fix(scheduler): rotate over connected users only for true fair share

The lead-user rotation rotated the full ListAllUsers set, so a connectionless
orphan identity ate a rotation slot — collapsing onto the next real user and
skewing the lead share (two real users got 2/3 vs 1/3 instead of 50/50). Filter
to connected users BEFORE rotating so the rotation is over exactly the users that
consume the caption budget. A dead identity can no longer skew fairness.

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

cmd/tapir/scheduler.go

@@ -73,22 +73,17 @@ func runDiscoveryPass(
 		return runner.Stats{}
 	}
 
-	// Rotate who goes first each pass. Caption fetches share one per-egress-IP
+	// Keep only users with a video connection. A pass for a connectionless user
-	// rate budget (ADR-014); whoever runs first each pass spends the pre-throttle
+	// (e.g. a stale Dex-era orphan identity) only tries to resolve a token that
-	// window, so a FIXED user order permanently starves whoever is last (a new
+	// was never minted, logging a spurious "ref not found" every tick. Filtering
-	// pilot user got 0 fetches for 12h while the first-listed user got all of
+	// here — BEFORE rotation — also keeps fairness honest: rotation is over the
-	// them). Rotation gives every user the lead slot in turn.
+	// users that actually consume the caption budget, so a dead identity can't eat
-	users = rotateUsers(users, pass)
+	// a rotation slot and skew the lead share.
-
+	var connected []store.UserIdentity
-	log.Info("scheduler: starting discovery pass", "users", len(users))
-	var total runner.Stats
 	for _, u := range users {
 		if ctx.Err() != nil {
-			break // shutting down: stop enumerating
+			return runner.Stats{} // shutting down
 		}
-		// Skip users with no video connection. A discovery pass for them only
-		// attempts to resolve a token that was never minted, logging a spurious
-		// "ref not found" every tick (e.g. stale Dex-era orphan identities).
 		conns, err := lister.ConnectionsForUser(ctx, u.UserID)
 		if err != nil {
 			log.Warn("scheduler: list connections failed", "user", u.UserID, "err", err)
@@ -98,6 +93,22 @@ func runDiscoveryPass(
 			log.Debug("scheduler: skipping user with no video connections", "user", u.UserID)
 			continue
 		}
+		connected = append(connected, u)
+	}
+
+	// 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 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 over the connected set gives each real user the lead in turn.
+	connected = rotateUsers(connected, pass)
+
+	log.Info("scheduler: starting discovery pass", "users", len(connected))
+	var total runner.Stats
+	for _, u := range connected {
+		if ctx.Err() != nil {
+			break // shutting down: stop enumerating
+		}
 		stats, err := runUser(ctx, u.UserID)
 		total = sumStats(total, stats)
 		if err != nil {

cmd/tapir/scheduler_test.go

@@ -128,6 +128,21 @@ func TestDiscoveryPassRotatesLeadUser(t *testing.T) {
 	require.Equal(t, 3, rc.count("c"))
 }
 
+// A connectionless orphan must not consume a rotation slot: rotation is over the
+// connected users only, so two real users alternate the lead 50/50 even with a
+// dead identity listed between them.
+func TestDiscoveryPassRotationIgnoresConnectionlessUsers(t *testing.T) {
+	lister := fakeLister{users: usersN("a", "orphan", "c"), noConn: map[string]bool{"orphan": true}}
+	rc := newCountingRunUser()
+
+	runDiscoveryPass(context.Background(), 0, lister, rc.run, quietLog())
+	runDiscoveryPass(context.Background(), 1, lister, rc.run, quietLog())
+
+	require.Equal(t, []string{"a", "c", "c", "a"}, rc.runOrder(),
+		"only connected users rotate; the orphan never runs and never holds a slot")
+	require.Equal(t, 0, rc.count("orphan"))
+}
+
 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.