[#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/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)
+	}
+}