Make copy of appender list when removing all, for increased thread-safety

[tmct]

Hello,

We have been witnessing this error now and again while using log4net 2.0.15:

Collection was modified; enumeration operation may not execute.
   at log4net.Appender.AppenderCollection.Enumerator.MoveNext()
   at log4net.Util.AppenderAttachedImpl.RemoveAllAppenders()
   at log4net.Repository.Hierarchy.Logger.RemoveAllAppenders()
   at log4net.Repository.Hierarchy.Hierarchy.Shutdown()
   at log4net.Core.LoggerManager.Shutdown()
   at log4net.LogManager.Shutdown()

It looks like the code on master is still susceptible to this, what do you think of this change? (The try-catch should be wel-equipped to handle appenders that have left the List, so long as we can still access their name for the error message...)

Thanks,
Tom

[FreeAndNil]

@tmct thanks for your pull request.

I would prefer the pattern already used in AppendLoopOnAppenders():

_appenderArray ??= _appenderList.ToArray();
foreach (IAppender appender in _appenderArray)

Can you adjust the code?

[tmct]

Yes of course, consistency can be very useful! Thank you.

[tmct]

Hi @FreeAndNil - I had a look and I don't think the private _appenderArray was actually doing any caching at all beyond the scope of each individual method. And it likely hinders the thread-safeness of anything that uses it. Most likely the original intent was to protect against exactly the thing we're seeing here.

I've removed it everywhere if that's ok?

[FreeAndNil]

@tmct the _appenderArray had the advantage to copy the contents only once into an array.
Now every method call (even those who only read) create a new Array.

I'm currently considering switching to System.ComponentModel.Collections.Collection as base class.
But that would probably be a breaking change.

[tmct]

Ah yes you could be right, this could specifically have a performance impact in AppendLoopOnAppenders, being the method among these that we expect to be called frequently without a change in the list, the problem being the repeated filing of the same array.

I can address that later today or Monday, thanks.