test: align Cluster.add_instance args types

test/conftest.py

@@ -480,7 +480,7 @@ class Cluster:
         self,
         wait_ready=True,
         peers=None,
-        instance_id: str | bool | None = True,
+        instance_id: str | bool = True,
         failure_domain=dict(),
         init_replication_factor=1,
     ) -> Instance:
@@ -489,30 +489,35 @@ class Cluster:
 
         `instance_id` specifies how the instance's id is generated in the
         following way:
-        - if `instance_id` is a string, it will be used as value for the
-          `--instance-id` command line option.
-        - If `instance_id` is `True` (default), this function will generate a
-          value for the `--instance-id` command line option.
-        - If `instance_id` is `False` or `None`, the instance will be started
-        without the `--instance-id` option, and the cluster will have to choose
-        the id.
+
+        - if `instance_id` is a string, it will be used as a value for the
+          `--instance-id` command-line option.
+
+        - If `instance_id` is `True` (default), the `--instance-id` command-line
+          option will be generated by the pytest according to the instances
+          sequence number in cluster.
+
+        - If `instance_id` is `False`, the instance will be started
+          without the `--instance-id` command-line option and the particular value
+          will be generated by the cluster.
         """
         i = 1 + len(self.instances)
 
+        generated_instance_id: str | None
         match instance_id:
-            case True:
-                instance_id = f"i{i}"
-            case False | None:
-                instance_id = None
             case str() as iid:
-                instance_id = iid
+                generated_instance_id = iid
+            case True:
+                generated_instance_id = f"i{i}"
+            case False:
+                generated_instance_id = None
             case _:
                 raise Exception("unreachable")
 
         instance = Instance(
             binary_path=self.binary_path,
             cluster_id=self.id,
-            instance_id=instance_id,
+            instance_id=generated_instance_id,
             data_dir=f"{self.data_dir}/i{i}",
             host=self.base_host,
             port=self.base_port + i,
@@ -534,7 +539,7 @@ class Cluster:
     def fail_to_add_instance(
         self,
         peers=None,
-        instance_id=None,
+        instance_id: str | bool = True,
         failure_domain=dict(),
         init_replication_factor=1,
     ):

test/int/test_joining.py

@@ -348,6 +348,7 @@ def test_fail_to_join(cluster: Cluster):
     # An instance with the given instance_id is already present in the cluster
     # so this instance cannot join
     # and therefore exits with failure
+    assert i1.instance_id is not None
     cluster.fail_to_add_instance(
         instance_id=i1.instance_id, failure_domain=dict(owner="Jim")
     )