Add Last db view in kvdb and statedb

Last db view is an opened pebble db which always contains a checkpoint from the
last batch.  Methods to access this last batch are thread safe so that views of
the last checkpoint can be made anywhere and with a consistent view of the
state.

db/kvdb/kvdb.go

@@ -24,6 +24,9 @@ const (
 	// PathCurrent defines the subpath of the current Batch in the subpath
 	// of the KVDB
 	PathCurrent = "current"
+	// PathLast defines the subpath of the last Batch in the subpath
+	// of the StateDB
+	PathLast = "last"
 )
 
 var (
@@ -42,6 +45,58 @@ type KVDB struct {
 	CurrentBatch common.BatchNum
 	keep         int
 	m            sync.Mutex
+	last         *Last
+}
+
+// Last is a consistent view to the last batch of the stateDB that can
+// be queried concurrently.
+type Last struct {
+	db   *pebble.Storage
+	path string
+	rw   sync.RWMutex
+}
+
+func (k *Last) setNew() error {
+	k.rw.Lock()
+	defer k.rw.Unlock()
+	if k.db != nil {
+		k.db.Close()
+	}
+	lastPath := path.Join(k.path, PathLast)
+	err := os.RemoveAll(lastPath)
+	if err != nil {
+		return tracerr.Wrap(err)
+	}
+	db, err := pebble.NewPebbleStorage(path.Join(k.path, lastPath), false)
+	if err != nil {
+		return tracerr.Wrap(err)
+	}
+	k.db = db
+	return nil
+}
+
+func (k *Last) set(kvdb *KVDB, batchNum common.BatchNum) error {
+	k.rw.Lock()
+	defer k.rw.Unlock()
+	if k.db != nil {
+		k.db.Close()
+	}
+	lastPath := path.Join(k.path, PathLast)
+	if err := kvdb.MakeCheckpointFromTo(batchNum, lastPath); err != nil {
+		return tracerr.Wrap(err)
+	}
+	db, err := pebble.NewPebbleStorage(lastPath, false)
+	if err != nil {
+		return tracerr.Wrap(err)
+	}
+	k.db = db
+	return nil
+}
+
+func (k *Last) close() {
+	k.rw.Lock()
+	defer k.rw.Unlock()
+	k.db.Close()
 }
 
 // NewKVDB creates a new KVDB, allowing to use an in-memory or in-disk storage.
@@ -58,6 +113,9 @@ func NewKVDB(pathDB string, keep int) (*KVDB, error) {
 		path: pathDB,
 		db:   sto,
 		keep: keep,
+		last: &Last{
+			path: pathDB,
+		},
 	}
 	// load currentBatch
 	kvdb.CurrentBatch, err = kvdb.GetCurrentBatch()
@@ -74,6 +132,13 @@ func NewKVDB(pathDB string, keep int) (*KVDB, error) {
 	return kvdb, nil
 }
 
+// LastRead is a thread-safe method to query the last KVDB
+func (kvdb *KVDB) LastRead(fn func(db *pebble.Storage) error) error {
+	kvdb.last.rw.RLock()
+	defer kvdb.last.rw.RUnlock()
+	return fn(kvdb.last.db)
+}
+
 // DB returns the *pebble.Storage from the KVDB
 func (kvdb *KVDB) DB() *pebble.Storage {
 	return kvdb.db
@@ -139,14 +204,21 @@ func (kvdb *KVDB) reset(batchNum common.BatchNum, closeCurrent bool) error {
 		kvdb.db = sto
 		kvdb.CurrentIdx = common.RollupConstReservedIDx // 255
 		kvdb.CurrentBatch = 0
+		if err := kvdb.last.setNew(); err != nil {
+			return tracerr.Wrap(err)
+		}
 
 		return nil
 	}
 
-	// copy 'BatchNumX' to 'current'
+	// copy 'batchNum' to 'current'
 	if err := kvdb.MakeCheckpointFromTo(batchNum, currentPath); err != nil {
 		return tracerr.Wrap(err)
 	}
+	// copy 'batchNum' to 'last'
+	if err := kvdb.last.set(kvdb, batchNum); err != nil {
+		return tracerr.Wrap(err)
+	}
 
 	// open the new 'current'
 	sto, err := pebble.NewPebbleStorage(currentPath, false)
@@ -334,6 +406,10 @@ func (kvdb *KVDB) MakeCheckpoint() error {
 	if err := kvdb.db.Pebble().Checkpoint(checkpointPath); err != nil {
 		return tracerr.Wrap(err)
 	}
+	// copy 'CurrentBatch' to 'last'
+	if err := kvdb.last.set(kvdb, kvdb.CurrentBatch); err != nil {
+		return tracerr.Wrap(err)
+	}
 	// delete old checkpoints
 	if err := kvdb.deleteOldCheckpoints(); err != nil {
 		return tracerr.Wrap(err)
@@ -456,4 +532,5 @@ func pebbleMakeCheckpoint(source, dest string) error {
 // Close the DB
 func (kvdb *KVDB) Close() {
 	kvdb.db.Close()
+	kvdb.last.close()
 }