Import speed: downloads pause ALL enrichment workers, discovery pauses the contention five

Measured during a live album download: ~4m15s per track in post-processing
(normal is ~20s), with the time vanishing silently inside embed_source_ids —
up to 5 MusicBrainz calls per track crawling against a degraded musicbrainz.org
while the MB enrichment worker kept eating the same ~1 req/s per-IP budget.
Only Spotify/Last.fm/Genius were in the yield set; MusicBrainz, Deezer, iTunes,
Discogs etc. kept grinding through downloads.

Policy (new core/enrichment/yield_policy, tested):
- downloads active  -> ALL enrichment workers yield (post-processing touches
  every metadata source). listening-stats (local-only) and repair
  (user-scheduled) intentionally keep running.
- discovery active  -> the API-contention five yield (spotify/itunes/deezer/
  discogs/hydrabase) — discovery never paused anything before, despite the
  pause helper literally defaulting to label='discovery'.
- user overrides and user-paused bookkeeping keep their existing semantics;
  the dashboard yield_reason label now says WHICH foreground work caused it.

Observability (the 4-minute silence can never come back):
- every source lookup is timed; >2s logs a warning NAMING the source and
  duration (core/metadata/source.py _call_source_lookup)
- the pipeline always logs "Metadata enhancement took X.Xs" per track

7 policy tests (incl. the motivating case: MB yields to downloads, keeps
running during discovery); 277 pipeline/enrichment tests pass.

core/enrichment/yield_policy.py

@@ -0,0 +1,57 @@
+"""Enrichment-worker yield policy: who pauses while the user's foreground
+work is running.
+
+Background enrichment workers share external API budgets with the foreground
+pipelines — most painfully MusicBrainz (~1 req/s per IP), where a worker
+grinding through the library can starve the import pipeline's per-track
+lookups into multi-minute crawls (measured: ~4m15s/track vs the normal ~20s).
+
+Policy (set with Boulder, 2026-06-06):
+  - downloads active   -> EVERYTHING yields (post-processing touches every
+                          metadata source: MusicBrainz, Spotify, iTunes,
+                          Deezer, Discogs, Last.fm, Genius, ...)
+  - discovery active   -> the API-contention five yield (discovery hammers
+                          the track-matching sources only)
+Workers the user explicitly resumed mid-yield are honored upstream (the
+override set lives in web_server's loop, as does the user-paused bookkeeping).
+"""
+
+from __future__ import annotations
+
+from typing import Optional
+
+# Everything that yields during active downloads. listening-stats (talks only
+# to the local media server) and repair (user-scheduled job runner, not a
+# background API drip) intentionally keep running.
+ALL_YIELD_WORKERS = (
+    'musicbrainz', 'audiodb', 'discogs', 'deezer',
+    'spotify-enrichment', 'itunes-enrichment', 'lastfm-enrichment',
+    'genius-enrichment', 'tidal-enrichment', 'qobuz-enrichment',
+    'amazon-enrichment', 'similar_artists', 'hydrabase', 'soulid',
+)
+
+# The sources discovery contends with (track matching APIs).
+API_CONTENTION_WORKERS = frozenset({
+    'spotify-enrichment', 'itunes-enrichment', 'deezer', 'discogs', 'hydrabase',
+})
+
+# Discovery state phases that mean "nothing running" (idle or terminal).
+_INACTIVE_PHASES = frozenset({'', 'idle', 'discovered', 'error', 'failed', 'cancelled'})
+
+
+def worker_yield_reason(name: str, downloading: bool, discovering: bool) -> Optional[str]:
+    """Why ``name`` should be paused right now, or None to run.
+    Downloads outrank discovery so the label reflects the stronger cause."""
+    if name not in ALL_YIELD_WORKERS:
+        return None
+    if downloading:
+        return 'downloads'
+    if discovering and name in API_CONTENTION_WORKERS:
+        return 'discovery'
+    return None
+
+
+def discovery_state_active(state: dict) -> bool:
+    """True when a per-playlist discovery state dict represents live work."""
+    phase = str((state or {}).get('phase', '') or '').lower()
+    return phase not in _INACTIVE_PHASES

core/imports/pipeline.py

@@ -734,7 +734,12 @@ def post_process_matched_download(context_key, context, file_path, runtime, meta
                 )
             else:
                 logger.info("[Metadata Input] album_info: None (single track)")
+            _enhance_started = time.time()
             enhance_file_metadata(file_path, context, artist_context, album_info, runtime=metadata_runtime)
+            # The enhancement block is the pipeline's biggest variable cost
+            # (external source lookups) and used to be a silent multi-minute
+            # gap in the log — always say how long it took.
+            logger.info(f"Metadata enhancement took {time.time() - _enhance_started:.1f}s")
         except Exception as meta_err:
             import traceback
             pp_logger.info(f"[inner] Metadata enhancement FAILED for {context_key}: {meta_err}\n{traceback.format_exc()}")

core/metadata/source.py

@@ -90,11 +90,19 @@ def _bounded_cache_set(cache, key, value, max_entries: int) -> None:
 
 
 def _call_source_lookup(label: str, func, *args, **kwargs):
+    # Timed: these lookups are where import post-processing silently stalls
+    # when an upstream is sick or contended (measured 4m+/track against a
+    # degraded MusicBrainz). A slow source must NAME itself in the log.
+    started = time.time()
     try:
         return func(*args, **kwargs)
     except _SOURCE_NETWORK_EXCEPTIONS as exc:
         logger.warning("%s lookup failed (network): %s", label, exc)
         return None
+    finally:
+        elapsed = time.time() - started
+        if elapsed > 2.0:
+            logger.warning("%s lookup took %.1fs — upstream slow or contended", label, elapsed)
 
 
 SOURCE_TAG_CONFIG = {

tests/test_yield_policy.py

@@ -0,0 +1,60 @@
+"""Tests for the enrichment-worker yield policy (downloads/discovery contention)."""
+
+from __future__ import annotations
+
+from core.enrichment.yield_policy import (
+    ALL_YIELD_WORKERS,
+    API_CONTENTION_WORKERS,
+    discovery_state_active,
+    worker_yield_reason,
+)
+
+
+def test_downloads_pause_everything():
+    for name in ALL_YIELD_WORKERS:
+        assert worker_yield_reason(name, downloading=True, discovering=False) == 'downloads'
+
+
+def test_discovery_pauses_only_the_contention_five():
+    for name in ALL_YIELD_WORKERS:
+        reason = worker_yield_reason(name, downloading=False, discovering=True)
+        if name in API_CONTENTION_WORKERS:
+            assert reason == 'discovery', name
+        else:
+            assert reason is None, name
+
+
+def test_downloads_outrank_discovery_for_the_label():
+    assert worker_yield_reason('spotify-enrichment', True, True) == 'downloads'
+
+
+def test_idle_pauses_nothing():
+    for name in ALL_YIELD_WORKERS:
+        assert worker_yield_reason(name, False, False) is None
+
+
+def test_unknown_and_excluded_workers_never_yield():
+    # listening-stats (local media server only) and repair (user-scheduled job
+    # runner) intentionally keep running through downloads.
+    for name in ('listening-stats', 'repair', 'definitely-not-a-worker'):
+        assert worker_yield_reason(name, True, True) is None
+
+
+def test_musicbrainz_yields_for_downloads_not_discovery():
+    # The case that motivated all of this: the MB worker starving the import
+    # pipeline's per-track lookups (~4m15s/track measured). It must yield to
+    # downloads — but keep running during discovery, which doesn't use MB.
+    assert worker_yield_reason('musicbrainz', True, False) == 'downloads'
+    assert worker_yield_reason('musicbrainz', False, True) is None
+
+
+def test_discovery_state_active_phases():
+    assert discovery_state_active({'phase': 'discovering'})
+    assert discovery_state_active({'phase': 'Matching tracks...'})
+    assert not discovery_state_active({'phase': 'idle'})
+    assert not discovery_state_active({'phase': ''})
+    assert not discovery_state_active({'phase': 'discovered'})
+    assert not discovery_state_active({'phase': 'error'})
+    assert not discovery_state_active({'phase': 'cancelled'})
+    assert not discovery_state_active({})
+    assert not discovery_state_active(None)

