[#1806] engine: Allow to flush write-cache

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

pkg/local_object_storage/shard/writecache.go

@@ -0,0 +1,36 @@
+package shard
+
+import (
+	"errors"
+
+	"github.com/nspcc-dev/neofs-node/pkg/local_object_storage/shard/mode"
+)
+
+// errWriteCacheDisabled is returned when an operation on write-cache is performed,
+// but write-cache is disabled.
+var errWriteCacheDisabled = errors.New("write-cache is disabled")
+
+// FlushWriteCache moves writecache in read-only mode and flushes all data from it.
+// After the operation writecache will remain read-only mode.
+func (s *Shard) FlushWriteCache() error {
+	if !s.hasWriteCache() {
+		return errWriteCacheDisabled
+	}
+
+	s.m.RLock()
+	defer s.m.RUnlock()
+
+	// To write data to the blobstor we need to write to the blobstor and the metabase.
+	if s.info.Mode.ReadOnly() {
+		return ErrReadOnlyMode
+	}
+	if s.info.Mode.NoMetabase() {
+		return ErrDegradedMode
+	}
+
+	if err := s.writeCache.SetMode(mode.ReadOnly); err != nil {
+		return err
+	}
+
+	return s.writeCache.Flush()
+}