Reset exception in WithCleanupFinish

[dchiquito]

Context managers have an __exit__ function that returns a boolean-like object. If the object is truthy, then exceptions are suppressed.

If an exception was thrown while resolving that boolean, it would leak and live on in the error stack, getting tacked on to all future exceptions. This caused several mysterious test failures which would only trigger after this very specific event was tested in test_with.

The solution is to move a call to vm.set_exception() before attempting the try_to_bool() which threw the error.

Minimal example to reproduce the bug:

import sys
import traceback

class cm(object):
    def __init__(self):
        pass

    def __enter__(self):
        return 3

    def __exit__(self, a, b, c):
        class Bool:
            def __bool__(self):
                1 // 0
        return Bool()

try:
    with cm():
        raise Exception("Should NOT see this")
except ZeroDivisionError:
    print("exception caught, as expected")

print("There should now be no exception")
traceback.print_exc()
print(sys.exc_info())

[youknowone]

👍

[youknowone]

Thank you so much! This fix seems hard to find the place to fix but related to many with bugs!