From 2f6caf65bbdec930cb404ae9e1ecdcf6a003b608 Mon Sep 17 00:00:00 2001
From: Tim Pillinger <26465611+wxtim@users.noreply.github.com>
Date: Tue, 27 Aug 2024 14:25:52 +0100
Subject: [PATCH] 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.

---
 changes.d/6330.fix.md                  |  1 +
 cylc/flow/task_job_mgr.py              |  6 ++--
 tests/integration/test_task_job_mgr.py | 49 ++++++++++++++++++++++++++
 3 files changed, 54 insertions(+), 2 deletions(-)
 create mode 100644 changes.d/6330.fix.md

diff --git a/changes.d/6330.fix.md b/changes.d/6330.fix.md
new file mode 100644
index 00000000000..190a9637a93
--- /dev/null
+++ b/changes.d/6330.fix.md
@@ -0,0 +1 @@
+Fix bug where broadcasting failed to change platform selected after host selection failure.
\ No newline at end of file
diff --git a/cylc/flow/task_job_mgr.py b/cylc/flow/task_job_mgr.py
index 185966ff12d..9eec82970be 100644
--- a/cylc/flow/task_job_mgr.py
+++ b/cylc/flow/task_job_mgr.py
@@ -306,10 +306,12 @@ 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:
diff --git a/tests/integration/test_task_job_mgr.py b/tests/integration/test_task_job_mgr.py
index 9265f2198dc..48a49eb30aa 100644
--- a/tests/integration/test_task_job_mgr.py
+++ b/tests/integration/test_task_job_mgr.py
@@ -187,3 +187,52 @@ async def test__prep_submit_task_job_impl_handles_execution_time_limit(
             schd.task_job_mgr._prep_submit_task_job(
                 schd.workflow, task_a)
         assert not task_a.summary.get('execution_time_limit', '')
+
+
+async def test_broadcast_platform_change(
+    mock_glbl_cfg,
+    flow,
+    scheduler,
+    start,
+    log_filter,
+):
+    """Broadcast can change task platform.
+
+    Even after host selection failure.
+
+    see https://github.com/cylc/cylc-flow/issues/6320
+    """
+    mock_glbl_cfg(
+        'cylc.flow.platforms.glbl_cfg',
+        '''
+            [platforms]
+                [[foo]]
+                    hosts = food
+        ''')
+
+    id_ = flow({
+        "scheduling": {"graph": {"R1": "mytask"}},
+        # Platform = None doesn't cause this issue!
+        "runtime": {"mytask": {"platform": "localhost"}}})
+
+    schd = scheduler(id_, run_mode='live')
+
+    async with start(schd) as log:
+        # Change the task platform with broadcast:
+        schd.broadcast_mgr.put_broadcast(
+            ['1'], ['mytask'], [{'platform': 'foo'}])
+
+        # Simulate prior failure to contact hosts:
+        schd.task_job_mgr.task_remote_mgr.bad_hosts = {'food'}
+
+        # Attempt job submission:
+        schd.task_job_mgr.submit_task_jobs(
+            schd.workflow,
+            schd.pool.get_tasks(),
+            schd.server.curve_auth,
+            schd.server.client_pub_key_dir)
+
+        # Check that task platform hasn't become "localhost":
+        assert schd.pool.get_tasks()[0].platform['name'] == 'foo'
+        # ... and that remote init failed because all hosts bad:
+        assert log_filter(log, contains="(no hosts were reachable)")