What would you like to be added?

As mentioned in the concurrency/Mutex TODO,

the current implementation causes unnecessary waiting when a session expires. The following example illustrates this issue.

Suppose three clients (A, B, and C) sequentially call the Lock method

to acquire a lock. At this point, A holds the lock, B is waiting for A (waiting for A’s key to be deleted), and C is waiting for B. Both client B and client C are blocked in the waitDeletes method. Now, if client C’s session cancels lease refresh using the Orphan method, the key held by client C in the waiting queue will be automatically deleted when the lease expires. However, client C will still remain blocked in the waitDeletes method until B’s key is deleted. Only then will client C be able to detect, through the check in the Lock method, that its key has already been deleted, thus returning the ErrSessionExpired error.

To resolve this issue, we can Watch for the deletion events of both the key with a smaller revision and the key held by the current client. This way, we can return immediately when the session expires instead of waiting until all keys with smaller revisions have been deleted.

Why is this needed?

Resolve the situation described in the TODO and reduce unnecessary lock waiting.