Serhiy Storchaka:
> I mean that after writing tests they can be tested manually by disabling conditions for posix_spawn one by one. I.e. some tests should fail if remove "stdout is None" and some tests should fail if remove "not close_fds", etc.

I made some manual tests on my PR 11452. I changed close_fds default value from True to False. I also modified my change to use posix_spawnp using Joannah's PR 11554 of bpo-35674.

The following tests fail *as expected*:

* test_close_fds_when_max_fd_is_lowered
* test_exception_errpipe_normal
* test_exception_errpipe_bad_data

The 2 errpipe tests mock subprocess to inject errors in the error pipe... but posix_spawn() doesn't expose its private "error pipe", so the test is not relevant for posix_spawn().

test_close_fds_when_max_fd_is_lowered() tests close_fds=True behavior. It's expected that it fails.

At least, I didn't notice any bug.