Regression in 4.0.0 vs 3.0.0 in coverage with multiprocessing

[segasai]

Summary

When using multiprocessing, coverage is lost with pytest-cov 4.0.0 comparing to 3.0.0

Expected vs actual result

Expected coverage 100%, actual 33%

Reproducer

We need two files

$ cat /tmp/test/test_x.py 
import multiprocessing as mp
import xtest

def test_x():
    pool = mp.Pool(2, xtest.initer)
    pool.map(xtest.doer, range(10))
    pool.close()
    pool.join()

$ cat /tmp/test/py/xtest.py 
def initer():
    x = 2
    y = 3
    z = 4

def doer(x):
    return x

When I run this with pytest-cov-4.0.0 I get this

$ env PYTHONPATH=py:$PYTHONPATH pytest --cov=xtest test_x.py 
============================= test session starts ==============================
platform linux -- Python 3.8.10, pytest-7.1.3, pluggy-1.0.0
rootdir: /tmp/test
plugins: profiling-1.7.0, forked-1.4.0, parallel-0.1.1, cov-4.0.0, anyio-3.6.1, xdist-2.5.0
collected 1 item                                                               

test_x.py .                                                              [100%]

---------- coverage: platform linux, python 3.8.10-final-0 -----------
Name          Stmts   Miss  Cover
---------------------------------
py/xtest.py       6      4    33%
---------------------------------
TOTAL             6      4    33%

============================== 1 passed in 0.04s ===============================

Now with the 3.0.0 version of pytest-cov

/tmp/test$ env PYTHONPATH=py:$PYTHONPATH pytest --cov=xtest test_x.py 
============================= test session starts ==============================
platform linux -- Python 3.8.10, pytest-7.1.3, pluggy-1.0.0
rootdir: /tmp/test
plugins: cov-3.0.0, profiling-1.7.0, forked-1.4.0, parallel-0.1.1, anyio-3.6.1, xdist-2.5.0
collected 1 item                                                               

test_x.py .                                                              [100%]

---------- coverage: platform linux, python 3.8.10-final-0 -----------
Name          Stmts   Miss  Cover
---------------------------------
py/xtest.py       6      0   100%
---------------------------------
TOTAL             6      0   100%

============================== 1 passed in 0.06s ===============================

Versions

$ python --version
Python 3.8.10
$ pytest --version
pytest 7.1.3
$ pip list| grep pytest
pytest                            7.1.3
pytest-cov                        3.0.0
pytest-forked                     1.4.0
pytest-parallel                   0.1.1
pytest-profiling                  1.7.0
pytest-xdist                      2.5.0
$ pip list| grep cover
coverage                          6.5.0
coveralls                         3.3.1

Config

default config

Code

given above

[segasai]

I see that multiprocessing support is dropped in 4.0.0. Sorry for the noise.

[azuline]

I see that multiprocessing support is dropped in 4.0.0. Sorry for the noise.

Where did you see this? I've been unsuccessfully trying to get multiprocessing to work.

[segasai]

I see that multiprocessing support is dropped in 4.0.0. Sorry for the noise.

Where did you see this? I've been unsuccessfully trying to get multiprocessing to work.

See here: https://github.com/pytest-dev/pytest-cov/blob/master/CHANGELOG.rst

[azuline]

Thanks so muchhhh