hey Viswa, I'm by no means an expert on this chunk of code, but I've done a bit of digging and it certainly seems that you've uncovered an issue.
Ultimately the root cause of the issue is the weirdness in the way that ZK is handling ephemeral nodes. I'm not sure if this is intentional or a bug, but I would have thought that if the ephemeral nodes are tied to a session then they should be removed as soon as the session has expired. >From the Curator standpoint, it appears that the InterProcessMutex has been written with the assumption that ephemeral nodes are deleted when their session expires. To fix it on the Curator side, I think that we would need to provide a way to interrupt the acquire() method, so that when the connection goes into a SUSPENDED state we can force the restart of the acquisition method. I guess you could just explicitly interrupt the thread when your ConnectionStateListener gets a SUSPENDED event, but this is a bit ugly. Might be worth raising the issue on the ZK lists to see if this is a bug or by design. Any other devs have any thoughts? cheers On Tue, Jul 20, 2021 at 3:45 AM Viswanathan Rajagopal <[email protected]> wrote: > Hi Team, > > Good day. > > Recently came across “Double Locking Issue (i.e. two clients acquiring > lock)” using Curator code ( InterProcessMutex lock APIs ) in our application > > Our use case: > > * Two clients attempts to acquire the zookeeper lock using Curator > InterProcessMutex and whoever owns it would release it once sees the > connection disconnect ( on receiving Connection.SUSPENDED / Connection.LOST > Curator Connection Events from Connection listener) > > Issue we noticed: > > * After session expired & reconnected with new session, both client > seems to have acquired the lock. Interesting thing that we found is that > one of the clients still holds the lock while its lock node (ephemeral) was > gone > > Things we found: > > * Based on our initial analysis and few test runs, we saw that Curator > acquire() method acquires the lock based on “about to be deleted lock node > of previous session”. Explanation : Ephemeral node created by previous > session was still seen by client that reconnected with new session id > until server cleans that up. If this happens, Curator acquire() would hold > the lock. > > > > * Clearly we could see the race condition (in zookeeper code) between > 1). Client reconnecting to server with new session id and 2). server > deleting the ephemeral nodes of client’s previous session. We were able to > reproduce this issue using the following approach, > * Artificially break the socket connection between client and > server for 30s > * Artificially pausing the set of server codes for a min and unpause > > > * On the above mentioned race condition, if client manage to reconnect > to server with new session id before server cleans up the ephemeral nodes > of client’s previous session, Curator lock acquire() who is trying to > acquire the lock will hold the lock as it still sees the lock node in > zookeeper directory. Eventually server would be cleaning up the ephemeral > nodes leaving the Curator local lock thread data stale giving the illusion > that it still hold the lock while its ephemeral node is gone > > > * Timeline events described below for better understanding, > * At t1, Client A and Client B establishes zookeeper session with > session id A1 and B1 respectively > * At t2, Client A creates the lock node N1 & acquires the lock > * At t3, Client B creates the lock node N2 & blocked on acquire() > to acquire the lock > * At t4, session timed out for both clients & server is about to > clean up the old session • Client A trying to release the lock > * At t5, Client A and Client B reconnects to server with new > session id A2 and B2 respectively before server deletes the ephemeral node > N1 & N2 of previous client session. Client A releases the lock, deleting N1 > and trying to acquire it again by creating N3 node and Client B who is > blocked on acquire() acquires the lock based on N2 (about to be deleted > node created by previous session) > * At t6, server cleans up the ephemeral node N2 created by Client > B’s previous session. Client A acquires the lock with node N3 as its > previous sequence N2 gets deleted, whereas Client B who incorrectly > acquired the lock at t5 timeline still holds the lock > > > Note: > > * We are not certain that this race condition that we noticed in > zookeeper code is intentional design. > > Questions: > > * Given this race condition seen in zookeeper code, we would like to > hear your recommendations / suggestions to avoid this issue while using > Curator lock code? > > > > * We also see that Interprocess Mutex has the makeRevocable() API that > enables application to revoke lock, but that handles Node change event only > but not for Node deleted event. I understand that this makes sense to > handle Node change event alone, as to enable application user to revoke > lock externally from application side. But would it be also to okay to have > one for Node delete event, so as application can register the listener for > Node delete event. I would like to hear your thoughts. > > > Many Thanks, > Viswa >
