feat(scrobbling): give a listen a queue it can leave the server by

[InstaZDLL]

Summary

The first slice of RFC-010: the durable half of external scrobbling, with no network in it. A listen is queued where it is recorded, the queue has states, and a background drain walks it — but this server still makes no outbound request, because there is no adapter to register yet. ListenBrainz, the native routes and the CLI are the next PR.

Nothing changes for an account that has linked nothing, which is every account. Cargo.toml gains no outbound dependency.

Two tables, one dated migration

scrobble_link is one generation of one account's authorisation at one destination, and the row is the generation. Relinking inserts a new one rather than updating this. The case that forces it: twenty listens are waiting, the account unlinks Last.fm and links a different one. The pair (account, destination) is unchanged and the authorisation is not, so a queue keyed on that pair would send the twenty to the second profile. A partial unique index keeps at most one live generation while past ones stay readable. The secret is sealed by SecretBox under the instance key, exactly as the dedicated Subsonic password is.

scrobble_outbox carries an immutable envelope rather than a pointer at the track. Decision 2 was rewritten for this after external review, and it bites harder here than it looks: since #186 a member corrects titles and artists, and a correction rewrites track_participant — so reading the track at drain time would submit what it has become rather than what was played. play_event_id is ON DELETE SET NULL for the same reason: a track deleted between the listen and the send must not empty the row.

Where the enqueue happens, and what is never queued

Inside the transaction that writes play_event, under the same writer gate. A listen cannot be recorded without being queued, nor queued without being recorded — and claim_operation already makes a replayed scrobble write neither, so the idempotence comes along for free.

A now-playing is deliberately never queued. It has value only while it is true; delivering it ten minutes late would announce a track the listener left long ago. A track the server cannot name — no title, or no credited artist — is not queued either.

The drain knows five words and no providers

An adapter answers Accepted, Retryable, AuthBroken, PermanentReject or Ambiguous. That is what a verdict has to be rather than an HTTP status: Last.fm answers 200 carrying an application error in its body, and Maloja reports refusals inside its JSON, so reading the status line would take a failure for a success.

Ambiguous is terminal and never retried on the server's own initiative. The request may have been recorded before the connection broke; a network failure and a success whose acknowledgement was lost are indistinguishable, and a duplicate in a public listening history is worse than a gap. A person may then discard the entry or retry it — once. A retry adds a row pointing back at the ambiguous one instead of reopening it, because erasing the first would falsify the only trace explaining why the destination may hold that listen twice.

Six properties worth naming because they are easy to lose later:

• No writer gate is ever held across a submission.
• One listen, one request. A batch of fifty that comes back Ambiguous makes fifty listens uncertain at once, which decision 5 then forbids retrying.
• One submission is bounded in time, in the drain rather than in an adapter, so every adapter written later inherits the bound.
• One silent destination costs one entry a pass, not the whole batch.
• A row nothing can carry steps aside instead of holding its place.
• A row is claimed before a byte is emitted, so it cannot be sent twice — by two passes, or by a restart that found it still waiting.

What the review changed

CodeRabbit was run against every commit of this branch before it was pushed — seven CLI passes, each re-reviewing the fix for the last — and then the GitHub app reviewed the pull request itself, which is a different surface and found something the seven had not. Between them: thirteen distinct findings — eleven accepted and fixed, one declined, one deferred with its reasoning recorded. Each fix is its own commit, so the history reads as the argument it was.

The part worth a reviewer's attention is that three of the accepted findings were reported narrower than they were, and chasing them found something worse underneath.

The wait on a submission was bounded by nothing

A destination that accepts the connection and then goes quiet would hold the drain for the life of the process — and a stopped queue is exactly what degraded exists to report, so the failure arrived by the one route that also stopped degraded from ever being recomputed. Same class as the defect the same reviewer caught on #189.

Two things the review did not settle: the deadline is a deployment setting, not a constant (decision 9 had already put the outbound timeout there — WAVEFLOW_SCROBBLE_REQUEST_TIMEOUT_SECS, 30s), and an expired wait is Ambiguous, never Retryable — the request left, and what failed to come back is the answer.

…and bounding it was not enough

Reported later as a latency problem: a silent destination cost the deadline per entry, so one dead link made a pass take twenty-five minutes. The latency is the smaller half. Each of those rows becomes uncertain, and an uncertain row is a decision decision 13 lets a person make exactly once — so one outage would have turned into fifty irreversible manual choices. That is the accident the RFC refuses when it declines to offer a "retry everything" button, arriving from the other end.

