Fix erroneous submissions to localhost when using deprecated batch system setting (#6990)

Author: wxtim

changes.d/6990.fix.md

@@ -0,0 +1 @@
+Fix a bug where setting 'batch system' and not setting host could lead to the wrong platform being selected.

cylc/flow/task_job_mgr.py

@@ -64,6 +64,7 @@
 from cylc.flow.job_runner_mgr import JOB_FILES_REMOVED_MESSAGE, JobPollContext
 from cylc.flow.pathutil import get_remote_workflow_run_job_dir
 from cylc.flow.platforms import (
+    FORBIDDEN_WITH_PLATFORM,
     get_host_from_platform,
     get_install_target_from_platform,
     get_localhost_install_target,
@@ -1222,14 +1223,21 @@ def _prep_submit_task_job(
 
         host_name, platform_name = None, None
         try:
-            if rtconfig['remote']['host'] is not None:
+            # We need to assume that we want a host if any of the items
+            # for the old host/batch system plaform selection system are set.
+            if any(
+                rtconfig[section][key] is not None
+                for section, values in FORBIDDEN_WITH_PLATFORM.items()
+                for key in values
+            ):
                 host_name = self.task_remote_mgr.eval_host(
                     rtconfig['remote']['host']
                 )
             else:
                 platform_name = self.task_remote_mgr.eval_platform(
                     rtconfig['platform']
                 )
+
         except PlatformError as exc:
             itask.waiting_on_job_prep = False
             itask.summary['platforms_used'][itask.submit_num] = ''

tests/integration/test_task_job_mgr.py

@@ -258,3 +258,38 @@ async def test_poll_job_deleted_log_folder(
     assert log_filter(
         logging.ERROR, f"job log directory {job_id} no longer exists"
     )
+
+
+async def test__prep_submit_task_job_impl_handles_all_old_platform_settings(
+    flow: Fixture,
+    scheduler: Fixture,
+    start: Fixture,
+    mock_glbl_cfg: Fixture,
+):
+    """Ensure that if the old host/batch system settings
+    are set that task job manager will wait for the any hostname to be
+    resolved.
+
+    https://github.com/cylc/cylc-flow/pull/6990
+    """
+    mock_glbl_cfg(
+        'cylc.flow.platforms.glbl_cfg',
+        '''
+            [platforms]
+                [[bakery]]
+                    hosts = localhost
+                    job runner = loaf
+        ''',
+    )
+    id_ = flow({
+        "scheduling": {"graph": {"R1": "a"}},
+        "runtime": {"a": {"job": {"batch system": "loaf"}}}
+    })
+
+    schd = scheduler(id_, run_mode='live')
+    async with start(schd):
+        task_a = schd.pool.get_tasks()[0]
+        with suppress(FileExistsError):
+            schd.task_job_mgr._prep_submit_task_job(task_a)
+
+        assert task_a.platform['name'] == 'bakery'