Exceptions thrown in asynchronous calls can be lost if the value of the future is never queried

[brycelelbach]

Typically, if an exception is thrown inside of an action/function invoked by hpx::async, the exception is stored in the shared state of the future and reported when the user calls .get() on the future (it is either rethrown or stored in the user-supplied hpx::error_code).

However, if the user does not call .get() on the future, the exception will not be reported (see testcase). I believe this is very dangerous behavior, because I don't believe most users would expect this - it's a "gotcha".

I had an issue with this today, and I only found out about the exception by going through the logs.

I think that we must abort if a future is being destroyed and it has an exception that has never been retrieved. Simply logging (even at the error log level) is not sufficient. Obviously we can't throw from the future dtor.

[brycelelbach]

Do we currently have a way of determining (from within the future) whether the value of the future has been queried?

[hkaiser]

We don't have that. Do we need it?

[hkaiser]

The added test is wrong. The only way to handle an exception received from a future is to call .get(). Please remove the test as it's misleading.

[hkaiser]

The test has been removed by 6cee12a