fix(worker): cancel abandoned slskd transfers + cap download fall-through

Two compounding causes of 'many downloads of one album from different sources' in
slskd: (1) the slskd client raised on timeout/error WITHOUT cancelling the transfer
it enqueued, so as the pipeline fell through to other peers each left a transfer
running; (2) the incomplete-download fall-through was uncapped, grinding through
every candidate of a popular album (~200 Soulseek peers), amplified now that Qobuz
pacing pushes most jobs to Soulseek.

- SlskdClient.download now cancels its enqueued transfers (DELETE) on any abandon.
- The download loop caps at _MAX_DOWNLOAD_ATTEMPTS (6) candidates.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

worker/tests/test_incomplete_fallthrough.py

@@ -89,3 +89,37 @@ def test_all_sources_incomplete_still_needs_attention(conn, tmp_path):
     run_pipeline(conn, job_id, [_Qobuz(deliver=2), _Soulseek(deliver=1)], dest_root=str(tmp_path))
     assert _state_error(conn, job_id) == ("needs_attention", "incomplete download")
     assert not (tmp_path / "Radiohead" / "In Rainbows").exists()  # nothing imported
+
+
+class _CountingIncompleteQobuz:
+    """Offers `n` candidates that all download but come back incomplete; counts download calls."""
+    name = "qobuz"
+    tier = 0
+
+    def __init__(self, n):
+        self.n = n
+        self.calls = 0
+
+    def health(self):
+        return True
+
+    def search(self, target):
+        q = _HIRES
+        return [Candidate(source="qobuz", source_ref=f"r{i}", matched_artist=target.artist,
+                          matched_album=target.album, quality=q, track_count=3, source_tier=0)
+                for i in range(self.n)]
+
+    def download(self, candidate, dest, on_progress):
+        self.calls += 1
+        _stage(dest, 1)  # 1 of 3 → always incomplete
+        return DownloadResult(ok=True, path=dest, track_count=1)
+
+
+def test_fall_through_is_capped(conn):
+    from lyra_worker.pipeline import _MAX_DOWNLOAD_ATTEMPTS
+    a = _CountingIncompleteQobuz(n=20)
+    job_id = insert_request(conn, artist="Radiohead", album="In Rainbows")
+    claim_next(conn)
+    run_pipeline(conn, job_id, [a], dest_root="/tmp/lib-cap")
+    assert _state_error(conn, job_id) == ("needs_attention", "incomplete download")
+    assert a.calls == _MAX_DOWNLOAD_ATTEMPTS  # capped, not all 20 peers tried

worker/tests/test_slskd_cancel.py

@@ -0,0 +1,72 @@
+"""When an slskd download is abandoned (times out or errors), the enqueued transfers must be
+cancelled — otherwise, as the pipeline falls through to other peers for the same album, abandoned
+transfers pile up in slskd (many downloads of one album from different sources)."""
+import json
+
+import pytest
+
+import lyra_worker.adapters._slskd as slskd_mod
+from lyra_worker.adapters._slskd import SlskdClient
+
+
+class _Resp:
+    def __init__(self, payload=None):
+        self._p = payload if payload is not None else {}
+
+    def raise_for_status(self):
+        pass
+
+    def json(self):
+        return self._p
+
+
+class _FakeRequests:
+    """Enqueue OK; polling always reports the file still in progress (never completes); records
+    DELETEs so the test can assert the abandoned transfer was cancelled."""
+    def __init__(self, state="InProgress"):
+        self.deleted = []
+        self._poll = {"directories": [{"files": [
+            {"filename": "f1.flac", "size": 5, "state": state, "id": "t1"},
+        ]}]}
+
+    def post(self, url, **kw):
+        return _Resp({})
+
+    def get(self, url, **kw):
+        return _Resp(self._poll)
+
+    def delete(self, url, **kw):
+        self.deleted.append(url)
+        return _Resp({})
+
+
+def _client(tmp_path):
+    return SlskdClient({"slskd.url": "http://x", "slskd.api_key": "k",
+                        "slskd.downloadsDir": str(tmp_path)})
+
+
+_REF = json.dumps({"username": "peerA", "files": [{"filename": "f1.flac", "size": 5}]})
+
+
+def test_cancels_transfer_on_timeout(tmp_path, monkeypatch):
+    fake = _FakeRequests(state="InProgress")
+    monkeypatch.setattr(slskd_mod, "requests", fake)
+    monkeypatch.setattr(slskd_mod.time, "sleep", lambda _s: None)
+    monkeypatch.setattr(slskd_mod, "_XFER_POLLS", 2)
+
+    with pytest.raises(TimeoutError):
+        _client(tmp_path).download(_REF, str(tmp_path / "dest"), lambda _p: None)
+
+    assert any(u.endswith("/api/v0/transfers/downloads/peerA/t1") for u in fake.deleted), fake.deleted
+
+
+def test_cancels_transfer_on_error_state(tmp_path, monkeypatch):
+    fake = _FakeRequests(state="Completed, Errored")
+    monkeypatch.setattr(slskd_mod, "requests", fake)
+    monkeypatch.setattr(slskd_mod.time, "sleep", lambda _s: None)
+    monkeypatch.setattr(slskd_mod, "_XFER_POLLS", 5)
+
+    with pytest.raises(RuntimeError):
+        _client(tmp_path).download(_REF, str(tmp_path / "dest"), lambda _p: None)
+
+    assert any(u.endswith("/peerA/t1") for u in fake.deleted), fake.deleted