#10973 creating a Subsystem without also registering via SubsystemRule leads to Issue created by tdyas Context: I created a

Subsystem

in #10889 to provide a streaming workunit handler, but the code did not initially register the

Subsystem

via

SubsystemRule

. I added the Subsystem as a streaming workunit handler using

--streaming-workunits-handlers='+["pants.reporting.metrics.MetricsReporting"]'

and got the following error message:

16:27:00.81 [ERROR] No such options scope: pants-metrics
Traceback (most recent call last):
  File "/Users/tdyas/TC/pants/src/python/pants/option/parser_hierarchy.py", line 73, in get_parser_by_scope
    return self._parser_by_scope[scope]
KeyError: 'pants-metrics'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/tdyas/TC/pants/src/python/pants/bin/daemon_pants_runner.py", line 148, in single_daemonized_run
    return runner.run(start_time)
  File "/Users/tdyas/TC/pants/src/python/pants/bin/local_pants_runner.py", line 259, in run
    callbacks = Subsystem.get_streaming_workunit_callbacks(streaming_handlers)
  File "/Users/tdyas/TC/pants/src/python/pants/option/subsystem.py", line 247, in get_streaming_workunit_callbacks
    subsystem = subsystem_class.global_instance()
  File "/Users/tdyas/TC/pants/src/python/pants/option/subsystem.py", line 157, in global_instance
    return cls._instance_for_scope(cls.options_scope)  # type: ignore[arg-type]  # MyPy is treating cls.options_scope as a Callable, rather than `str`
  File "/Users/tdyas/TC/pants/src/python/pants/option/subsystem.py", line 182, in _instance_for_scope
    cls._scoped_instances[key] = cls(scope, cls._options.for_scope(scope))
  File "/Users/tdyas/TC/pants/src/python/pants/util/memo.py", line 123, in memoize
    result = func(*args, **kwargs)
  File "/Users/tdyas/TC/pants/src/python/pants/option/options.py", line 425, in for_scope
    values = self._parser_hierarchy.get_parser_by_scope(scope).parse_args(parse_args_request)
  File "/Users/tdyas/TC/pants/src/python/pants/option/parser_hierarchy.py", line 75, in get_parser_by_scope
    raise Config.ConfigValidationError(f"No such options scope: {scope}")
pants.option.config.Config.ConfigValidationError: No such options scope: pants-metrics

A better error message would have highlighted that the

Subsystem

had not been registered and then how to register it. pantsbuild/pants