Ensure that platform from group selection checks broadcast manager

for updates to task config.
Ensure that 255 callback gives a sensible platform in warning messages.

cylc/flow/subprocpool.py

@@ -557,12 +557,12 @@ def _run_callback(callback, args_=None):
  # Backup, get a platform name from the config:
  for arg in callback_args:
  if isinstance(arg, TaskProxy):
- platform_name = arg.tdef.rtconfig['platform']
+ platform_name = arg.platform['name']
  elif (
  isinstance(arg, list)
  and isinstance(arg[0], TaskProxy)
  ):
- platform_name = arg[0].tdef.rtconfig['platform']
+ platform_name = arg[0].platform['name']
 
  if cls.ssh_255_fail(ctx) or cls.rsync_255_fail(ctx, platform) is True:
  # Job log retrieval passes a special object as a command key

cylc/flow/task_job_mgr.py

@@ -144,8 +144,15 @@ class TaskJobManager:
  REMOTE_INIT_IN_PROGRESS: REMOTE_INIT_MSG
  }
 
- def __init__(self, workflow, proc_pool, workflow_db_mgr,
- task_events_mgr, data_store_mgr, bad_hosts):
+ def __init__(
+ self,
+ workflow,
+ proc_pool,
+ workflow_db_mgr,
+ task_events_mgr,
+ data_store_mgr,
+ bad_hosts,
+ ):
  self.workflow = workflow
  self.proc_pool = proc_pool
  self.workflow_db_mgr = workflow_db_mgr
@@ -306,10 +313,12 @@ def submit_task_jobs(self, workflow, itasks, curve_auth,
 
  # Get another platform, if task config platform is a group
  use_next_platform_in_group = False
- if itask.tdef.rtconfig['platform']:
+ bc_mgr = self.task_events_mgr.broadcast_mgr
+ rtconf = bc_mgr.get_updated_rtconfig(itask)
+ if rtconf['platform']:
  try:
  platform = get_platform(
- itask.tdef.rtconfig['platform'],
+ rtconf['platform'],
  bad_hosts=self.bad_hosts
  )
  except PlatformLookupError: