[InstaZDLL]

Summary

Step C of the enrichment plan, which implements RFC-011. No surface changes: nothing reads the cache yet, so every Subsonic container answers what it answers today. D (#266) is the first reader.

• Settings (src/config.rs, MetadataSettings): WAVEFLOW_METADATA_PROVIDERS (empty, the default, is off), WAVEFLOW_METADATA_IMAGES, WAVEFLOW_METADATA_LASTFM_KEY, the two TTLs, the rate and the timeout. Naming lastfm without its key, an unknown name, or a name given twice refuses to boot. Scrobbling's Last.fm key does not switch this on, per decision 3. The key is redacted from Config's Debug.
• Two migrations in one file: metadata_artist and metadata_artist_similar, keyed on (provider, name_canonical), where name_canonical is the form artist.canonical_name stores. metadata_album waits for the RFC's open question. Portrait columns come with F.
• The engine (src/enrichment.rs). Enrichment::artist(name) returns what the cache holds, in the operator's order, stale rows included. For every configured provider with nothing fresh, it schedules a fill in the background and does not wait for it. Each provider's requests are spaced to the rate (a token bucket holding one token). Concurrent first visits for one name collapse onto one fill, and no more than MAX_IN_FLIGHT (64) fills wait or run at once: pacing refuses no request, so without that ceiling a page opening thousands of artists, or an outage that writes no row, would grow the queue without bound. Beyond the ceiling nothing is scheduled, and the next visit asks again. A hit stands for the TTL, a miss (Last.fm's code 6) for the miss TTL, and an unreached lookup writes no row. The writer gate is taken for the row write only, after the requests are over (Database::write_metadata_artist).
• What is logged: the provider and a verdict (answered HTTP 503, refused with code 10, timed out…), never a URL or a body.
• The outbound client moves out of scrobblers/ into src/outbound.rs, which both subsystems use. scrobblers::outbound_client keeps its signature as a thin wrapper.
• RFC-011's erratum: a dated Revised line, and marked corrections to decisions 4 (the key's normalisation, and the columns that shipped), 6 (only the desktop is told when a lookup lands) and 10 (the *ImageUrl fields stay absent). The Implemented by line now names the steps so far.
• docs/operations.md: how to switch enrichment on, what the server starts contacting, and what it stores and logs.

Only Last.fm exists as a provider. Deezer arrives with portraits in F, and the merge across providers is read in D.

Closes #265.

How I tested

• cargo test --all-features: every target passes, including the new enrichment target (8 tests against a fake Last.fm on a loopback listener) and config::tests::metadata_providers_are_read_strictly.
• cargo clippy --all-targets --all-features -- -D warnings, cargo fmt --all --check.
• Each test was made to fail under the change it guards, by an inversion applied and reverted mechanically:

test	inversion
an_empty_provider_list_asks_nothing_even_with_a_key	build a provider whenever a key is present
a_provider_that_knows_nothing_is_cached_as_a_miss_for_the_miss_ttl	give a miss the hit TTL
a_provider_that_cannot_be_reached_writes_no_row	write an unreached lookup as a miss
ten_concurrent_first_visits_make_one_request	disable the single-flight map
no_more_than_the_ceiling_of_fills_is_ever_queued	disable the ceiling
a_redirect_from_the_provider_is_not_followed	build the client with Policy::limited(10)
an_answer_over_the_ceiling_is_refused	lift MAX_RESPONSE_BYTES in the core, through a temporary [patch] onto a local checkout of bd8d360c
the_key_never_reaches_the_log	add the key to the unreached log line
metadata_providers_are_read_strictly	accept lastfm without a key

• The log test captures through a global subscriber at TRACE, the HTTP stack's own events included. A first version used a thread-scoped set_default, which kept two lines out of hundreds because tracing caches callsite interest for the whole process. The test now also asserts that both failure verdicts appear, so a capture that sees nothing cannot pass it.
• CI caught a flaw in that test on the first push (e4e3d94 fixes it). It asked about one artist twice, so the second engine found the first one's fresh row and sent nothing. The 503 it checked for was only in the log when another test in the binary had produced one. That happened locally and not in CI. Each mode now asks about its own artist. Run alone, the old version fails and the new one passes.
• Local CodeRabbit review: one finding, fixed. The fill's guard is now built before spawn, so a task dropped before its first poll still releases its key.

