[Linux] ppid() is incorrect in case of Process thread

[giampaolo]

Note: Linux is the only platform allowing to instantiate a thread via Process class as in thread = Process(tid), and query the thread as if it was a process.

This test case fails:

    def test_thread_process_ppid(self):
        with ThreadTask():
            p1 = psutil.Process()
            threads = p1.threads()
            assert len(threads) == 2
            tid = threads[1].id
            p2 = psutil.Process(tid)
            assert p2.ppid() == os.getpid()

In this case p2.ppid() (name) is bash instead of python (which is "us", the process who actually started the thread).
Apparently, when Linux deals with a process thread, /proc/{TID}/status behaves differently in terms of Tgid and PPid columns. E.g. this is a normal process:

$ cat /proc/$PID/status
Name:   cat
Tgid:   125294
Pid:    125294
PPid:   22482

And this is a thread:

$ cat /proc/$TID/status
Name:	python3
Tgid:	125724
Pid:	125725
PPid:	125723

When Pid and Tgid are different, that indicates we're dealing with a thread (this I knew already). The problem is that PPid does not point to the process which spawned the thread ("python"in my case) but its parent ("sh"):

(Pdb++) psutil.Process(125723)  # PPid column returns 'sh' (wrong)
psutil.Process(pid=125723, name='sh', status='sleeping', started='15:17:43')
(Pdb++) psutil.Process(125724)  # Tgid columnt returns 'python3' (correct)
psutil.Process(pid=125724, name='python3', status='running', started='15:17:43')

This exact condition is also handled by forkstat util:
https://github.com/ColinIanKing/forkstat/blob/6d3bea2af4d7fa923b4647feb40244f154f9f5ad/forkstat.c#L700-L733
When dealing with a thread, it takes the ppid from Tgid column instead of PPid. This seems to return the correct result.

[giampaolo]

Also kind of related: #1989.