[#1825] writecache: Flush cache when moving to the DEGRADED mode

Degraded mode allows us to operate without an SSD,
thus writecache should be unavailable in this mode.

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

pkg/local_object_storage/writecache/mode.go

@@ -18,9 +18,11 @@ func (c *cache) SetMode(m mode.Mode) error {
 	c.modeMtx.Lock()
 	defer c.modeMtx.Unlock()
 
-	if m.ReadOnly() == c.readOnly() {
-		c.mode = m
-		return nil
+	if m.NoMetabase() && !c.mode.NoMetabase() {
+		err := c.flush(true)
+		if err != nil {
+			return err
+		}
 	}
 
 	if c.db != nil {
@@ -37,6 +39,11 @@ func (c *cache) SetMode(m mode.Mode) error {
 		time.Sleep(time.Second)
 	}
 
+	if m.NoMetabase() {
+		c.mode = m
+		return nil
+	}
+
 	if err := c.openStore(m.ReadOnly()); err != nil {
 		return err
 	}