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https://issues.apache.org/jira/browse/MESOS-980?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13897328#comment-13897328
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Benjamin Mahler commented on MESOS-980:
---------------------------------------
Part 1: https://reviews.apache.org/r/17685/
Part 2: https://reviews.apache.org/r/17686/
> Revisit Future discard semantics to enforce that transitions occur through a
> Promise.
> -------------------------------------------------------------------------------------
>
> Key: MESOS-980
> URL: https://issues.apache.org/jira/browse/MESOS-980
> Project: Mesos
> Issue Type: Improvement
> Components: libprocess
> Reporter: Benjamin Mahler
> Assignee: Benjamin Hindman
>
> Previously, invoking Future::discard caused a future to transition
> into the DISCARDED state which meant that any callbacks associated wit
> such a transition (i.e., Future::onAny, Future::onDiscarded) got
> executed _even if the computation associated with the future had not
> yet completed_! This meant certain patterns where difficult if not all
> together impossible. Consider:
>
> ExpensiveResource* resource = new ExpensiveResource();
>
> Future<int> future = function()
> .then([=] () { ... *resource ...; })
> .onDiscarded([=] () { delete resource; });
>
> future.discard();
>
> In the above code, it's possible that the onDiscarded lambda would
> execute _concurrently_ with the lambda in the then! With the new
> semantics, a discard is more advisorial: rather than transition a
> future it instead allows a subsequent computation to decide whether or
> not to cancel (or even begin) some computation. For example, the
> Future::then implementation uses the "discard" information to decide
> whether or not to execute the next lambda after the previous future
> has been satisfied.
>
> A few new helpers have been added to support this refactor:
>
> Future::onDiscard(callback):
>
> Adds a callback for when Future::discard was executed on this future.
>
> Future::hasDiscard():
>
> Checks to see if Future::discard was executed on this future.
>
> Promise::discard():
>
> Transitions this promises underlying future to DISCARDED.
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