[#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>

CHANGELOG.md

@@ -16,6 +16,7 @@ Changelog for NeoFS Node
 ### Changed
 
 - Allow to evacuate shard data with `EvacuateShard` control RPC (#1800)
+- Flush write-cache when moving shard to DEGRADED mode (#1825)
 
 ### Fixed
 - Description of command `netmap nodeinfo` (#1821)

pkg/local_object_storage/shard/put.go

@@ -49,10 +49,11 @@ func (s *Shard) Put(prm PutPrm) (PutRes, error) {
 
 	// exist check are not performed there, these checks should be executed
 	// ahead of `Put` by storage engine
-	if s.hasWriteCache() {
+	tryCache := s.hasWriteCache() && !m.NoMetabase()
+	if tryCache {
 		res, err = s.writeCache.Put(putPrm)
 	}
-	if err != nil || !s.hasWriteCache() {
+	if err != nil || !tryCache {
 		if err != nil {
 			s.log.Debug("can't put object to the write-cache, trying blobstor",
 				zap.String("err", err.Error()))

pkg/local_object_storage/writecache/flush.go

@@ -50,7 +50,7 @@ func (c *cache) runFlushLoop() {
 		for {
 			select {
 			case <-tt.C:
-				c.flush()
+				c.flushDB()
 				tt.Reset(defaultFlushInterval)
 			case <-c.closeCh:
 				return
@@ -59,7 +59,7 @@ func (c *cache) runFlushLoop() {
 	}()
 }
 
-func (c *cache) flush() {
+func (c *cache) flushDB() {
 	lastKey := []byte{}
 	var m []objectInfo
 	for {
@@ -241,6 +241,10 @@ func (c *cache) Flush(ignoreErrors bool) error {
 		return errMustBeReadOnly
 	}
 
+	return c.flush(ignoreErrors)
+}
+
+func (c *cache) flush(ignoreErrors bool) error {
 	var prm common.IteratePrm
 	prm.IgnoreErrors = ignoreErrors
 	prm.LazyHandler = func(addr oid.Address, f func() ([]byte, error)) error {

pkg/local_object_storage/writecache/flush_test.go

@@ -109,10 +109,6 @@ func TestFlush(t *testing.T) {
 		wc, bs, mb := newCache(t)
 		objects := putObjects(t, wc)
 
-		t.Run("must be read-only", func(t *testing.T) {
-			require.ErrorIs(t, wc.Flush(false), errMustBeReadOnly)
-		})
-
 		require.NoError(t, wc.SetMode(mode.ReadOnly))
 		require.NoError(t, bs.SetMode(mode.ReadWrite))
 		require.NoError(t, mb.SetMode(mode.ReadWrite))
@@ -135,6 +131,36 @@ func TestFlush(t *testing.T) {
 		check(t, mb, bs, objects[2:])
 	})
 
+	t.Run("flush on moving to degraded mode", func(t *testing.T) {
+		wc, bs, mb := newCache(t)
+		objects := putObjects(t, wc)
+
+		// Blobstor is read-only, so we expect en error from `flush` here.
+		require.Error(t, wc.SetMode(mode.Degraded))
+
+		// First move to read-only mode to close background workers.
+		require.NoError(t, wc.SetMode(mode.ReadOnly))
+		require.NoError(t, bs.SetMode(mode.ReadWrite))
+		require.NoError(t, mb.SetMode(mode.ReadWrite))
+
+		wc.(*cache).flushed.Add(objects[0].addr.EncodeToString(), true)
+		wc.(*cache).flushed.Add(objects[1].addr.EncodeToString(), false)
+
+		require.NoError(t, wc.SetMode(mode.Degraded))
+
+		for i := 0; i < 2; i++ {
+			var mPrm meta.GetPrm
+			mPrm.SetAddress(objects[i].addr)
+			_, err := mb.Get(mPrm)
+			require.Error(t, err)
+
+			_, err = bs.Get(common.GetPrm{Address: objects[i].addr})
+			require.Error(t, err)
+		}
+
+		check(t, mb, bs, objects[2:])
+	})
+
 	t.Run("ignore errors", func(t *testing.T) {
 		testIgnoreErrors := func(t *testing.T, f func(*cache)) {
 			wc, bs, mb := newCache(t)

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() {
+	if m.NoMetabase() && !c.mode.NoMetabase() {
-		c.mode = m
+		err := c.flush(true)
-		return nil
+		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
 	}