atenet/router: make parkOutcome a named type

Replace the bare-string park-outcome constants with a parkOutcome string type and rename the classifier to parkOutcomeFor, so the parking.wait.duration outcome label is type-checked rather than an arbitrary string.

cmd/atenet/internal/router/extproc.go

@@ -162,8 +162,7 @@ func (s *ExtProcServer) handleRequestHeaders(
 
 	slog.InfoContext(ctx, "ResumeActor", slog.Any("actor", actorRef))
 	actor, err := s.resumer.ResumeActor(ctx, actorRef)
-	release(parkOutcome(err))
-
+	release(parkOutcomeFor(err))
 
 	if err != nil {
 		return nil, metadata, "", "", "", mapResumeError(actorRef, err)

cmd/atenet/internal/router/metrics.go

@@ -115,11 +115,11 @@ func (m *parkingMetrics) addActive(ctx context.Context, delta int64) {
 	m.active.Add(ctx, delta)
 }
 
-func (m *parkingMetrics) recordWait(ctx context.Context, d time.Duration, outcome string) {
+func (m *parkingMetrics) recordWait(ctx context.Context, d time.Duration, outcome parkOutcome) {
 	if m == nil || m.wait == nil {
 		return
 	}
-	m.wait.Record(ctx, d.Seconds(), metric.WithAttributes(attribute.String("outcome", outcome)))
+	m.wait.Record(ctx, d.Seconds(), metric.WithAttributes(attribute.String("outcome", string(outcome))))
 }
 
 func (m *parkingMetrics) recordRejected(ctx context.Context) {

cmd/atenet/internal/router/parking.go

@@ -30,12 +30,16 @@ const (
 	defaultParkingMaxParked = 2048
 )
 
-// Parking-wait outcome labels, recorded on the parking.wait.duration histogram.
+// parkOutcome is the terminal disposition of a parked request. It is recorded
+// as the `outcome` label on the parking.wait.duration histogram.
+type parkOutcome string
+
+// Park-wait outcomes, recorded on the parking.wait.duration histogram.
 const (
-	parkOutcomeServed   = "served"   // resume succeeded and the request was routed
-	parkOutcomeTimeout  = "timeout"  // the request's deadline elapsed while parked
-	parkOutcomeCanceled = "canceled" // the client disconnected while parked
-	parkOutcomeError    = "error"    // resume failed (including park-budget exhaustion)
+	parkOutcomeServed   parkOutcome = "served"   // resume succeeded and the request was routed
+	parkOutcomeTimeout  parkOutcome = "timeout"  // the request's deadline elapsed while parked
+	parkOutcomeCanceled parkOutcome = "canceled" // the client disconnected while parked
+	parkOutcomeError    parkOutcome = "error"    // resume failed (including park-budget exhaustion)
 )
 
 // parkingConfig controls how the router parks resume-gated requests.
@@ -87,9 +91,9 @@ func newParkingLot(cfg parkingConfig, m *parkingMetrics) *parkingLot {
 // ok=false means the lot is full and the request should be shed without
 // waiting. When parking is disabled every request is admitted and no slot
 // accounting or metrics are recorded.
-func (l *parkingLot) enter(ctx context.Context) (release func(outcome string), ok bool) {
+func (l *parkingLot) enter(ctx context.Context) (release func(outcome parkOutcome), ok bool) {
 	if l == nil || !l.cfg.enabled {
-		return func(string) {}, true
+		return func(parkOutcome) {}, true
 	}
 
 	for {
@@ -108,7 +112,7 @@ func (l *parkingLot) enter(ctx context.Context) (release func(outcome string), o
 	l.metrics.addActive(ctx, 1)
 
 	var once sync.Once
-	return func(outcome string) {
+	return func(outcome parkOutcome) {
 		once.Do(func() {
 			atomic.AddInt64(&l.active, -1)
 			l.metrics.addActive(ctx, -1)
@@ -138,10 +142,10 @@ func (l *parkingLot) status() ParkingStatus {
 	}
 }
 
-// parkOutcome classifies a completed resume attempt for the wait-duration
+// parkOutcomeFor classifies a completed resume attempt for the wait-duration
 // metric. A budget-exhausted park surfaces the underlying capacity error and is
 // reported as parkOutcomeError.
-func parkOutcome(err error) string {
+func parkOutcomeFor(err error) parkOutcome {
 	switch {
 	case err == nil:
 		return parkOutcomeServed

cmd/atenet/internal/router/parking_test.go

@@ -111,7 +111,7 @@ func TestParkingLot_ConcurrentEntryRespectsCapacity(t *testing.T) {
 
 	var admitted int64
 	var mu sync.Mutex
-	releases := make([]func(string), 0, capacity)
+	releases := make([]func(parkOutcome), 0, capacity)
 
 	var wg sync.WaitGroup
 	wg.Add(goroutines)
@@ -142,11 +142,11 @@ func TestParkingLot_ConcurrentEntryRespectsCapacity(t *testing.T) {
 	}
 }
 
-func TestParkOutcome(t *testing.T) {
+func TestParkOutcomeFor(t *testing.T) {
 	tests := []struct {
 		name string
 		err  error
-		want string
+		want parkOutcome
 	}{
 		{"nil is served", nil, parkOutcomeServed},
 		{"canceled", context.Canceled, parkOutcomeCanceled},
@@ -155,8 +155,8 @@ func TestParkOutcome(t *testing.T) {
 	}
 	for _, tc := range tests {
 		t.Run(tc.name, func(t *testing.T) {
-			if got := parkOutcome(tc.err); got != tc.want {
-				t.Errorf("parkOutcome(%v) = %q, want %q", tc.err, got, tc.want)
+			if got := parkOutcomeFor(tc.err); got != tc.want {
+				t.Errorf("parkOutcomeFor(%v) = %q, want %q", tc.err, got, tc.want)
 			}
 		})
 	}