RFR: 8340572: ConcurrentModificationException when sorting ArrayList sublists

[Daniel Fuchs]

On Sun, 29 Sep 2024 17:44:30 GMT, Attila Szegedi <attila at openjdk.org> wrote:

> Fixes a regression with #17818 where `ArrayList.subList(…).sort()` started incrementing `ArrayList.modCount` resulting in some cases throwing a `ConcurrentModificationException` where none was thrown before.
> 
> This change keeps the optimization from #17818 but restores the behavior where only sorting the `ArrayList` changes the mod count, but sorting its sublists does not.

I am not a spcialist here. But if sorting the parent list is considered as a modification, and if the sublist is just a view of the parent list, then surely sorting/modifying the sublist should be considered as a modification of the parent list? 

In which case the issue here is probably deeper: SubList and parent list appear to maintain separate `modcount` variables which are only loosely coupled. Maybe that's the bug that ought to be fixed?

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PR Comment: https://git.openjdk.org/jdk/pull/21250#issuecomment-2383407392