spy.reset_mock(side_effect=True) makes spies stop calling the spied code #636

mocker.spy() calls the spied code from the spy's side_effect. #633 made mocker.resetall(side_effect=True) restore it after the reset, but calling reset_mock(side_effect=True) on the spy itself still clears it, so the spy returns a MagicMock from then on:

class Foo:
    @classmethod
    def class_bar(cls, x):
        return x * 3

def test_spy_reset_mock(mocker):
    spy = mocker.spy(Foo, "class_bar")
    spy.reset_mock(side_effect=True)
    assert Foo.class_bar(2) == 6  # fails: returns <MagicMock name='class_bar()' ...>

Reproduced on 3.16.1. It affects spies whose mock is a MagicMock: classmethods, staticmethods, builtins and classes. Spies on functions and methods are autospecced functions whose reset_mock() takes no arguments, so they aren't affected today.

Fixing it means handling the reset on the spy object itself (for example overriding reset_mock on spies), since resetall() is not involved here.