Recommendation relevance, the rollback unit, and a version that names what shipped (#131)

internal/api/radio.go

@@ -101,10 +101,6 @@ func (h *handlers) handleRadio(w http.ResponseWriter, r *http.Request) {
 	candidates, err := recommendation.LoadCandidatesFromSimilarity(
 		r.Context(), q, user.ID, seedID,
 		h.recCfg.RecentlyPlayedHours, currentVec, exclude, limits,
-		// A fresh seed per request (#3889): radio is a new session each time
-		// and SHOULD draw differently. The system mixes are the surfaces that
-		// promise repeatability; this is not one of them.
-		strconv.FormatInt(time.Now().UnixNano(), 36),
 	)
 	if err != nil {
 		h.logger.Warn("api: radio: similarity-pool failed; falling back to whole-library", "err", err)

internal/db/dbq/recommendation.sql.go

@@ -829,7 +829,7 @@ similar_artists AS (
     JOIN seed_artist sa ON asim.artist_a_id = sa.artist_id
     WHERE asim.source = 'listenbrainz'
       AND t.id NOT IN (SELECT id FROM excluded_ids)
-    ORDER BY asim.score DESC, md5(t.id::text || $12::text)
+    ORDER BY asim.score DESC, random()
     LIMIT $6
 ),
 tag_overlap AS (
@@ -857,7 +857,7 @@ likes_overlap AS (
           WHERE t.id = gl.track_id
             AND trim(g_overlap.g) IN (SELECT tag FROM seed_tags)
       )
-    ORDER BY md5(gl.track_id::text || $12::text)
+    ORDER BY random()
     LIMIT $8
 ),
 taste_overlap AS (
@@ -884,7 +884,7 @@ coplay_artists AS (
     WHERE asim.source = 'user_cooccurrence'
       AND t.id NOT IN (SELECT id FROM excluded_ids)
       AND t.id <> $2
-    ORDER BY asim.score DESC, md5(t.id::text || $12::text)
+    ORDER BY asim.score DESC, random()
     LIMIT $11
 ),
 random_fill AS (
@@ -900,7 +900,7 @@ random_fill AS (
           UNION SELECT track_id FROM taste_overlap
           UNION SELECT track_id FROM coplay_artists
       )
-    ORDER BY md5(t.id::text || $12::text)
+    ORDER BY random()
     LIMIT $9
 )
 SELECT
@@ -949,7 +949,6 @@ type LoadRadioCandidatesV2Params struct {
 	Limit_5 int32
 	Limit_6 int32
 	Limit_7 int32
-	Column12 string
 }
 
 type LoadRadioCandidatesV2Row struct {
@@ -972,22 +971,8 @@ type LoadRadioCandidatesV2Row struct {
 //	enter the pool even when the similarity/random arms miss them; scored
 //	in Go via TasteMatch, so sim_score here is 0 pool-inclusion),
 //	$11 coplay_artists K (#1533 — tracks by artists co-played across the
-//	instance with the seed's artist; source='user_cooccurrence'),
-//	$12 order_seed (text) — see below.
+//	instance with the seed's artist; source='user_cooccurrence').
 //
-// $12 REPLACES `ORDER BY random()` IN FOUR ARMS (#3889). Those arms returned
-// a stable set only while their LIMIT exceeded the rows eligible for them: at
-// that point they returned all of them and the order stopped mattering,
-// because the caller sorts by track id before scoring. Below that threshold
-// they returned a random SUBSET, and two builds on the same day drew
-// different ones — so "daily determinism" held by accident, and only for
-// libraries smaller than the limits.
-//
-// md5(id || seed) keeps the intent — an arbitrary spread that changes when
-// the seed does — while making it reproducible for a given seed. The CALLER
-// decides what that means: system mixes pass a per-(user, day) string and get
-// the determinism they promise; radio passes a fresh value per request and
-// keeps varying, which is what a radio should do.
 // Returns same shape as LoadRadioCandidates plus similarity_score column.
 func (q *Queries) LoadRadioCandidatesV2(ctx context.Context, arg LoadRadioCandidatesV2Params) ([]LoadRadioCandidatesV2Row, error) {
 	rows, err := q.db.Query(ctx, loadRadioCandidatesV2,
@@ -1002,7 +987,6 @@ func (q *Queries) LoadRadioCandidatesV2(ctx context.Context, arg LoadRadioCandid
 		arg.Limit_5,
 		arg.Limit_6,
 		arg.Limit_7,
-		arg.Column12,
 	)
 	if err != nil {
 		return nil, err

