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We should make sure that users don't have to call getResults on the returned ResultCollector. If we want to make it easier for users to implement a ResultCollector that waits for all of the results to arrive, we should provide helper classes like a BlockingResultCollector that waits for all results to arrive in getResults.

Part of the reason is is required right now is the way that exceptions get handled. If a function throws an exception or even just sends an exception with ResultSender.sendResult, the user's ResultCollector will never see it. The only way to get the exception is to call getResult on the result collector returned by execute. See

Summary of changes

New interfaces

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