[#1085] writecache: add read-only mode

In read-only mode modifying operations are immediately returned with
error and all background operations are suspended.

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

pkg/local_object_storage/shard/mode.go

@@ -1,6 +1,10 @@
 package shard
 
-import "errors"
+import (
+	"errors"
+
+	"github.com/nspcc-dev/neofs-node/pkg/local_object_storage/writecache"
+)
 
 // Mode represents enumeration of Shard work modes.
 type Mode uint32
@@ -40,6 +44,15 @@ func (s *Shard) SetMode(m Mode) error {
 	s.m.Lock()
 	defer s.m.Unlock()
 
+	if s.hasWriteCache() {
+		switch m {
+		case ModeReadOnly:
+			s.writeCache.SetMode(writecache.ModeReadOnly)
+		case ModeReadWrite:
+			s.writeCache.SetMode(writecache.ModeReadWrite)
+		}
+	}
+
 	s.info.Mode = m
 
 	return nil

pkg/local_object_storage/writecache/delete.go

@@ -12,6 +12,12 @@ import (
 
 // Delete removes object from write-cache.
 func (c *cache) Delete(addr *objectSDK.Address) error {
+	c.modeMtx.RLock()
+	defer c.modeMtx.RUnlock()
+	if c.mode == ModeReadOnly {
+		return ErrReadOnly
+	}
+
 	saddr := addr.String()
 
 	// Check memory cache.

pkg/local_object_storage/writecache/flush.go

@@ -63,6 +63,13 @@ func (c *cache) flush() {
 		m = m[:0]
 		sz := 0
 
+		c.modeMtx.RLock()
+		if c.mode == ModeReadOnly {
+			c.modeMtx.RUnlock()
+			time.Sleep(time.Second)
+			continue
+		}
+
 		// We put objects in batches of fixed size to not interfere with main put cycle a lot.
 		_ = c.db.View(func(tx *bbolt.Tx) error {
 			b := tx.Bucket(defaultBucket)
@@ -90,6 +97,7 @@ func (c *cache) flush() {
 			select {
 			case c.flushCh <- obj:
 			case <-c.closeCh:
+				c.modeMtx.RUnlock()
 				return
 			}
 		}
@@ -98,6 +106,7 @@ func (c *cache) flush() {
 		for i := range m {
 			c.flushed.Add(m[i].addr, true)
 		}
+		c.modeMtx.RUnlock()
 
 		c.log.Debug("flushed items from write-cache",
 			zap.Int("count", len(m)),
@@ -116,8 +125,13 @@ func (c *cache) flushBigObjects() {
 	for {
 		select {
 		case <-tick.C:
-			evictNum := 0
+			c.modeMtx.RLock()
+			if c.mode == ModeReadOnly {
+				c.modeMtx.RUnlock()
+				break
+			}
 
+			evictNum := 0
 			_ = c.fsTree.Iterate(func(addr *objectSDK.Address, data []byte) error {
 				sAddr := addr.String()
 
@@ -150,6 +164,7 @@ func (c *cache) flushBigObjects() {
 
 			// evict objects which were successfully written to BlobStor
 			c.evictObjects(evictNum)
+			c.modeMtx.RUnlock()
 		case <-c.closeCh:
 		}
 	}