internal/db/queries/recommendation.sql

@@ -45,22 +45,7 @@ WHERE t.id <> $2
 --   enter the pool even when the similarity/random arms miss them; scored
 --   in Go via TasteMatch, so sim_score here is 0 pool-inclusion),
 --   $11 coplay_artists K (#1533 — tracks by artists co-played across the
---   instance with the seed's artist; source='user_cooccurrence'),
---   $12 order_seed (text) — see below.
---
--- $12 REPLACES `ORDER BY random()` IN FOUR ARMS (#3889). Those arms returned
--- a stable set only while their LIMIT exceeded the rows eligible for them: at
--- that point they returned all of them and the order stopped mattering,
--- because the caller sorts by track id before scoring. Below that threshold
--- they returned a random SUBSET, and two builds on the same day drew
--- different ones — so "daily determinism" held by accident, and only for
--- libraries smaller than the limits.
---
--- md5(id || seed) keeps the intent — an arbitrary spread that changes when
--- the seed does — while making it reproducible for a given seed. The CALLER
--- decides what that means: system mixes pass a per-(user, day) string and get
--- the determinism they promise; radio passes a fresh value per request and
--- keeps varying, which is what a radio should do.
+--   instance with the seed's artist; source='user_cooccurrence').
 -- Returns same shape as LoadRadioCandidates plus similarity_score column.
 
 WITH
@@ -102,7 +87,7 @@ similar_artists AS (
     JOIN seed_artist sa ON asim.artist_a_id = sa.artist_id
     WHERE asim.source = 'listenbrainz'
       AND t.id NOT IN (SELECT id FROM excluded_ids)
-    ORDER BY asim.score DESC, md5(t.id::text || $12::text)
+    ORDER BY asim.score DESC, random()
     LIMIT $6
 ),
 tag_overlap AS (
@@ -130,7 +115,7 @@ likes_overlap AS (
           WHERE t.id = gl.track_id
             AND trim(g_overlap.g) IN (SELECT tag FROM seed_tags)
       )
-    ORDER BY md5(gl.track_id::text || $12::text)
+    ORDER BY random()
     LIMIT $8
 ),
 taste_overlap AS (
@@ -157,7 +142,7 @@ coplay_artists AS (
     WHERE asim.source = 'user_cooccurrence'
       AND t.id NOT IN (SELECT id FROM excluded_ids)
       AND t.id <> $2
-    ORDER BY asim.score DESC, md5(t.id::text || $12::text)
+    ORDER BY asim.score DESC, random()
     LIMIT $11
 ),
 random_fill AS (
@@ -173,7 +158,7 @@ random_fill AS (
           UNION SELECT track_id FROM taste_overlap
           UNION SELECT track_id FROM coplay_artists
       )
-    ORDER BY md5(t.id::text || $12::text)
+    ORDER BY random()
     LIMIT $9
 )
 SELECT

internal/playlists/system.go

@@ -276,17 +276,6 @@ func SetTasteConfig(c taste.Config) {
 	systemTasteConfig = c
 }
 
-// dailyOrderSeed is the value the randomised candidate arms order by (#3889).
-//
-// Per (user, day) so a same-day rebuild draws the SAME set — which is what
-// TestBuildSystemPlaylists_DailyNonceDeterminism asserts and what those arms
-// only ever achieved by accident before, when their limits happened to exceed
-// the eligible rows. It changes on the day boundary, so the mixes still move
-// daily.
-func dailyOrderSeed(userID pgtype.UUID, dateStr string) string {
-	return uuidStringPL(userID) + ":" + dateStr
-}
-
 func currentSongsLikeWeights() recommendation.ScoringWeights {
 	systemTuningMu.RLock()
 	defer systemTuningMu.RUnlock()
@@ -640,7 +629,6 @@ func produceForYou(
 			zeroVec,
 			seeds,
 			systemForYouSourceLimits(),
-			dailyOrderSeed(userID, dateStr),
 		)
 		if cerr != nil {
 			logger.Warn("system playlist: for-you candidates load failed for seed; continuing",
@@ -728,7 +716,6 @@ func produceSeedMixes(
 			recommendation.ScaleForLibrary(
 				recommendation.SongsLikeCandidateSourceLimits(), librarySize,
 			),
-			dailyOrderSeed(userID, dateStr),
 		)
 		if cerr != nil {
 			logger.Warn("system playlist: seed candidates load failed; skipping",

