[rfc] [icedtea-web] Fix EDT hanging on OpenJDK 11

[Jiri Vanek]

...
> 
>      >     3. It seems that methods that use "observable" are synchronized.
>      >     However, the EDT will use "observable" and it's not synchronized. Is
>      >     this safe?
>      >
>      >
>      > Ok, fixed:
>      >                 synchronized(observable) {
>      >                     if (observable.hasChanged() ||
>      > (Boolean.TRUE.equals(force))) {
>      >                         observable.notifyObservers(force);
>      >                     }
>      >                 }
> 
>     Right, but I don't think this addresses the issue, one is synchronized
>     on the observable, the other methods are on the JavaConsole, so we
>     should probably pick one.
> 
> 
> Sorry, I did not catch the problem.
> My fix is very minimal and this patch does not fix synchronization issues already present in ITW.
> I tested javaws with the console opened and did not notice any problem.
> 
> 
>     By going through the code though, it seems like we could just ensure
>     that updateModel() always run in the EDT and remove all other
>     synchronised keywords from the methods, the only user of this method
>     which can execute outside the EDT is addMessage, but otherwise all other
>     consumers are executing in the EDT.
> 
> 
> Could we postpone this point later as I did not want to rewrite too much the JavaConsole ?
> You propose to remove all 'synchronized' keywords in JavaConsole, I am pretty sure the 

I would strongly advice against removing the synchronizations. The logging bottleneck is lazily 
initiated singleton, and there were many troubles without it. Maybe with plugin gone, the troubles 
will decrease a lot, But i doubt.

On contrary:

 >on the observable, the other methods are on the JavaConsole, so we
 >     should probably pick one.

Sounds like straightforward fix. Anyway, this is on your judgement.

> synchronization overhead does not hurt here...
> If you could propose an alternative, I am OK to integrate it.

Please, as another changeset (if any).



Thanx!
   J.