0.14: ConcurrentModificationException in MemorySubscriptionsRepository

[hylkevds]

The MemorySubscriptionsRepository implementation does not synchronise write access to the subscriptions list. As a result, Iterations over the list will break when another Thread modifies the list.

I'm not sure what the best way to solve this is:

• the easy way is to use a CopyOnWriteArrayList to protect iterations, and to synchronise add and remove operations. This would not be good for add/remove performance.
• Another option is to use a ConcurrentSkipListSet that has better add/remove performance, but that would mean the interface of the public List<Subscription> listAllSubscriptions() method would need to change to return a Set.

What do you think?
I'll make a PR for the change to ConcurrentSkipListSet. The other one is pretty trivial.

Expected behavior

No exceptions

Actual behavior

ConcurrentModificationExceptions in MemorySubscriptionsRepository

Steps to reproduce

Have many clients subscribe and unsubscribe often, while publishes are in progress.

Minimal yet complete reproducer code (or URL to code) or complete log file

• https://github.com/hylkevds/moquetteTests/
• In MoquetteTest
  • Set CLIENT_LIVE_MILLIS = 100;
  • Set threadCountListen = 200;
  • Comment-out line 213 that sets the PERSISTENT_STORE_PROPERTY_NAME, so that the MemorySubscriptionsRepository is used

Moquette MQTT version

0.14

JVM version (e.g. java -version)

OpenJDK 11

OS version (e.g. uname -a)

Linux

[hylkevds]

PR is here: #572