py39: branch coverage reports uncovered return branch

[a-feld]

Describe the bug

Coverage reports return statement as uncovered when branch coverage is enabled in py39.

To Reproduce

example.py

def main():
    try:
        return object()
    finally:
        try:
            pass
        except:
            pass

main()

command

coverage run --branch example.py && coverage html

environment

Python Version: Python 3.9.0
coverage.py version: Coverage.py, version 5.3 with C extension (although without C extension reports the same results)

Expected behavior

coverage reports the return statement as covered in Python 3.9, the same as Python 3.8

Additional context

From what I can see, the disassembly of main in example.py is different in py38 versus py39
Coverage is expecting an arc from the return statement to the first code line. In py38, this arc occurs. In py39, this arc does not occur.

When using sys.settrace with a debug function, the outputs are different between py38 and py39.

import sys

from example import main


def trace(frame, event, arg):
    lineno = frame.f_lineno
    print(event, lineno)
    return trace


sys.settrace(trace)
main()

py38 output:

call 1
line 2
line 3
line 5
line 6
line 3
return 3

py39 output:

call 1
line 2
line 3
line 5
line 6
return 6

[tirkarthi]

On bisecting this seems to have been due to changes made in python/cpython#6641

[yaythomas]

thanks for a detailed bug report @a-feld and analysis @tirkarthi! you both just saved me a lot of digging! 🙌

I'm dealing with something similar here https://github.com/pypyr/pypyr/blob/d4a36f0dbf6a3e47773dadfb8deebd73aa1e0bcf/pypyr/dsl.py#L463

Code is definitely covered and used to pass 100% covered with py 3.6 through to py 3.8.

As of py 3.9, reports 463->exit as uncovered.