Re: CompletableFuture.cancel vs Future.cancel bug?

Doug,

Thanks for the clarification!

Yes, my interpretation was exactly "successful attempt" == "return true",
"failed attempt" == "return false". Would be nice to have this spec
improved.

Also, could you please clarify why the decision was made not to have
CAS-like semantics for CompletableFuture.cancel() while it was clearly
possible? Maybe there are some hidden benefits I don't see?



On Sun, Aug 23, 2020 at 3:34 PM Doug Lea via Concurrency-interest <
[email protected]> wrote:

> I agree that the Future.cancel javadoc should be improved to better
> distinguish the return value spec from the method description. It was not
> the intent to require the return value to be a CAS-like indication of
> whether the current invocation was responsible for cancellation. It is nice
> when it can do so (and some implementations do), but the intention for this
> high-level interface spec was to allow a wide range of implementations,.
> Perhaps it would have been better to define cancel as void, requiring a
> call to isCancelled to check. In other words...
> On 8/20/20 3:06 PM, Sergi Vladykin via Concurrency-interest wrote:
>
>
> Here is the part with "typically":
>
>      * @return {@code false} if the task could not be cancelled,
>      * typically because it has already completed normally;
>      * {@code true} otherwise
>
> I read it as "usually it is impossible to cancel the task because it has
> already been completed normally, but there are other reasons: for example
> it has already been completed exceptionally or cancelled".
>
> The case left out here is "already cancelled", which can go either way.
>
>
> And I've already cited the beginning of that javadoc which explicitly
> states that the attempt WILL fail if the task has been cancelled:
>
>      * Attempts to cancel execution of this task.  This attempt will
>      * fail if the task has already completed, has already been cancelled,
>      * or could not be cancelled for some other reason.
>
> I see no room for wiggling here.
>
> "The attempt" is not a description of return value. Sorry for the
> confusion.
>
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