web_server.py

@@ -34911,6 +34911,23 @@ def _has_active_downloads():
 # Track whether we auto-paused workers so we only resume ones we paused (not user-paused ones)
 _download_auto_paused = set()
 _download_yield_override = set()  # Workers the user explicitly resumed during downloads — don't re-pause
+_auto_yield_cause = {}            # name -> 'downloads' | 'discovery' (status label for the UI)
+
+
+def _has_active_discovery():
+    """True while any playlist discovery is actively running (any platform)."""
+    from core.enrichment.yield_policy import discovery_state_active
+    try:
+        for states in (tidal_discovery_states, qobuz_discovery_states,
+                       deezer_discovery_states, youtube_playlist_states,
+                       beatport_chart_states, listenbrainz_playlist_states,
+                       spotify_public_discovery_states, itunes_link_discovery_states):
+            for state in list(states.values()):
+                if discovery_state_active(state):
+                    return True
+    except Exception as e:
+        logger.debug("active discovery check failed: %s", e)
+    return False
 
 
 # ---------------------------------------------------------------------------
@@ -35114,41 +35131,46 @@ def _emit_enrichment_status_loop():
         'repair': lambda: repair_worker,
     }
 
-    # Workers to auto-pause during downloads (rate-limit sensitive services)
+    # Yield policy: downloads pause EVERYTHING (post-processing touches every
-    yield_workers = {
+    # metadata source — measured 4m+/track when MusicBrainz contended with its
-        'spotify-enrichment': lambda: spotify_enrichment_worker,
+    # own worker); discovery pauses the API-contention five. The name lists +
-        'lastfm-enrichment': lambda: lastfm_worker,
+    # decision live in core.enrichment.yield_policy (tested); this loop owns
-        'genius-enrichment': lambda: genius_worker,
+    # the pause/resume side effects and the user-override bookkeeping.
-    }
+    from core.enrichment.yield_policy import ALL_YIELD_WORKERS, worker_yield_reason
+    yield_workers = {name: workers[name] for name in ALL_YIELD_WORKERS if name in workers}
 
     while not globals().get('IS_SHUTTING_DOWN', False):
         socketio.sleep(2)
 
-        # Auto-pause/resume rate-limited workers during downloads
+        # Auto-pause/resume workers while foreground work runs
         try:
             downloading = _has_active_downloads()
-            if not downloading:
+            discovering = _has_active_discovery()
-                _download_yield_override.clear()  # Reset overrides when downloads finish
+            if not downloading and not discovering:
+                _download_yield_override.clear()  # Reset overrides when work finishes
             for name, get_w in yield_workers.items():
                 w = get_w()
-                if w is None:
+                if w is None or not hasattr(w, 'paused'):
                     continue
-                if downloading and not w.paused and name not in _download_yield_override:
+                reason = worker_yield_reason(name, downloading, discovering)
+                if reason and not w.paused and name not in _download_yield_override:
                     w.paused = True
                     _download_auto_paused.add(name)
-                    logger.debug(f"Auto-paused {name} during active downloads")
+                    _auto_yield_cause[name] = reason
-                elif not downloading and name in _download_auto_paused:
+                    logger.debug(f"Auto-paused {name} during active {reason}")
+                elif not reason and name in _download_auto_paused:
                     # Don't override an explicit user pause. If config says the worker
                     # was paused via the UI, leave it paused and just drop the auto-pause
                     # marker so the next auto-pause/resume cycle behaves normally.
                     config_key = f"{name.replace('-', '_')}_paused"
                     user_paused = config_manager.get(config_key, False)
                     _download_auto_paused.discard(name)
+                    _auto_yield_cause.pop(name, None)
                     if not user_paused:
                         w.paused = False
-                        logger.debug(f"Auto-resumed {name} after downloads finished")
+                        logger.debug(f"Auto-resumed {name} after foreground work finished")
                     else:
-                        logger.debug(f"Downloads finished but {name} remains paused by user")
+                        logger.debug(f"Foreground work finished but {name} remains paused by user")
         except Exception as e:
             logger.debug(f"Error in download-yield check: {e}")
 
@@ -35158,9 +35180,9 @@ def _emit_enrichment_status_loop():
                 if worker is None:
                     continue
                 status = worker.get_stats()
-                # Flag workers that were auto-paused for downloads
+                # Flag workers that were auto-paused for foreground work
                 if name in _download_auto_paused:
-                    status['yield_reason'] = 'downloads'
+                    status['yield_reason'] = _auto_yield_cause.get(name, 'downloads')
                 socketio.emit(f'enrichment:{name}', status)
             except Exception as e:
                 logger.debug(f"Error emitting {name} status: {e}")