internal/playlists/you_might_like.go

@@ -102,7 +102,6 @@ func buildYouMightLike(
 	cands, err := recommendation.LoadCandidatesFromSimilarity(
 		ctx, q, userID, seed, 1, zeroVec,
 		[]pgtype.UUID{seed}, ymlLimits,
-		dailyOrderSeed(userID, dateStr),
 	)
 	if err != nil {
 		logger.Warn("you-might-like: candidate load failed; skipping",

internal/recommendation/candidates.go

@@ -139,41 +139,46 @@ func DefaultCandidateSourceLimits() CandidateSourceLimits {
 // would produce a short mix or none at all, and "no playlist" is a worse
 // answer than "a few tracks further from the seed than we would like".
 //
-// The seed-independent arms are trimmed hardest, because on this surface they
-// are noise: `taste_overlap` (tracks by the user's top taste artists) and
-// `random_fill` (any track not already in the pool) both carry
-// `0.0::float8 AS sim_score`, so nearly a third of the default pool had no
-// relationship to the seed at all.
+// DO NOT SHRINK AN ARM ORDERED BY UNSEEDED random(). This is the constraint
+// that shapes the numbers below, and it is not obvious from reading them.
 //
-// THESE TRIMS WERE BLOCKED UNTIL #3889. `likes_overlap` and `random_fill`
-// used to end in a bare `ORDER BY random()`, which made their output a stable
-// SET only while the limit exceeded the eligible rows — so SHRINKING them
-// changed pool membership between same-day rebuilds and broke daily
-// determinism. Those arms now order by md5(id || seed), so a smaller limit
-// takes a smaller but REPRODUCIBLE slice, and the trim is safe.
+// `likes_overlap` and `random_fill` both end in a bare `ORDER BY random()`
+// (recommendation.sql:118, :161) with no daily seed. Such an arm returns a
+// STABLE set only while its LIMIT exceeds the rows eligible for it — at that
+// point it returns all of them and the random order is irrelevant, because
+// the caller sorts by id before scoring. Drop the limit below the eligible
+// count and the arm starts returning a random SUBSET, which differs between
+// two builds on the same day.
 //
-// Reduced, never removed. Rule 131: the two seed-independent arms are the
-// tier-3 floor, and zeroing them would leave a seed with thin ListenBrainz
-// coverage producing a short mix or none at all. The weights (SimilarityWeight
-// 4.0, everything seed-independent demoted) keep them ranked last, so they
-// surface only when the closer tiers cannot fill the mix.
+// That is a real defect (#3889) rather than a quirk of this function, and it
+// bit here: cutting RandomFill to 10 broke
+// TestBuildSystemPlaylists_DailyNonceDeterminism, whose library is smaller
+// than the default limit and whose determinism was therefore accidental.
+// Growing an arm is always safe; only shrinking one is.
 //
-// likes_overlap is cut hardest of the tier-2 arms for a specific reason: its
-// SQL assigns a FLAT 0.6 sim_score (recommendation.sql) rather than measuring
-// anything. It is a collaborative signal wearing similarity's clothes, and a
-// raised SimilarityWeight amplifies it — if real ListenBrainz scores commonly
-// land below 0.6 it would outrank genuine matches. Halved pending the
-// fill-rate measurement in #3879; the honest fix is to stop it claiming a
-// similarity score it never computed.
+// So the seed-independent arms are trimmed only where the ordering is
+// deterministic: `taste_overlap` sorts by `tpa.weight DESC, t.id` and can be
+// cut, `random_fill` cannot. The reduction is consequently modest — and it
+// matters less than it looks, because the WEIGHTS are what demote sim_score-0
+// candidates now. The pool change biases the draw; the songs_like profile is
+// what actually keeps unrelated tracks out of the result.
+//
+// One arm is left alone that arguably should not be: `likes_overlap` assigns
+// a FLAT 0.6 sim_score (recommendation.sql:108) rather than measuring
+// anything — a collaborative signal wearing similarity's clothes, which a
+// raised SimilarityWeight amplifies. If real ListenBrainz scores commonly
+// land below 0.6 it will outrank genuine matches. It cannot be trimmed here
+// without the determinism fix landing first; the honest repair is to stop it
+// claiming a similarity score it never computed (#3879).
 func SongsLikeCandidateSourceLimits() CandidateSourceLimits {
 	return CandidateSourceLimits{
 		LBSimilar:     60, // tier 1 — doubled; the only arm that measures the seed
 		SimilarArtist: 40, // tier 2 — raised; growing is always safe
 		TagOverlap:    20, // tier 2
 		UserCoplay:    20, // tier 2
-		LikesOverlap:  10, // tier 2, halved — flat 0.6 sim_score, see above
-		TasteOverlap:  10, // tier 3 floor — halved, not removed
-		RandomFill:    10, // tier 3 floor — cut hard, never to zero
+		LikesOverlap:  20, // tier 2 — NOT trimmed: unseeded random(), see above
+		TasteOverlap:  10, // tier 3 floor — halved; deterministic ordering, safe
+		RandomFill:    30, // tier 3 floor — NOT trimmed: unseeded random(), see above
 	}
 }
 
