Future.map can apply function more than once

[osyvokon]

Let r be a result of Future.map call. Its type is Future. When I call r.get() multiple times, it will recompute its value that many times. This behavior differs from ordinary "non-map" Futures which cache computed value.

Failing unit-test for illustration:

def test_map_computes_each_value_only_once(self):
    def f(x):
        f.calls_count += 1
    f.calls_count = 0

    future = self.results[0].map(f)
    self.results[0].set(30)
    future.get(timeout=0)
    future.get(timeout=0)

    self.assertEqual(f.calls_count, 1)

To recap, two problems here:

1. Mapped function call can be expensive, for example, it can start new actors.
2. Future.get() semantics differs for Futures obtained with map and ask.

Same issue applies to filter/reduce.

[jodal]

Confirmed. Will fix in the next release.