ConcurrentModificationException in getLogs()

[AndreyNudko]

We tried LogCaptor in some integration-style tests, where logging actually happens in background thread.
Occasionally getLogs() throws ConcurrentModificationException -
quick look into code shows that ListAppender.list is not thread-safe, so it's almost certainly a race between test and application threads. I think it's not a problem for actual logging since AppenderBase::doAppend is synchronized, but it is a problem when scanning the list.
One possible fix would be to wrap all access to list into synchronized(listAppender). Or, maybe even better, swap it for CopyOnWrite / Collections::synchronizedList.
Any views on that? Happy to submit PR as well.

[Hakky54]

Hi Andrey

Thank you for mentioning this issue. It looks like it is indeed not thread safe. I think both suggestions sounds as a good solution and I am ok with you creating and submitting a pull request. Looking forward to it!

[Hakky54]

I just tried it out it it looks like synchronize will do the trick, just like you suggested by wrapping it with synchronized(listAppender) before accessing it. So I don't think synchronizedList is needed with this approach. What do you think?

[AndreyNudko]

Thanks for taking a look!
I was thinking using synchronizedList would be a slightly cleaner approach - it's contract clearly states that It is imperative that the user manually synchronize on the returned list when traversing it via Iterator, Spliterator or Stream. Synchronising on appender relies on implementation details of the base class, which are sort of explained in http://logback.qos.ch/manual/appenders.html#AppenderBase, but in less strict terms.
That said, I don't think it's a big deal - either approach will work.
I'll try to find some time to do PR in the next couple of days, unless you're going to make changes.

[Hakky54]

Sounds good, I am looking forward to your PR with the synchronizedList

[Hakky54]

@AndreyNudko I will publish this version in the coming days, I will keep you updated

[AndreyNudko]

@Hakky54 Awesome, thanks for the quick turnaround!

[Hakky54]

@AndreyNudko The latest changes are now available within version 2.7.2