pkg/local_object_storage/writecache/mode.go

@@ -0,0 +1,52 @@
+package writecache
+
+import (
+	"errors"
+	"time"
+)
+
+// Mode represents write-cache mode of operation.
+type Mode uint32
+
+const (
+	// ModeReadWrite is a default mode allowing objects to be flushed.
+	ModeReadWrite Mode = iota
+
+	// ModeReadOnly is a mode in which write-cache doesn't flush anything to a metabase.
+	ModeReadOnly
+)
+
+// ErrReadOnly is returned when Put/Write is performed in a read-only mode.
+var ErrReadOnly = errors.New("write-cache is in read-only mode")
+
+// SetMode sets write-cache mode of operation.
+// When shard is put in read-only mode all objects in memory are flushed to disk
+// and all background jobs are suspended.
+func (c *cache) SetMode(m Mode) {
+	c.modeMtx.Lock()
+	defer c.modeMtx.Unlock()
+	if c.mode == m {
+		return
+	}
+
+	c.mode = m
+	if m == ModeReadWrite {
+		return
+	}
+
+	// Because modeMtx is taken no new objects will arrive an all other modifying
+	// operations are completed.
+	// 1. Persist objects already in memory on disk.
+	c.persistMemoryCache()
+
+	// 2. Suspend producers to ensure there are channel send operations in fly.
+	// metaCh and directCh can be populated either during Put or in background memory persist thread.
+	// Former possibility is eliminated by taking `modeMtx` mutex and
+	// latter by explicit persist in the previous step.
+	// flushCh is populated by `flush` with `modeMtx` is also taken.
+	// Thus all producers are shutdown and we only need to wait until all channels are empty.
+	for len(c.metaCh) != 0 || len(c.directCh) != 0 || len(c.flushCh) != 0 {
+		c.log.Info("waiting for channels to flush")
+		time.Sleep(time.Second)
+	}
+}

pkg/local_object_storage/writecache/persist.go

@@ -19,10 +19,28 @@ func (c *cache) persistLoop() {
 	for {
 		select {
 		case <-tick.C:
+			c.modeMtx.RLock()
+			if c.mode == ModeReadOnly {
+				c.modeMtx.RUnlock()
+				continue
+			}
+			c.persistMemoryCache()
+			c.modeMtx.RUnlock()
+		case <-c.closeCh:
+			return
+		}
+	}
+}
+
+func (c *cache) persistMemoryCache() {
 	c.mtx.RLock()
 	m := c.mem
 	c.mtx.RUnlock()
 
+	if len(m) == 0 {
+		return
+	}
+
 	sort.Slice(m, func(i, j int) bool { return m[i].addr < m[j].addr })
 
 	start := time.Now()
@@ -46,10 +64,6 @@ func (c *cache) persistLoop() {
 		c.curMemSize += uint64(len(c.mem[i].data))
 	}
 	c.mtx.Unlock()
-		case <-c.closeCh:
-			return
-		}
-	}
 }
 
 // persistSmallObjects persists small objects to the write-cache database and

pkg/local_object_storage/writecache/put.go

@@ -12,6 +12,12 @@ var ErrBigObject = errors.New("too big object")
 
 // Put puts object to write-cache.
 func (c *cache) Put(o *object.Object) error {
+	c.modeMtx.RLock()
+	defer c.modeMtx.RUnlock()
+	if c.mode == ModeReadOnly {
+		return ErrReadOnly
+	}
+
 	sz := uint64(o.ToV2().StableSize())
 	if sz > c.maxObjectSize {
 		return ErrBigObject

pkg/local_object_storage/writecache/writecache.go

@@ -21,6 +21,7 @@ type Cache interface {
 	Head(*objectSDK.Address) (*object.Object, error)
 	Delete(*objectSDK.Address) error
 	Put(*object.Object) error
+	SetMode(Mode)
 	DumpInfo() Info
 
 	Init() error
@@ -35,6 +36,9 @@ type cache struct {
 	mtx sync.RWMutex
 	mem []objectInfo
 
+	mode    Mode
+	modeMtx sync.RWMutex
+
 	// compressFlags maps address of a big object to boolean value indicating
 	// whether object should be compressed.
 	compressFlags map[string]struct{}
@@ -83,6 +87,7 @@ func New(opts ...Option) Cache {
 		metaCh:   make(chan *object.Object),
 		closeCh:  make(chan struct{}),
 		evictCh:  make(chan []byte),
+		mode:     ModeReadWrite,
 
 		compressFlags: make(map[string]struct{}),
 		options: options{