cql-pytest: Verify RF is changes by at most 1 when tablets on

This commit adds a test verifying that we can only
change the RF of a keyspace for any DC by at most 1
when using tablets.

Fixes scylladb#18029

test/cql-pytest/test_keyspace.py

@@ -99,7 +99,13 @@ def test_drop_keyspace_nonexistent(cql):
 # Test trying to ALTER a keyspace.
 def test_alter_keyspace(cql, this_dc):
     with new_test_keyspace(cql, "WITH REPLICATION = { 'class' : 'NetworkTopologyStrategy', '" + this_dc + "' : 1 }") as keyspace:
-        cql.execute(f"ALTER KEYSPACE {keyspace} WITH REPLICATION = {{ 'class' : 'NetworkTopologyStrategy', '{this_dc}' : 3 }} AND DURABLE_WRITES = false")
+        cql.execute(f"ALTER KEYSPACE {keyspace} WITH REPLICATION = {{ 'class' : 'NetworkTopologyStrategy', '{this_dc}' : 2 }} AND DURABLE_WRITES = false")
+
+# Test trying to ALTER a keyspace's RF by more than 1 at a time
+def test_alter_keyspace(cql, this_dc):
+    with new_test_keyspace(cql, "WITH REPLICATION = { 'class' : 'NetworkTopologyStrategy', '" + this_dc + "' : 1 }") as keyspace:
+        with pytest.raises(InvalidRequest):
+            cql.execute(f"ALTER KEYSPACE {keyspace} WITH REPLICATION = {{ 'class' : 'NetworkTopologyStrategy', '{this_dc}' : 3 }} AND DURABLE_WRITES = false")
 
 # Test trying to ALTER a keyspace with invalid options.
 def test_alter_keyspace_invalid(cql, this_dc):

test/cql-pytest/test_tablets.py

@@ -219,3 +219,35 @@ def test_lwt_support_with_tablets(cql, test_keyspace, skip_without_tablets):
             cql.execute(f"DELETE FROM {table} WHERE key = 1 IF EXISTS")
         res = cql.execute(f"SELECT val FROM {table} WHERE key = 1").one()
         assert res.val == 0
+
+
+# We want to ensure that we can only change the RF of any DC by at most 1 at a time
+# if we use tablets. That provides us with the guarantee that the old and the new QUORUM
+# overlap by at least one node.
+def test_alter_tablet_keyspace(cql, this_dc):
+    with new_test_keyspace(cql, f"WITH REPLICATION = {{ 'class' : 'NetworkTopologyStrategy', '{this_dc}' : 1 }} "
+                                f"AND TABLETS = {{ 'enabled': true, 'initial': 128 }}") as keyspace:
+        def change_opt_rf(rf_opt, new_rf):
+            cql.execute(f"ALTER KEYSPACE {keyspace} WITH REPLICATION = {{ 'class' : 'NetworkTopologyStrategy', '{rf_opt}' : {new_rf} }}")
+        def change_dc_rf(new_rf):
+            change_opt_rf(this_dc, new_rf)
+        def change_default_rf(new_rf):
+            change_opt_rf("replication_factor", new_rf)
+
+        change_dc_rf(2)
+        change_dc_rf(3)
+
+        with pytest.raises(InvalidRequest):
+            change_dc_rf(5)
+        with pytest.raises(InvalidRequest):
+            change_dc_rf(1)
+        with pytest.raises(InvalidRequest):
+            change_dc_rf(10)
+
+        # Changing the default replication factor doesn't affect any of the existing DCs,
+        # so we can change it arbitrarily.
+        change_default_rf(4)
+        change_default_rf(2)
+        change_default_rf(8)
+        change_default_rf(1)
+        change_default_rf(10)