fix(scheduler): rotate lead user each pass so caption budget is shared

Caption fetches share one per-egress-IP rate budget; whoever runs first each pass
spends the pre-throttle window before YouTube starts 429ing. ListAllUsers order
is unspecified and was stable, so the last-listed user was permanently starved —
a friendly-pilot user got 0 fetches in 12h (all rate_limited) while the
first-listed user got every successful fetch. rotateUsers left-rotates the user
order by pass index so each user leads 1/N passes and the lead slot is shared.

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

cmd/tapir/scheduler.go

@@ -36,6 +36,11 @@ func buildUserRunner(cfg config.Config, st *store.Store, secretStore ports.Secre
 	}, secretStore)
 
 	engine := usecase.NewEngine(src, buildSummarizer(cfg), st)
+	// Share the transcript cache (ADR-021) on the scheduler path too — without
+	// this every scheduled pass re-fetches transcripts it already had, burning the
+	// scarce per-IP caption budget (ADR-014) and starving other users. The web
+	// "Summarize now" path already sets this; the scheduler omitting it was a bug.
+	engine.Transcripts = st
 
 	return runner.New(src, st, engine, userID, log,
 		runner.WithBackoff(cfg.FetchBackoff),
@@ -57,6 +62,7 @@ type userLister interface {
 // isolation). Returns the stats summed across users.
 func runDiscoveryPass(
 	ctx context.Context,
+	pass int,
 	lister userLister,
 	runUser func(context.Context, string) (runner.Stats, error),
 	log *slog.Logger,
@@ -67,6 +73,13 @@ func runDiscoveryPass(
 		return runner.Stats{}
 	}
 
+	// Rotate who goes first each pass. Caption fetches share one per-egress-IP
+	// rate budget (ADR-014); whoever runs first each pass spends the pre-throttle
+	// window, so a FIXED user order permanently starves whoever is last (a new
+	// pilot user got 0 fetches for 12h while the first-listed user got all of
+	// them). Rotation gives every user the lead slot in turn.
+	users = rotateUsers(users, pass)
+
 	log.Info("scheduler: starting discovery pass", "users", len(users))
 	var total runner.Stats
 	for _, u := range users {
@@ -116,7 +129,8 @@ func runScheduler(
 		return // disabled
 	}
 
-	runDiscoveryPass(ctx, lister, runUser, log)
+	pass := 0
+	runDiscoveryPass(ctx, pass, lister, runUser, log)
 
 	ticker := time.NewTicker(interval)
 	defer ticker.Stop()
@@ -125,11 +139,32 @@ func runScheduler(
 		case <-ctx.Done():
 			return
 		case <-ticker.C:
-			runDiscoveryPass(ctx, lister, runUser, log)
+			pass++
+			runDiscoveryPass(ctx, pass, lister, runUser, log)
 		}
 	}
 }
 
+// rotateUsers left-rotates users by pass positions so a different user leads each
+// pass. With n users, user i leads on every pass where pass ≡ i (mod n). A pass
+// offset that is negative or exceeds n is normalised. Order within the rotation
+// is otherwise preserved, so the set of users run is unchanged — only who is
+// first (and thus wins the scarce caption-fetch budget) rotates.
+func rotateUsers(users []store.UserIdentity, pass int) []store.UserIdentity {
+	n := len(users)
+	if n <= 1 {
+		return users
+	}
+	off := ((pass % n) + n) % n
+	if off == 0 {
+		return users
+	}
+	out := make([]store.UserIdentity, 0, n)
+	out = append(out, users[off:]...)
+	out = append(out, users[:off]...)
+	return out
+}
+
 // sumStats adds two passes' stats field-wise, so runDiscoveryPass can report a
 // per-tick aggregate across all users.
 func sumStats(a, b runner.Stats) runner.Stats {

cmd/tapir/scheduler_test.go

@@ -45,6 +45,7 @@ func (f fakeLister) ConnectionsForUser(_ context.Context, userID string) ([]stor
 type countingRunUser struct {
 	mu      sync.Mutex
 	calls   map[string]int
+	order   []string // userIDs in the order they were run, across all passes
 	failFor map[string]bool
 }
 
