AAP-90120: add _mesh_all_ready_nodes_visible() to block peer-judgment
while KnownConnectionCosts is empty after a controller restart. Gate
fails open when mesh_status is unavailable. Eliminates double receptor
status call per heartbeat.
AAP-90121: wrap each instance's reap+mark_offline block in
advisory_lock('task_manager_lock', wait=False) so the task manager
cannot dispatch to a node being concurrently marked offline. Extracts
_reap_and_mark_lost_instance() helper.
Tests: rewrite new test classes to use real Django DB (functional/)
instead of MagicMock objects.
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Signed-off-by: Hui Song <hsong@redhat.com>
Co-authored-by: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>
