Three findings from a final review pass:
1. **Worker clobbered Cancelled with Errored when impl returned
None / raised mid-cancel.** The legacy per-client thread workers
each had a guard (``if state != 'Cancelled': state = 'Errored'``);
the shared worker dropped it. Fix: new ``_mark_terminal`` helper
in BackgroundDownloadWorker reads current state before writing
the terminal one and leaves Cancelled alone. SoundCloud test
updated back to the strict Cancelled-only assertion (had been
loosened to accept Errored as a workaround). Two new pinning
tests catch the regression.
2. **Dead code in engine.py.** ``find_record`` and
``iter_all_records`` had no production callers — only tests.
Removed them. Concurrent-add stress test rewritten to use the
per-source iterator that's actually in use.
3. **Silent ``except Exception: pass`` in cross-source query
methods.** Faithful to legacy behavior (one source failing
shouldn't take down aggregation) but Cin's standard is "log
even when you swallow." Each silent-swallow site now logs at
debug level so the source name + exception are inspectable
without adding warning-level noise.
Suite still green (2049 passed).
`engine.search_with_fallback(query, source_chain, ...)` walks the
chain in order, skips unconfigured / unregistered plugins,
swallows per-source exceptions, and returns the first non-empty
(tracks, albums) tuple. Replaces orchestrator's hand-rolled
hybrid search loop.
`engine.download_with_fallback(username, filename, file_size,
source_chain)` falls through the chain when a source returns
None / raises. Username hint promotes a matching source-chain
entry to head of order. NOT yet wired into orchestrator.download
— today's username comes from a search result and represents
the user's explicit source pick, so silently falling through
would override their choice. Engine method is available for
future callers that want fallback semantics
(search_and_download_best, automation).
Orchestrator gains _resolve_source_chain helper that builds
the ordered list (hybrid_order config, falling back to legacy
primary/secondary pair). Orchestrator.search hands chain off
to engine.search_with_fallback for hybrid mode.
8 new tests pin the fallback semantics: chain ordering,
unconfigured-skip, exception-continue, empty-when-exhausted,
username-hint promotion. Suite still green (2050 passed).
`core/download_engine/rate_limit.py` introduces a per-source
policy declaration: download_concurrency + download_delay_seconds.
Plugins declare via `RATE_LIMIT_POLICY` class attribute or a
`rate_limit_policy()` method.
Engine applies the declared policy to engine.worker at
register_plugin time — set_concurrency + set_delay get pushed
in automatically. Plugins without a declaration get the
conservative default (1 / 0). The set_engine callback fires
AFTER policy registration so config-driven sources (YouTube
reads user-tunable youtube.download_delay) can override.
Plan doc updated to reflect Phase D skip (search code is 90%
source-specific, not 60% — lifting it would be lossy or
bloated).
Pure additive — no plugin migrated yet. 8 tests pin the
resolution priority + engine wire-up + override semantics.
Suite still green (327 download tests).
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.
`BackgroundDownloadWorker` lives on the engine and owns the
boilerplate every streaming download client currently
hand-rolls: thread spawn, per-source semaphore, rate-limit
delay, state lifecycle (Initializing → InProgress → Completed
or Errored), exception capture.
Plugins provide only the atomic download op (`impl_callable`).
Per-source rate-limit policy (concurrency, delay) is configured
on the worker via `set_concurrency` / `set_delay`. Source-
specific record fields merge in via `extra_record_fields` so
existing consumer code that reads `video_id`, `track_id`,
`permalink_url`, etc. keeps working post-migration. Username
slot supports override (Deezer's legacy `'deezer_dl'`).
Phase C1 scope: worker exists. No client migrated yet — C2-C7
migrate sources one at a time, each gated by the Phase A
pinning tests so per-source contract drift fails fast.
10 new tests pin the worker contract: UUID id format, initial
record shape, extra-fields merge, username override, state
transitions on success / impl-returns-None / impl-raises,
semaphore serialization (default + parallel), rate-limit
delay between successive downloads.
Suite still green (308 download tests). Pure additive.
`DownloadEngine` grows async query methods that wrap plugin
iteration: `get_all_downloads` (concatenates every plugin's
active downloads), `get_download_status` (first plugin to
recognize the id wins), `cancel_download` (with source-hint
routing — streaming sources go direct, unknown hints route to
Soulseek as peer username), and `clear_all_completed_downloads`
(skips unconfigured plugins).
Code moved from the orchestrator's hand-iterated loops into the
engine. Orchestrator delegation comes in B3 — for B2 the engine
methods exist but nothing calls them yet.
Per-plugin behavior preserved verbatim (defensive `try ... except`
swallows per-iteration, unconfigured-skip on clear, source-hint
routing semantics). Phase A pinning tests + 8 new engine query
tests catch any drift.
Pure additive — zero behavior change for users.
`core/download_engine/` package with the engine class that will own
cross-source state, threading, search retry, rate-limits, and
fallback chains. Orchestrator constructs an engine and registers
each plugin with it.
Phase B1 scope: skeleton only. Engine stores active_downloads
records keyed by (source, download_id), provides thread-safe
add/update/remove/iterate primitives, and holds plugin references
for later phases. NOT on any code path yet — pure additive
scaffolding so subsequent commits can introduce engine-driven
behavior one piece at a time without a big-bang switchover.
15 new tests pin the engine's state-storage contract: shallow-copy
reads, partial-patch updates, no-op-on-missing semantics,
per-source iteration, id-only find, concurrent-add safety.
Suite still 290 (download subset) green. Zero behavior change.
