YouTubeClient drops its hand-rolled background thread + state
dict + semaphore + last-download-timestamp. download() now
delegates to engine.worker.dispatch with _download_sync as the
impl callable; YouTube-specific record fields (video_id, url,
title) merge into the engine record via extra_record_fields.
Engine wires itself in via plugin.set_engine(engine) callback
on register_plugin. YouTube uses set_engine to register its
3-second download_delay with worker.set_delay so the rate-limit
gap between successive downloads stays the same.
Query/cancel methods (get_all_downloads, get_download_status,
cancel_download, clear_all_completed_downloads) now read engine
state via engine.iter_records_for_source / get_record /
update_record / remove_record. Net: ~120 LOC of thread+state
boilerplate removed from youtube_client.py.
Phase A pinning tests updated to assert engine state instead of
client.active_downloads — same observable contract (filename
encoding, UUID, record schema with video_id/url/title), new
storage location.
Suite still green (2025 passed). Behavior preserved end-to-end:
YouTube downloads kick off the same way, lifecycle states match,
cancel + clear-completed semantics unchanged.
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