Summary by CodeRabbit

• Nouvelles fonctionnalités
  • Ajout de l’enrichissement facultatif des métadonnées d’artistes via Last.fm, avec récupération des biographies et des artistes similaires.
  • Les résultats sont mis en cache, y compris les absences de résultat. L’enrichissement est désactivé par défaut et nécessite une clé Last.fm pour être activé.
• Documentation
  • Ajout des instructions de configuration et de précisions sur les données, les requêtes et le comportement des clients.

[coderabbitai]

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📝 Walkthrough

Le serveur ajoute un enrichissement facultatif des métadonnées d’artistes via Last.fm. Il configure ce service, conserve ses réponses dans un cache SQLite et partage la création du client HTTP avec les scrobblers.

Changes: Enrichissement des métadonnées d’artistes

Layer / File(s)	Summary
Configuration et stockage du cache src/config.rs, migrations-v2/20261015000000_metadata_artist.sql, src/database.rs	La configuration ajoute les paramètres du service et reste désactivée par défaut. La migration crée les tables des réponses et des artistes similaires. La base expose les opérations de lecture et d’écriture du cache.
Client HTTP partagé et état applicatif src/outbound.rs, src/scrobblers/mod.rs, src/lib.rs	src/outbound.rs centralise la création du client HTTP. Les scrobblers délèguent à ce client. L’état applicatif initialise et expose le moteur d’enrichissement.
Remplissage Last.fm et validation src/enrichment.rs, tests/enrichment.rs, Cargo.toml, CLAUDE.md, docs/operations.md, docs/rfcs/RFC-011-metadata-enrichment.md	Le moteur renvoie les entrées en cache et programme le remplissage des entrées absentes ou expirées. Il distingue les réponses Hit, Miss et les erreurs sans écriture, et fusionne les remplissages simultanés. Les tests couvrent notamment le cache, les erreurs HTTP, les redirections, les réponses trop volumineuses et la journalisation. La documentation décrit la configuration et rectifie l’état d’implémentation du RFC.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant Appelant
  participant Enrichment
  participant Database
  participant LastFm
  Appelant->>Enrichment: Demander les métadonnées d’un artiste
  Enrichment->>Database: Lire les entrées du cache
  Enrichment-->>Appelant: Renvoyer les entrées disponibles
  Enrichment->>LastFm: Demander les informations et les artistes similaires
  LastFm-->>Enrichment: Renvoyer les réponses
  Enrichment->>Database: Enregistrer la réponse et les voisins

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Merge Risk: ⚪ Minimal · up to b68d9

The previously reported fill-limit race is fixed. No actionable merge-blocking risk remains after normal checks.

Security architecture risk: 🔵 Low · up to b68d9

The integration is disabled by default and has no current application reader. A fixed production destination, bounded background work, transactional cache updates, and preserved outbound restrictions limit immediate exposure. Provider-library internals and deployed recovery behavior were not fully verified.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The new cache is instance-wide rather than account-scoped, and provider requests use an operator-supplied metadata key. Once a reader exists, artist names can cross the provider boundary and cached facts can be shared across libraries. Catalogue visibility must be enforced where a reader resolves those names; the current PR does not supply that reader.

Trust Boundaries and Controls

• observed — The production constructor selects a fixed HTTPS endpoint rather than accepting a request-controlled destination. The injected HTTP client refuses redirects and environment proxies, and each provider request is enclosed by the configured caller-owned timeout. Custom endpoints remain available to compiled callers through the public constructor seam.