@@ -60,12 +61,19 @@ func (c *countingRunUser) run(_ context.Context, userID string) (runner.Stats, e
 	c.mu.Lock()
 	defer c.mu.Unlock()
 	c.calls[userID]++
+	c.order = append(c.order, userID)
 	if c.failFor[userID] {
 		return runner.Stats{Errors: 1}, errors.New("boom")
 	}
 	return runner.Stats{Summarized: 1}, nil
 }
 
+func (c *countingRunUser) runOrder() []string {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	return append([]string(nil), c.order...)
+}
+
 func (c *countingRunUser) count(userID string) int {
 	c.mu.Lock()
 	defer c.mu.Unlock()
@@ -94,7 +102,7 @@ func TestDiscoveryPassRunsEveryUserOnce(t *testing.T) {
 	lister := fakeLister{users: usersN("a", "b", "c")}
 	rc := newCountingRunUser()
 
-	stats := runDiscoveryPass(context.Background(), lister, rc.run, quietLog())
+	stats := runDiscoveryPass(context.Background(), 0, lister, rc.run, quietLog())
 
 	require.Equal(t, 1, rc.count("a"))
 	require.Equal(t, 1, rc.count("b"))
@@ -102,13 +110,31 @@ func TestDiscoveryPassRunsEveryUserOnce(t *testing.T) {
 	require.Equal(t, 3, stats.Summarized, "stats are summed across users")
 }
 
+// Caption fetches share one per-IP budget; a fixed user order starves whoever is
+// last. Each pass must rotate which user leads so the lead slot is shared.
+func TestDiscoveryPassRotatesLeadUser(t *testing.T) {
+	lister := fakeLister{users: usersN("a", "b", "c")}
+	rc := newCountingRunUser()
+
+	runDiscoveryPass(context.Background(), 0, lister, rc.run, quietLog())
+	runDiscoveryPass(context.Background(), 1, lister, rc.run, quietLog())
+	runDiscoveryPass(context.Background(), 2, lister, rc.run, quietLog())
+
+	require.Equal(t, []string{"a", "b", "c", "b", "c", "a", "c", "a", "b"}, rc.runOrder(),
+		"each pass left-rotates the user order so every user leads in turn")
+	// Fairness: over a full rotation cycle every user ran the same number of times.
+	require.Equal(t, 3, rc.count("a"))
+	require.Equal(t, 3, rc.count("b"))
+	require.Equal(t, 3, rc.count("c"))
+}
+
 func TestDiscoveryPassSkipsUsersWithoutConnections(t *testing.T) {
 	// b never connected a video source (e.g. a stale Dex-era orphan identity).
 	// It must be skipped silently — not run and logged as a token error every pass.
 	lister := fakeLister{users: usersN("a", "b", "c"), noConn: map[string]bool{"b": true}}
 	rc := newCountingRunUser()
 
-	stats := runDiscoveryPass(context.Background(), lister, rc.run, quietLog())
+	stats := runDiscoveryPass(context.Background(), 0, lister, rc.run, quietLog())
 
 	require.Equal(t, 1, rc.count("a"))
 	require.Equal(t, 0, rc.count("b"), "a user with no connection must be skipped, not run")
@@ -121,7 +147,7 @@ func TestDiscoveryPassOneUserFailureDoesNotStopOthers(t *testing.T) {
 	lister := fakeLister{users: usersN("a", "b", "c")}
 	rc := newCountingRunUser("b") // user b's pass errors
 
-	stats := runDiscoveryPass(context.Background(), lister, rc.run, quietLog())
+	stats := runDiscoveryPass(context.Background(), 0, lister, rc.run, quietLog())
 
 	require.Equal(t, 1, rc.count("a"))
 	require.Equal(t, 1, rc.count("b"))
@@ -134,7 +160,7 @@ func TestDiscoveryPassListerErrorIsContained(t *testing.T) {
 	lister := fakeLister{err: errors.New("db down")}
 	rc := newCountingRunUser()
 
-	stats := runDiscoveryPass(context.Background(), lister, rc.run, quietLog())
+	stats := runDiscoveryPass(context.Background(), 0, lister, rc.run, quietLog())
 
 	require.Equal(t, 0, rc.total(), "no users enumerated → no passes")
 	require.Equal(t, runner.Stats{}, stats)