A stalled_links set now mirrors broken_links. The entry that actually timed out stays Ambiguous; the ones behind it were never emitted, so they stay pending and come back next pass.

A row with no adapter kept its place at the head of the queue

Reported as "these rows consume batch capacity". The mechanism is worse: the row was skipped without spending an attempt and without advancing its date, so the next pass fetched it again, and again. A destination that does have an adapter was never reached — not late, never.

The suggested SQL filter sits badly against this repository's rule that sqlx takes static SQL only: the registered set is a DashMap filled after construction. So the row steps aside instead, deferred by one drain interval with attempts untouched.

A retry could be asked for without bound

retry_uncertain_scrobble checked only that the entry was uncertain under a live link. The original stays uncertain by design, so nothing in the row said it had been answered, and a second call queued a second copy. The exactly-once index could not help: its predicate is WHERE retry_of IS NULL.

This is the one path in the whole design that manufactures duplicates on demand. Now shut twice: a NOT EXISTS clause so the refusal is an ordinary 404, and a unique index on retry_of so the schema holds it.

A row was not claimed before being emitted

The review framed this as two overlapping drain passes — narrower than it sounds, since the ticker awaits each pass. But looking for it turned up a second window the review did not name: between a submission leaving and its verdict being committed, the process can stop, and a row left pending there is simply sent again at the next boot. A duplicate arriving with nobody having chosen it.

One mechanism closes both. A row moves pending → sending before a byte is emitted, so exactly one caller can take it; and a row still sending well past the outbound deadline belongs to a process that stopped mid-flight, which is uncertain by decision 5.

Two things that were latent, and cheap only now

• A destination's name was two words on the wire and one everywhere else. rename_all = "snake_case" serialised ListenBrainz as listen_brainz, against the listenbrainz that as_str, FromStr and the CHECK constraint all use — so the link-state route would have published a name a client could read back and not send in.
• scrobble_outbox.id was a sequential integer, and the two gestures of decision 13 took it as their public argument, against this repository's rule that public ids are UUIDs. public_id now exists and the rowid stays internal, where retry_of, the ordering and the jitter want it.

Neither was reachable yet, since the routes are not written. Both were fixed now because while this migration is unmerged a column costs one line, and the moment it merges it costs a second migration and a backfill.

The attempt cap covered a tenth of what its comment claimed

"Eight doublings from a minute carry a listen across most of a day" — eight gives eight submissions and seven waits, 60s through 3600s where the ceiling bites: two hours and three minutes.

The review offered both repairs; the sentence was the honest statement of what the cap is for, so the number moved to meet it. Giving up early buys nothing here — abandoning a listen is a permanent hole in someone's history, and unlike a retry after an ambiguous answer it risks no duplicate at all. Thirty submissions span a little over a day, and a unit test sums the schedule so the prose and the arithmetic cannot drift apart again.

A missing index, and a missing test

scrobble_outbox gains a plain index on play_event_id: that column is ON DELETE SET NULL, so deleting a play_event makes SQLite look for the rows pointing at it, and the only other index leading with it is partial and blind to retry rows. An ordinary rescan finding files gone cascades track → play_event → this lookup. And PermanentReject was the one verdict of five with no test.

A verdict was counted even when it wrote nothing

Found by the GitHub app's review of the PR, not by any of the seven CLI passes.

recover_stale_sending computes its cutoff before taking the writer gate, while settle_scrobble must wait for that gate before writing — a wait a long scan can stretch past the ninety-second margin. Since drain_scrobble_outbox is public, a manual or test pass can settle the row first. The UPDATE … WHERE state='sending' then touches nothing, and everything downstream carried on regardless: the counter incremented, reschedule_scrobble answered "will retry", and an accepted submission stamped last_success_at on the link.

Where the row ends up is defensible either way. The reporting is not: decision 12 says what the API shows is a state rather than a guess, and a counter describing a write that did not happen is a guess — the healthy it feeds most of all. settle_scrobble now answers whether the row moved and rolls back if it did not, every arm counts only what it wrote, and reschedule_scrobble gained a third word because there were three outcomes.

Declined, four times

Reformatting the partial indexes' WHERE onto their own lines (SQLFluff LT14). Re-rated across the passes, with the same absence of any semantic change.