Resilience and Maintainability Implications

• observed — An upstream failure or incomplete two-request lookup performs no cache replacement, avoiding a long-lived negative answer manufactured from an outage. Successful answers and misses receive separate TTLs. Transactional parent-and-neighbor replacement prevents partial refresh data from becoming the committed cache state; abrupt-process recovery was not demonstrated.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 64 functions across 7 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Title check		Le titre décrit clairement l’ajout du cache de métadonnées et du moteur d’enrichissement. Il est concis et directement lié aux changements principaux.
Description check		La description couvre le résumé, les changements principaux, les tests exécutés, les décisions RFC et les points importants pour la revue. Elle utilise « How I tested » au lieu de « Test plan » et ome…
Linked Issues check		Les exigences codées de #265 sont couvertes. MetadataSettings désactive l’enrichissement par défaut et valide les fournisseurs, la clé Last.fm, les TTL, le débit et le délai. Les migrations ajoutent…
Out of Scope Changes check		Les changements restent dans le périmètre de #265. Le déplacement du client sortant sert directement l’enrichissement et le scrobbling existant. Les changements de configuration, de base de données, d…

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[coderabbitai]

🔇 Additional comments: 12

src/config.rs: 1

192-259: LGTM!

Also applies to: 1054-1112

migrations-v2/20261015000000_metadata_artist.sql: 1

1-53: LGTM!

src/database.rs: 1

1903-2031: LGTM!

src/outbound.rs: 1

1-47: LGTM!

src/scrobblers/mod.rs: 1

8-12: LGTM!

Also applies to: 122-125

src/lib.rs: 1

9-13: LGTM!

Also applies to: 81-83, 682-684, 703-703

tests/enrichment.rs: 1

1-435: LGTM!

Cargo.toml: 1

157-160: LGTM!

CLAUDE.md: 1

47-47: LGTM!

docs/operations.md: 1

172-212: LGTM!

docs/rfcs/RFC-011-metadata-enrichment.md: 1

4-15: LGTM!

Also applies to: 181-188, 223-232, 296-302, 451-460

src/enrichment.rs-284-300: 1

284-300: 🩺 Stability & Availability | 🟡 Minor | ⚡ Quick win

⚠️ Unverified finding
Verification ran but could not confirm this finding. It is shown for review, not as a verified issue.

Borner le nombre de fills en attente.

schedule crée une tâche pour chaque couple (provider, nom) absent de in_flight. Rien ne limite le nombre de couples distincts. Pace::wait réserve un créneau par appel et ne refuse jamais de créneau. Avec rate_per_sec = 4, une page qui ouvre 2 000 artistes distincts crée 2 000 tâches et 4 000 créneaux, soit environ 17 minutes de file d’attente. Chaque tâche garde sa clé dans in_flight pendant toute l’attente.

Pendant une panne du provider, aucune ligne n’est écrite. Chaque nouvelle visite remet donc ces fills en file. La file croît alors sans limite tant que les visites continuent. Ce chemin n’est pas encore atteint aujourd’hui, car la façade ne lit pas le cache (D, #266). Le premier lecteur rendra ce chemin actif.

Correction proposée : ne rien planifier au-delà d’un plafond. La visite suivante redemandera le fill.

Correctif proposé

     fn schedule(self: Arc<Self>, index: usize, name: &str, canonical: &str) {
         let key = (self.sources[index].provider, canonical.to_owned());
+        // Au-delà du plafond, on ne planifie rien : la prochaine visite redemandera.
+        if self.in_flight.len() >= MAX_IN_FLIGHT {
+            return;
+        }
         if self.in_flight.insert(key.clone(), ()).is_some() {
             return;
         }

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Inline comments:
Review comments at @src/enrichment.rs:
- Around line 297-302: Rendez atomique la réservation de la limite dans
`schedule` en ajoutant un compteur atomique qui complète `in_flight` : réservez
une place avant l’insertion et libérez-la si la limite est dépassée ou si la clé
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