)
+        assert_exception(session, 'DROP TABLE system_distributed.repair_history;', expected=Unauthorized)
+        assert_exception(session, 'DROP TABLE system_traces.sessions;', expected=Unauthorized)