A colleague packaging Python for Red Hat Entreprise Linux reported me that tests hang randomly on test_exception_errpipe_bad_data() of test_subprocess. I don't know why exactly, but using strace, I noticed that the "unit test" calls os.waitpid() with the pid returned by the mocked fork_exec(): pid 0. So the test calls os.waitpid(0, 0).

In a REPL on my Fedora 26, os.waitpid(0, 0) raises "ChildProcessError: [Errno 10] No child processes". I'm not sure that waitpid() is the cause of the hang, but it doesn't seem correct to me to call waitpid() with the result of a mock function, since the mock doesn't create a real child process.

Attached PR 3896 mocks also os.waitpid() to fix this bug. I reopen the issue to discuss this bug in the new test added in this issue.