@@ -182,10 +187,6 @@ func SongsLikeCandidateSourceLimits() CandidateSourceLimits {
 // likes-overlap / random fill) + dedup-by-max sim_score. Returns
 // []Candidate (same shape as LoadCandidates) so Shuffle is unchanged.
 //
-// orderSeed decides whether the randomised arms repeat their draw — see
-// Column12 below and #3889. Pass a stable per-(user, day) value where the
-// selection must be reproducible, and a varying one where it should not be.
-//
 // Caller (radio handler) falls back to LoadCandidates on error.
 func LoadCandidatesFromSimilarity(
 	ctx context.Context,
@@ -195,7 +196,6 @@ func LoadCandidatesFromSimilarity(
 	currentVector SessionVector,
 	exclude []pgtype.UUID,
 	limits CandidateSourceLimits,
-	orderSeed string,
 ) ([]Candidate, error) {
 	if exclude == nil {
 		exclude = []pgtype.UUID{}
@@ -212,12 +212,6 @@ func LoadCandidatesFromSimilarity(
 		Limit_5: int32(limits.RandomFill),
 		Limit_6: int32(limits.TasteOverlap),
 		Limit_7: int32(limits.UserCoplay),
-		// #3889. Four arms used to end in a bare ORDER BY random(), which made
-		// their output a stable SET only while the limit exceeded the eligible
-		// rows. They now order by md5(id || this), so the caller decides
-		// whether the draw repeats: a per-(user, day) seed for the system
-		// mixes that promise daily determinism, a fresh one per radio request.
-		Column12: orderSeed,
 	})
 	if err != nil {
 		return nil, err

internal/recommendation/candidates_v2_test.go

@@ -2,10 +2,6 @@ package recommendation
 
 import (
 	"context"
-	"fmt"
-	"reflect"
-	"sort"
-	"strings"
 	"testing"
 
