Propagate abandoned future errors via future<T>::then()

[coryan]

Consider this code:

std::future<int> F() { return std::promise<int>().get_future(); }
google::cloud::future<int> G() { return google::cloud::promise<int>().get_future(); }

The expression F().get() will throw a std::future_error(std::future_errc::broken_promise)¹:

https://en.cppreference.com/w/cpp/thread/promise/%7Epromise

The expression G().get() behaves the same way, that all sounds good. Now consider this function:

google::cloud::future<int> H() {
  return google::cloud::promise<int>().get_future().then([](auto g) { return 2 * g.get(); });
}

Currently H().get() deadlocks. That is not a good thing. I can fix it, the fix is trivial, but unfortunately we have unit tests that depend on the existing behavior.

We should fix the tests, unfortunately it is not that easy. These tests (and the code they are testing) needs to work with exceptions disabled (i.e., the noex builds aka -fno-exceptions). With exceptions disabled .get() terminates the program if the promise was abandoned (or the future<T> contains any exception).

So we need to find what promise is being destroyed and abandoning the future, and most likely, set the value to some kind of google::cloud::Status before destroying the promise.

Footnotes

1. this is the only sensible choice. Once cannot leave .get() blocked forever, and there is no valid int value to return. ↩