update according to comments

metaflow/cli.py

@@ -724,7 +724,7 @@ def wrapper(*args, **kwargs):
     default=None,
     show_default=True,
     hidden=True,
-    help="If specified, it identifies the task that started this resume call",
+    help="If specified, it identifies the task that started this resume call. It is in the form of {step_name}-{task_id}",
 )
 @click.argument("step-to-rerun", required=False)
 @cli.command(help="Resume execution of a previous run of this flow.")

metaflow/runtime.py

@@ -787,12 +787,6 @@ def __init__(
             # At this point, we know we are going to clone
             self._is_cloned = True
 
-            if resume_identifier:
-                self.log(
-                    "Resume identifier is %s." % resume_identifier,
-                    system_msg=True,
-                )
-
             task_id_exists_already = False
             task_completed = False
             if reentrant:
@@ -821,6 +815,15 @@ def __init__(
                         "Reentrant clone-only resume does not allow for explicit task-id"
                     )
 
+                if resume_identifier:
+                    self.log(
+                        "Resume identifier is %s." % resume_identifier,
+                        system_msg=True,
+                    )
+                else:
+                    raise MetaflowInternalError(
+                        "Reentrant clone-only resume needs a resume identifier."
+                    )
                 # We will use the same task_id as the original task
                 # to use it effectively as a synchronization key
                 clone_task_id = origin.task_id
@@ -1028,7 +1031,7 @@ def mark_resume_done(self):
 
         # Mark the resume as done. This is called at the end of the resume flow and after
         # the _parameters step was successfully cloned, so we need to 'dangerously' save
-        # this done file, but the risk should be minimum.
+        # this done file, but the risk should be minimal.
         self._ds._dangerous_save_metadata_post_done(
             {"_resume_done": True}, add_attempt=False
         )