 	"github.com/jackc/pgx/v5/pgtype"
@@ -54,7 +50,7 @@ func TestLoadCandidatesFromSimilarity_LBSimilarSourceContributes(t *testing.T) {
 	target := f.tracks[1]
 	helperLBSimilarity(t, f, seed.ID, target.ID, 0.85)
 	got, err := LoadCandidatesFromSimilarity(
-		context.Background(), f.q, f.user, seed.ID, 1, SessionVector{Seed: true}, nil, defaultLimits(), "test-seed",
+		context.Background(), f.q, f.user, seed.ID, 1, SessionVector{Seed: true}, nil, defaultLimits(),
 	)
 	if err != nil {
 		t.Fatalf("load: %v", err)
@@ -86,7 +82,7 @@ func TestLoadCandidatesFromSimilarity_SimilarArtistTracksContribute(t *testing.T
 	})
 	helperArtistSimilarity(t, f, seed.ArtistID, otherArtist.ID, 0.8)
 	got, err := LoadCandidatesFromSimilarity(
-		context.Background(), f.q, f.user, seed.ID, 1, SessionVector{Seed: true}, nil, defaultLimits(), "test-seed",
+		context.Background(), f.q, f.user, seed.ID, 1, SessionVector{Seed: true}, nil, defaultLimits(),
 	)
 	if err != nil {
 		t.Fatalf("load: %v", err)
@@ -110,7 +106,7 @@ func TestLoadCandidatesFromSimilarity_TagOverlapContributes(t *testing.T) {
 	helperSetTrackGenre(t, f, seed.ID, "Rock; Pop")
 	helperSetTrackGenre(t, f, target.ID, "Rock")
 	got, err := LoadCandidatesFromSimilarity(
-		context.Background(), f.q, f.user, seed.ID, 1, SessionVector{Seed: true}, nil, defaultLimits(), "test-seed",
+		context.Background(), f.q, f.user, seed.ID, 1, SessionVector{Seed: true}, nil, defaultLimits(),
 	)
 	if err != nil {
 		t.Fatalf("load: %v", err)
@@ -137,7 +133,7 @@ func TestLoadCandidatesFromSimilarity_LikesOverlapContributes(t *testing.T) {
 		t.Fatalf("like: %v", err)
 	}
 	got, err := LoadCandidatesFromSimilarity(
-		context.Background(), f.q, f.user, seed.ID, 1, SessionVector{Seed: true}, nil, defaultLimits(), "test-seed",
+		context.Background(), f.q, f.user, seed.ID, 1, SessionVector{Seed: true}, nil, defaultLimits(),
 	)
 	if err != nil {
 		t.Fatalf("load: %v", err)
@@ -159,7 +155,7 @@ func TestLoadCandidatesFromSimilarity_RandomFillReturnsTracks(t *testing.T) {
 	f := newFixture(t, 10) // 10 tracks; no similarity data
 	seed := f.tracks[0]
 	got, err := LoadCandidatesFromSimilarity(
-		context.Background(), f.q, f.user, seed.ID, 1, SessionVector{Seed: true}, nil, defaultLimits(), "test-seed",
+		context.Background(), f.q, f.user, seed.ID, 1, SessionVector{Seed: true}, nil, defaultLimits(),
 	)
 	if err != nil {
 		t.Fatalf("load: %v", err)
@@ -180,7 +176,7 @@ func TestLoadCandidatesFromSimilarity_ExcludeListRespected(t *testing.T) {
 	excluded := f.tracks[1].ID
 	got, err := LoadCandidatesFromSimilarity(
 		context.Background(), f.q, f.user, seed.ID, 1, SessionVector{Seed: true},
-		[]pgtype.UUID{excluded}, defaultLimits(), "test-seed",
+		[]pgtype.UUID{excluded}, defaultLimits(),
 	)
 	if err != nil {
 		t.Fatalf("load: %v", err)
@@ -196,7 +192,7 @@ func TestLoadCandidatesFromSimilarity_SeedAlwaysExcluded(t *testing.T) {
 	f := newFixture(t, 5)
 	seed := f.tracks[0]
 	got, err := LoadCandidatesFromSimilarity(
-		context.Background(), f.q, f.user, seed.ID, 1, SessionVector{Seed: true}, nil, defaultLimits(), "test-seed",
+		context.Background(), f.q, f.user, seed.ID, 1, SessionVector{Seed: true}, nil, defaultLimits(),
 	)
 	if err != nil {
 		t.Fatalf("load: %v", err)
@@ -226,7 +222,7 @@ func TestLoadCandidatesFromSimilarity_RecentlyPlayedExcluded(t *testing.T) {
 		t.Fatalf("play_event: %v", err)
 	}
 	got, err := LoadCandidatesFromSimilarity(
-		context.Background(), f.q, f.user, seed.ID, 1, SessionVector{Seed: true}, nil, defaultLimits(), "test-seed",
+		context.Background(), f.q, f.user, seed.ID, 1, SessionVector{Seed: true}, nil, defaultLimits(),
 	)
 	if err != nil {
 		t.Fatalf("load: %v", err)
@@ -246,7 +242,7 @@ func TestLoadCandidatesFromSimilarity_DedupTakesMaxScore(t *testing.T) {
 	helperSetTrackGenre(t, f, target.ID, "Rock")      // jaccard 1/1 = 1.0 from tag-overlap
 	helperLBSimilarity(t, f, seed.ID, target.ID, 0.5) // weaker LB signal
 	got, err := LoadCandidatesFromSimilarity(
-		context.Background(), f.q, f.user, seed.ID, 1, SessionVector{Seed: true}, nil, defaultLimits(), "test-seed",
+		context.Background(), f.q, f.user, seed.ID, 1, SessionVector{Seed: true}, nil, defaultLimits(),
 	)
 	if err != nil {
 		t.Fatalf("load: %v", err)
@@ -299,7 +295,7 @@ func TestLoadCandidatesFromSimilarity_TasteOverlapArm(t *testing.T) {
 	// Only the taste_overlap arm is enabled.
 	limits := CandidateSourceLimits{TasteOverlap: 10}
 	got, err := LoadCandidatesFromSimilarity(
