[#1636] qos: Add semaphore limiter

If no tags specified, then limiter could be optimized to use atomic semaphore.

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

internal/qos/semaphore.go

@@ -0,0 +1,39 @@
+package qos
+
+import (
+	"context"
+	"errors"
+	"sync/atomic"
+
+	"git.frostfs.info/TrueCloudLab/frostfs-qos/scheduling"
+)
+
+var (
+	_                         scheduler = (*semaphore)(nil)
+	errSemaphoreLimitExceeded           = errors.New("semaphore limit exceeded")
+)
+
+type semaphore struct {
+	count atomic.Int64
+	limit int64
+}
+
+func (s *semaphore) Close() {}
+
+func (s *semaphore) RequestArrival(ctx context.Context, _ string) (scheduling.ReleaseFunc, error) {
+	select {
+	case <-ctx.Done():
+		return nil, ctx.Err()
+	default:
+	}
+
+	v := s.count.Add(1)
+	if v > s.limit {
+		s.count.Add(-1)
+		return nil, errSemaphoreLimitExceeded
+	}
+
+	return func() {
+		s.count.Add(-1)
+	}, nil
+}