[#1367] writecache: Add background flushing objects limiter

To limit memory usage by background flush.

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

pkg/local_object_storage/writecache/limiter.go

@@ -0,0 +1,70 @@
+package writecache
+
+import (
+	"errors"
+	"sync"
+)
+
+var errLimiterClosed = errors.New("acquire failed: limiter closed")
+
+// flushLimiter is used to limit the total size of objects
+// being flushed to blobstore at the same time. This is a necessary
+// limitation so that the flushing process does not have
+// a strong impact on user requests.
+type flushLimiter struct {
+	count, size uint64
+	maxSize     uint64
+	cond        *sync.Cond
+	closed      bool
+}
+
+func newFlushLimiter(maxSize uint64) *flushLimiter {
+	return &flushLimiter{
+		maxSize: maxSize,
+		cond:    sync.NewCond(&sync.Mutex{}),
+	}
+}
+
+func (l *flushLimiter) acquire(size uint64) error {
+	l.cond.L.Lock()
+	defer l.cond.L.Unlock()
+
+	// it is allowed to overflow maxSize to allow flushing objects with size > maxSize
+	for l.count > 0 && l.size+size > l.maxSize && !l.closed {
+		l.cond.Wait()
+		if l.closed {
+			return errLimiterClosed
+		}
+	}
+	l.count++
+	l.size += size
+	return nil
+}
+
+func (l *flushLimiter) release(size uint64) {
+	l.cond.L.Lock()
+	defer l.cond.L.Unlock()
+
+	if l.size >= size {
+		l.size -= size
+	} else {
+		panic("flushLimiter: invalid size")
+	}
+
+	if l.count > 0 {
+		l.count--
+	} else {
+		panic("flushLimiter: invalid count")
+	}
+
+	l.cond.Broadcast()
+}
+
+func (l *flushLimiter) close() {
+	l.cond.L.Lock()
+	defer l.cond.L.Unlock()
+
+	l.closed = true
+
+	l.cond.Broadcast()
+}