-		ctx, f.q, f.user, seed.ID, 1, SessionVector{Seed: true}, nil, limits, "test-seed",
+		ctx, f.q, f.user, seed.ID, 1, SessionVector{Seed: true}, nil, limits,
 	)
 	if err != nil {
 		t.Fatalf("load: %v", err)
@@ -325,7 +321,7 @@ func TestLoadCandidatesFromSimilarity_EmptyLibrary_NoError(t *testing.T) {
 	f := newFixture(t, 1) // just the seed
 	seed := f.tracks[0]
 	got, err := LoadCandidatesFromSimilarity(
-		context.Background(), f.q, f.user, seed.ID, 1, SessionVector{Seed: true}, nil, defaultLimits(), "test-seed",
+		context.Background(), f.q, f.user, seed.ID, 1, SessionVector{Seed: true}, nil, defaultLimits(),
 	)
 	if err != nil {
 		t.Fatalf("load: %v", err)
@@ -335,97 +331,3 @@ func TestLoadCandidatesFromSimilarity_EmptyLibrary_NoError(t *testing.T) {
 		t.Errorf("got %d candidates from seed-only library, want 0", len(got))
 	}
 }
-
-// The randomised arms must draw REPRODUCIBLY for a given seed (#3889).
-//
-// Four arms used to end in a bare `ORDER BY random()`. That returned a stable
-// set only while the arm's LIMIT exceeded the rows eligible for it — at that
-// point it returned all of them and the order stopped mattering, because the
-// caller sorts by track id before scoring. Below that threshold it returned a
-// random SUBSET, so two calls drew different candidates.
-//
-// It therefore held by ACCIDENT, and only for libraries smaller than the
-// limits. Any real library is larger, so same-day rebuilds had been drawing
-// different mixes since the arm was written — invisible, because a mix that
-// changes after a refresh looks like a feature.
-//
-// Limits deliberately smaller than the fixture, because that is the only
-// regime where the bug existed at all: with limits above the eligible count
-// the old code passes this too.
-func TestLoadCandidatesFromSimilarity_SameSeedDrawsTheSameSet(t *testing.T) {
-	f := newFixture(t, 12)
-	seed := f.tracks[0]
-
-	tight := CandidateSourceLimits{
-		LBSimilar: 2, SimilarArtist: 2, TagOverlap: 2,
-		LikesOverlap: 2, RandomFill: 3, TasteOverlap: 2, UserCoplay: 2,
-	}
-	ids := func(cs []Candidate) []string {
-		out := make([]string, 0, len(cs))
-		for _, c := range cs {
-			out = append(out, fmt.Sprintf("%x", c.Track.ID.Bytes))
-		}
-		sort.Strings(out) // membership, not order — order is settled downstream
-		return out
-	}
-
-	first, err := LoadCandidatesFromSimilarity(
-		context.Background(), f.q, f.user, seed.ID, 1, SessionVector{Seed: true}, nil, tight, "day-one",
-	)
-	if err != nil {
-		t.Fatalf("load: %v", err)
-	}
-	if len(first) == 0 {
-		t.Fatal("no candidates, so this test asserts nothing")
-	}
-
-	for i := 0; i < 3; i++ {
-		again, err := LoadCandidatesFromSimilarity(
-			context.Background(), f.q, f.user, seed.ID, 1, SessionVector{Seed: true}, nil, tight, "day-one",
-		)
-		if err != nil {
-			t.Fatalf("load %d: %v", i, err)
-		}
-		if !reflect.DeepEqual(ids(first), ids(again)) {
-			t.Fatalf("same seed drew a different set on call %d:\n  first %v\n  again %v",
-				i, ids(first), ids(again))
-		}
-	}
-}
-
-// ...and a different seed is free to draw differently, or the ordering would
-// be fixed rather than seeded and every day would serve the same mix.
-//
-// Asserted as "not pinned to one answer" rather than "always differs": with a
-// small fixture two seeds can legitimately collide, so requiring a difference
-// on any single pair would be flaky. Several seeds producing exactly one
-// distinct set is the real regression — that is what a constant ORDER BY
-// looks like.
-func TestLoadCandidatesFromSimilarity_DifferentSeedsCanDrawDifferently(t *testing.T) {
-	f := newFixture(t, 12)
-	seed := f.tracks[0]
-	tight := CandidateSourceLimits{
-		LBSimilar: 2, SimilarArtist: 2, TagOverlap: 2,
-		LikesOverlap: 2, RandomFill: 3, TasteOverlap: 2, UserCoplay: 2,
-	}
-
-	seen := map[string]bool{}
-	for _, orderSeed := range []string{"a", "b", "c", "d", "e", "f"} {
-		cs, err := LoadCandidatesFromSimilarity(
-			context.Background(), f.q, f.user, seed.ID, 1, SessionVector{Seed: true}, nil, tight, orderSeed,
-		)
-		if err != nil {
-			t.Fatalf("load %q: %v", orderSeed, err)
-		}
-		ids := make([]string, 0, len(cs))
-		for _, c := range cs {
-			ids = append(ids, fmt.Sprintf("%x", c.Track.ID.Bytes))
-		}
-		sort.Strings(ids)
-		seen[strings.Join(ids, ",")] = true
-	}
-	if len(seen) < 2 {
-		t.Errorf("six different seeds produced %d distinct set(s); the ordering is not "+
-			"varying with the seed at all", len(seen))
-	}
-}

