[HIVE-21469] Review of ZooKeeperHiveLockManager

• Type: Improvement
• Status: Patch Available
• Priority: Major
• Resolution: Unresolved
• Affects Version/s: 3.2.0, 4.0.0
• Component/s: Locking
• Flags: Patch
• Assignee: David Mollitor
• Reporter: David Mollitor
• Created: 19/Mar/19 02:08
• Updated: 24/Mar/19 18:18

Description

A lot of sins in this class to resolve:

  @Override
  public void setContext(HiveLockManagerCtx ctx) throws LockException {
 try {
      curatorFramework = CuratorFrameworkSingleton.getInstance(conf);
      parent = conf.getVar(HiveConf.ConfVars.HIVE_ZOOKEEPER_NAMESPACE);
      try{
        curatorFramework.create().withMode(CreateMode.PERSISTENT).forPath("/" +  parent, new byte[0]);
      } catch (Exception e) {
        // ignore if the parent already exists
        if (!(e instanceof KeeperException) || ((KeeperException)e).code() != KeeperException.Code.NODEEXISTS) {
          LOG.warn("Unexpected ZK exception when creating parent node /" + parent, e);
        }
      }

Every time a new session is created and this setContext method is called, it attempts to create the root node. I have seen that, even though the root node exists, an create node action is written to the ZK logs. Check first if the node exists before trying to create it.

      try {
        curatorFramework.delete().forPath(zLock.getPath());
      } catch (InterruptedException ie) {
        curatorFramework.delete().forPath(zLock.getPath());
      }

There has historically been a quite a few bugs regarding leaked locks. The Driver will signal the session Thread by performing an interrupt. That interrupt can happen any time and it can kill a create/delete action within the ZK framework. We can see one example of workaround for this. If the ZK action is interrupted, simply do it again. Well, what if it's interrupted yet again? The lock will be leaked. Also, when the InterruptedException is caught in the try block, the thread's interrupted flag is cleared. The flag is not reset in this code and therefore we lose the fact that this thread has been interrupted. This flag should be preserved so that other code paths will know that it's time to exit.

        if (tryNum > 1) {
          Thread.sleep(sleepTime);
        }
        unlockPrimitive(hiveLock, parent, curatorFramework);
        break;
      } catch (Exception e) {
        if (tryNum >= numRetriesForUnLock) {
          String name = ((ZooKeeperHiveLock)hiveLock).getPath();
          throw new LockException("Node " + name + " can not be deleted after " + numRetriesForUnLock + " attempts.",
              e);
        }
      }

... related... the sleep here may be interrupted, but we still need to delete the lock (again, for fear of leaking it). This sleep should be uninterruptible. If we need to get the lock deleted, and there's a problem, interrupting the sleep will cause the code to eventually exit and locks will be leaked.

It also requires a bunch more TLC.