[#563] writecache: Fix metrics and bolt delete

Estimate cache size after delete objects to update metric.
Update counters on small object deletion.
Do not count bbolt DB file as FSTree object.

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

pkg/local_object_storage/writecache/writecachebbolt/flush.go

@@ -246,7 +246,7 @@ func (c *cache) workerFlushSmall() {
 			continue
 		}
 
-		c.deleteFromDB([]string{objectCore.AddressOf(obj).EncodeToString()})
+		c.deleteFromDB(objectCore.AddressOf(obj).EncodeToString())
 	}
 }
 

pkg/local_object_storage/writecache/writecachebbolt/state.go

@@ -68,6 +68,9 @@ func (c *cache) initCounters() error {
 	if err != nil {
 		return fmt.Errorf("could not read write-cache FS counter: %w", err)
 	}
+	if inFS > 0 {
+		inFS-- //small.bolt DB file
+	}
 
 	c.objCounters.cDB.Store(inDB)
 	c.objCounters.cFS.Store(inFS)

pkg/local_object_storage/writecache/writecachebbolt/storage.go

@@ -63,36 +63,24 @@ func (c *cache) openStore(readOnly bool) error {
 	return nil
 }
 
-func (c *cache) deleteFromDB(keys []string) []string {
-	if len(keys) == 0 {
-		return keys
-	}
-
-	var errorIndex int
+func (c *cache) deleteFromDB(key string) {
 	err := c.db.Batch(func(tx *bbolt.Tx) error {
 		b := tx.Bucket(defaultBucket)
-		for errorIndex = range keys {
-			if err := b.Delete([]byte(keys[errorIndex])); err != nil {
-				return err
-			}
-		}
-		return nil
+		return b.Delete([]byte(key))
 	})
-	for i := 0; i < errorIndex; i++ {
+
+	if err == nil {
 		c.objCounters.DecDB()
 		c.metrics.Evict(writecache.StorageTypeDB)
 		storagelog.Write(c.log,
-			storagelog.AddressField(keys[i]),
+			storagelog.AddressField(key),
 			storagelog.StorageTypeField(wcStorageType),
 			storagelog.OpField("db DELETE"),
 		)
-	}
-	if err != nil {
+		c.estimateCacheSize()
+	} else {
 		c.log.Error(logs.WritecacheCantRemoveObjectsFromTheDatabase, zap.Error(err))
 	}
-
-	copy(keys, keys[errorIndex:])
-	return keys[:len(keys)-errorIndex]
 }
 
 func (c *cache) deleteFromDisk(ctx context.Context, keys []string) []string {
@@ -125,6 +113,7 @@ func (c *cache) deleteFromDisk(ctx context.Context, keys []string) []string {
 			)
 			c.metrics.Evict(writecache.StorageTypeFSTree)
 			c.objCounters.DecFS()
+			c.estimateCacheSize()
 		}
 	}