fix opening some files after download finished

lib/torrent.js

@@ -1603,9 +1603,29 @@ Torrent.prototype._checkDone = function () {
     if (!done) break
   }
   if (!self.done && done) {
+    const done = () => {
     self.done = true
-    self._debug('torrent done: ' + self.infoHash)
-    self.emit('done')
+      self._debug('torrent done: ' + self.infoHash)
+      self.emit('done')
+    }
+    // flush files to be able to open after download finish
+    if(self._store === FSChunkStore)
+    {
+      let tasks = self.store.store.files.map(function (file) {
+        return function (cb) {
+            file.open(function (err, file) {
+             // an open error is okay because that means the file is not open
+             if (err) return cb(null)
+               file.close(cb)
+            })
+          }
+      })
+      parallel(tasks, done)
+    }
+    else
+    {
+      done()
+    }
   }
   self._gcSelections()