Go ahead with PR 1569 to exhaust the inner iterator when the outer iterator advances.

Rationale:  The OP expected the inner iterator to be exhausted.  That is also what the PyPy team originally implemented and it is what I would have expected.

Also, there are some loose analogies elsewhere in the language.  Raising StopIteration is what next(somefileobject) does when there is a seek-to-end between next() calls.  A list iterator raises StopIteration when there is a del somelist[:] between next() calls.  When zip(it1, it2) terminates on the shortest input, it leaves the other iterator alive, allowing it to run and terminally normally with StopIteration. And though I don't have a use case for it, I would expect that next(inner_iterator, default_value) would return a value from the input stream or the default value but would not fail with a RuntimeError (this would apply to islice() as well).