internal/recommendation/songs_like_limits_test.go

@@ -59,3 +59,41 @@ func TestSongsLikeLimits_KeepThePoolRoughlyTheSameSize(t *testing.T) {
 			"this was meant to re-weight the pool, not starve it", s, d)
 	}
 }
+
+// The constraint that is invisible in the numbers, and that this file exists
+// to keep visible.
+//
+// `likes_overlap` and `random_fill` end in a bare `ORDER BY random()` with no
+// daily seed (recommendation.sql:118, :161). Such an arm returns a stable set
+// only while its LIMIT exceeds the eligible rows; below that it returns a
+// random SUBSET that differs between two builds on the same day, and the
+// daily-determinism promise quietly stops holding.
+//
+// This is not hypothetical — it is how this change first failed CI. Cutting
+// RandomFill to 10 broke TestBuildSystemPlaylists_DailyNonceDeterminism,
+// whose library is smaller than the default limit and whose determinism was
+// therefore an accident of the limit exceeding the library.
+//
+// Growing these arms is always safe. Only shrinking is, and the fix that
+// would make shrinking safe is a seeded ordering (#3889), not a smaller
+// number here.
+func TestSongsLikeLimits_DoNotShrinkTheUnseededRandomArms(t *testing.T) {
+	d := DefaultCandidateSourceLimits()
+	s := SongsLikeCandidateSourceLimits()
+
+	for _, tc := range []struct {
+		arm             string
+		songsLike, dflt int
+	}{
+		{"RandomFill", s.RandomFill, d.RandomFill},
+		{"LikesOverlap", s.LikesOverlap, d.LikesOverlap},
+	} {
+		if tc.songsLike < tc.dflt {
+			t.Errorf("%s cut from %d to %d. That arm is ordered by unseeded random(), "+
+				"so a smaller limit makes pool membership vary between same-day "+
+				"rebuilds — it breaks daily determinism rather than merely narrowing "+
+				"the mix. Fix the ordering (#3889) before trimming this.",
+				tc.arm, tc.dflt, tc.songsLike)
+		}
+	}
+}