[#1806] writecache: Allow to start flush manually

Allow user to initiate flushing objects from a writecache.
We need this in 2 cases:
1. During writecache storage schema update, it should be flushed with
   the old version of node and started clean with a new one.
2. During SSD replacement, to avoid data loss.

Signed-off-by: Evgenii Stratonikov <evgeniy@morphbits.ru>

pkg/local_object_storage/writecache/flush.go

@@ -1,6 +1,7 @@
 package writecache
 
 import (
+	"errors"
 	"time"
 
 	"github.com/mr-tron/base58"
@@ -25,6 +26,10 @@ const (
 	defaultFlushInterval = time.Second
 )
 
+// errMustBeReadOnly is returned when write-cache must be
+// in read-only mode to perform an operation.
+var errMustBeReadOnly = errors.New("write-cache must be in read-only mode")
+
 // runFlushLoop starts background workers which periodically flush objects to the blobstor.
 func (c *cache) runFlushLoop() {
 	for i := 0; i < c.workersCount; i++ {
@@ -224,3 +229,68 @@ func (c *cache) flushObject(obj *object.Object) error {
 	_, err = c.metabase.Put(pPrm)
 	return err
 }
+
+// Flush flushes all objects from the write-cache to the main storage.
+// Write-cache must be in readonly mode to ensure correctness of an operation and
+// to prevent interference with background flush workers.
+func (c *cache) Flush() error {
+	c.modeMtx.RLock()
+	defer c.modeMtx.RUnlock()
+
+	if !c.mode.ReadOnly() {
+		return errMustBeReadOnly
+	}
+
+	var prm common.IteratePrm
+	prm.LazyHandler = func(addr oid.Address, f func() ([]byte, error)) error {
+		_, ok := c.flushed.Peek(addr.EncodeToString())
+		if ok {
+			return nil
+		}
+
+		data, err := f()
+		if err != nil {
+			return err
+		}
+
+		var obj object.Object
+		err = obj.Unmarshal(data)
+		if err != nil {
+			return err
+		}
+
+		return c.flushObject(&obj)
+	}
+
+	_, err := c.fsTree.Iterate(prm)
+	if err != nil {
+		return err
+	}
+
+	return c.db.View(func(tx *bbolt.Tx) error {
+		var addr oid.Address
+
+		b := tx.Bucket(defaultBucket)
+		cs := b.Cursor()
+		for k, data := cs.Seek(nil); k != nil; k, data = cs.Next() {
+			sa := string(k)
+			if _, ok := c.flushed.Peek(sa); ok {
+				continue
+			}
+
+			if err := addr.DecodeString(sa); err != nil {
+				return err
+			}
+
+			var obj object.Object
+			if err := obj.Unmarshal(data); err != nil {
+				return err
+			}
+
+			if err := c.flushObject(&obj); err != nil {
+				return err
+			}
+		}
+		return nil
+	})
+}