Raised, and correct as it stands

Two behaviours were questioned and are documented rather than changed:

• unlink_scrobble leaves a row already in flight alone, where the AuthBroken arm had to settle its own. Deliberate: decision 4 finishes what is waiting, and a submission that has left is not waiting — calling it cancelled would be a lie about something that really was sent.
• If settle_scrobble fails after a submission has left, the pass aborts with the row still sending, and recovery later reads it as uncertain. That is the correct resting place, not an oversight, and the doc comment now says so.

Verification

Eighteen integration tests in a new scrobbling target, four unit tests, and fifteen inversions — each guard removed on its own, its test watched falling, every file restored byte-for-byte:

Removed	Test that fell
the now-playing guard	a_listen_is_queued_with_what_was_heard_and_a_now_playing_is_not
the refusal to submit an unnameable track	a_track_the_server_cannot_name_is_never_queued
the cancel on unlink	unlinking_leaves_its_queue_behind_and_a_new_link_does_not_inherit_it
the skip behind a broken authorisation	a_broken_authorisation_finishes_the_queue_rather_than_asking_again
the terminal uncertain state	an_ambiguous_answer_is_terminal_and_waits_for_a_person
the retry_of linkage	retrying_an_uncertain_entry_adds_to_the_history_instead_of_rewriting_it
the outbound deadline	a_destination_that_never_answers_does_not_hold_the_queue_forever
the stalled-link guard	a_silent_destination_costs_one_entry_a_pass_and_not_the_whole_queue
the deferral of an uncarryable row	a_destination_with_no_adapter_does_not_starve_one_that_has_it
the one-retry rule, both defences	an_ambiguous_entry_may_be_retried_once_and_not_twice
the mid-flight recovery	a_listen_interrupted_mid_flight_is_uncertain_rather_than_sent_again
the terminal refusal	a_refused_listen_is_not_offered_to_the_destination_again
the per-variant destination names	the_four_spellings_of_a_destination_agree
the raised attempt cap	the_default_attempt_cap_carries_a_listen_across_a_day_of_outage
the rows_affected check on a settled row	a_verdict_that_arrives_after_its_row_was_settled_is_not_counted

The inversions ran through a Bun script with absolute paths, printing whether the inverted text had actually reached the disk before the test result — an inversion that silently did nothing once produced a green "proof" here with no defect reintroduced.

Three are shaped deliberately rather than by simple deletion:

• The one-retry rule comes out twice at once. That property is held by a SQL guard and a unique index, so removing either alone leaves the test green with the other doing the work — and a green result there would say nothing about whether the test covers the behaviour, which is the whole question.
• The deadline is widened, not deleted, because deleting it makes the test hang instead of fail. The test bounds its own call at five seconds so the loss reads as a destination that never answers must not hold the drain: Elapsed(()).
• The deferral's UPDATE is made to match no row rather than removed, keeping its placeholder count at three so the failure is the one under examination and not a bind error.

Two properties have no inversion, and that is stated rather than papered over. The queue keying on a generation instead of on (account, destination) is held by the shape of the table — there is no column to remove. And naming a row by UUID instead of by rowid is a type change: reverting it does not fail a test, it stops the target compiling, and a compiler error is not evidence that a test covers a behaviour.

cargo fmt --all --check, cargo clippy --all-targets --all-features -- -D warnings and all eighteen test targets are green. main has not moved since this branch was cut, and its last three runs are green.

Four notes for a reviewer

The queue has no retention, deliberately, and not in this PR. Terminal rows — sent, rejected, abandoned, cancelled, discarded — are never purged, so the table grows for an active listener. The seventh review raised it and it is real. It is left out on the same reasoning that pulled public_id in, applied in reverse: retention needs no migration change, only a Config field and a purge task on the spawn_library_event_purge pattern, both of which cost exactly the same after this migration freezes as before. It is an absent feature rather than a defect in what is written, RFC-010 says nothing about it, and whatever shape it takes must never purge an uncertain row — that one is waiting on a person.

The review sequence had almost, but not quite, converged. The sixth pass raised two substantive claims that five earlier passes over the same code had not; the seventh was the first with no major finding and nothing at all against the Rust sources. That is an argument for reading this rather than trusting the count of passes.

