We were testing mongoid-locker, because we detected consistency issues (two threads entered lock at the same time). After simplifying the test scenario and removing mongoid-locker and mongoid dependencies we concluded that in following case two threads are able to update the same document:

db[:wallets].update_one({
  :_id => id,
  '$or' => [
    # not locked
    { locked_until: nil },
    # expired
    { locked_until: { '$lte' => time } }
   ]
  },

  '$set' => {
     locked_at: time,
     locked_until: expiration
   }
)

I haven't found the exact reason for that. I prepared a test case in this repo:

https://github.com/mrzasa/mongoid-locker-debugging/ 

THREAD_COUNT=500 PROCESS_COUNT=2 bundle exec ruby test/raw_test.rb

The issue happens when several threads tries to run updates to acquire the lock (as quoted above) and other unrelated updates on the same document.

I am not sure if it's a bug in the ruby driver or mongo itself. As I reproduced it only in Ruby, I report it here. Further investigation is required to determine if it may happen in other drivers as well.

I have tested it on mongo: 3.2.16, 3.2.17 and 4.0.5 on docker and ubuntu 18.04, Linux  4.15.0-33-generic #36-Ubuntu x86_64. I'm eager to share any further details of my test and provide assistance in fixing it.