RFR: 8343426: ConcurrentSkipListMap.spliterator() can no longer split the stream

[Viktor Klang]

On Fri, 1 Nov 2024 10:13:15 GMT, kabutz <duke at openjdk.org> wrote:

> Since Java 10, spliterators for the ConcurrentSkipListMap were pointing to the head, which has item == null, rather than to the first element. The trySplit() method no longer worked, and always returned null. Therefore, parallel streams have not worked for ConcurrentSkipListMap and ConcurrentSkipListSet since Java 10. It worked correctly in Java 8 and 9.
> 
> The problem lies in the constructor calls for the various spliterators. In Java 9, we pass in head.node.next as "origin" into the  constructor. In Java 10, this was changed to just head.node as "origin". Since the "item" field is always null on head.node, we never enter the first if() statement in the trySplit() method and thus it always returns null.

Hi @kabutz,
Interesting catch!
A regression test (splitability) and a test which verifies correct operation under parallel evaluation would be appropriate here.

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PR Comment: https://git.openjdk.org/jdk/pull/21820#issuecomment-2451687772