The migration was edited in place across these commits. It is unmerged and this branch was never pushed before now, so the rule about immutable migrations is not in play — but anyone who checked out an intermediate commit and ran the server will have 20260913000000 applied under an older checksum, and the server will refuse to start until that row is cleared. A fresh database is unaffected.

The test accounts are inserted with a placeholder in password_hash rather than hashed from a literal. This target has no HTTP surface to log in to, and the repository's usual fixture password is what the main ruleset blocks a pull request over — the rule fires on every new occurrence, so avoiding one here costs nothing and saves a dismissal.

https://claude.ai/code/session_019coGCzcX775GmG9kYz8fft

Summary by CodeRabbit

• Nouvelles fonctionnalités

  • Ajout de la mise en file des écoutes pour le scrobbling externe.
  • Les informations d’écoute sont figées au moment de la soumission et associées aux destinations actives.
  • Ajout du suivi des connexions, tentatives et résultats de traitement.
  • Les soumissions incertaines peuvent être relancées ou abandonnées manuellement.

• Fiabilité

  • Les erreurs temporaires sont réessayées automatiquement selon des limites configurables.
  • Les secrets de connexion sont protégés lors de leur stockage.
  • Les entrées interrompues peuvent être récupérées automatiquement.

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📝 Walkthrough

Walkthrough

Le PR ajoute une file transactionnelle de scrobbling. Il persiste les liens et les écoutes, capture les soumissions terminées, draine les entrées selon les verdicts des destinations et expose une configuration dédiée. Des tests d’intégration couvrent les transitions principales.

Changes

Scrobbling durable

Layer / File(s)	Summary
Stockage et configuration migrations-v2/20260913000000_scrobble_outbox.sql, src/config.rs, src/services/mod.rs	Le schéma ajoute les tables scrobble_link et scrobble_outbox, leurs contraintes et leurs index. La configuration ajoute les limites de drainage et les variables d’environnement associées. DomainServices initialise le support du scrobbling.
Capture et gestion des liens src/services/scrobbling.rs, src/services/playback.rs	Les écoutes terminées sont mises en file dans la même transaction que play_event. Le module gère les liens, les secrets chiffrés, les états de file et les opérations manuelles sur les entrées uncertain.
Drain et transitions d’état src/services/scrobbling.rs, src/main.rs, docs/rfcs/RFC-010-external-scrobbling.md	Le serveur démarre un drain périodique. Le drain réclame les entrées, applique les verdicts, gère les expirations, les reprises, les destinations absentes et les autorisations invalides. Le RFC décrit l’implémentation livrée et la rétention des entrées terminales.
Validation et câblage du test tests/scrobbling.rs, src/services/scrobbling.rs, Cargo.toml	Les tests couvrent les verdicts, les délais, les reprises, les secrets, les générations de liens, la capture des métadonnées et les actions manuelles. Cargo ajoute la cible de test scrobbling.

Priority: ➖ Normal

Estimated code review effort: 4 (Complex) | ~60 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant PlaybackService
  participant SQLite
  participant ScrobbleDrain
  participant ScrobbleTarget
  PlaybackService->>SQLite: insère play_event et scrobble_outbox
  ScrobbleDrain->>SQLite: réclame une entrée échue
  ScrobbleDrain->>ScrobbleTarget: soumet l’enveloppe
  ScrobbleTarget-->>ScrobbleDrain: retourne un verdict
  ScrobbleDrain->>SQLite: finalise l’entrée et le lien

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Merge Risk: ⚪ Minimal · up to a4631

The durable scrobbling queue changes are covered by integration tests, including late verdict and recovery behavior. No unresolved merge-blocking risk is identified.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 79.37% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 63 functions across 6 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	Le titre décrit la mise en file des écoutes pour le scrobbling externe. La formulation est inhabituelle, mais elle reste liée au changement principal et permet d’identifier l’objectif de la PR.
Description check	✅ Passed	La description couvre le résumé, les changements, la stratégie de test et les points d’attention. Elle est très complète et cohérente avec la PR, même si elle n’utilise pas exactement les sections « C…

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Inline comments:
In `@src/services/scrobbling.rs`:
- Around line 908-917: Update settle_scrobble and reschedule_scrobble to inspect
rows_affected() and return a result distinguishing a persisted transition from
an already-recovered row. In drain_scrobble_outbox, do not increment verdict
counters or classify zero-row transitions as abandoned/retrying; preserve
uncertain, skip scrobble_link updates, and log the discrepancy.

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