[#1201] writecache: Cancel background flush without lock

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

pkg/local_object_storage/writecache/cachebbolt.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"os"
 	"sync"
+	"sync/atomic"
 
 	"git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/blobstor/fstree"
 	"git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/internal/metaerr"
@@ -30,7 +31,7 @@ type cache struct {
 	// flushCh is a channel with objects to flush.
 	flushCh chan objectInfo
 	// cancel is cancel function, protected by modeMtx in Close.
-	cancel func()
+	cancel atomic.Value
 	// wg is a wait group for flush workers.
 	wg sync.WaitGroup
 	// store contains underlying database.
@@ -54,7 +55,10 @@ const (
 	defaultMaxCacheSize    = 1 << 30          // 1 GiB
 )
 
-var defaultBucket = []byte{0}
+var (
+	defaultBucket                    = []byte{0}
+	dummyCanceler context.CancelFunc = func() {}
+)
 
 // New creates new writecache instance.
 func New(opts ...Option) Cache {
@@ -114,20 +118,19 @@ func (c *cache) Open(_ context.Context, mod mode.Mode) error {
 func (c *cache) Init() error {
 	c.metrics.SetMode(mode.ConvertToComponentModeDegraded(c.mode))
 	ctx, cancel := context.WithCancel(context.Background())
-	c.cancel = cancel
+	c.cancel.Store(cancel)
 	c.runFlushLoop(ctx)
 	return nil
 }
 
 // Close closes db connection and stops services. Executes ObjectCounters.FlushAndClose op.
 func (c *cache) Close() error {
+	if cancelValue := c.cancel.Swap(dummyCanceler); cancelValue != nil {
+		cancelValue.(context.CancelFunc)()
+	}
 	// We cannot lock mutex for the whole operation duration
 	// because it is taken by some background workers, so `wg.Wait()` is done without modeMtx.
 	c.modeMtx.Lock()
-	if c.cancel != nil {
-		c.cancel()
-		c.cancel = nil
-	}
 	c.mode = mode.DegradedReadOnly // prevent new operations from being processed
 	c.modeMtx.Unlock()