Fix #2239 / proc name(): don't fail with ZombieProcess on cmdline()

A recent failure observed on OpenBSD led me to an interesting consideration.

```
======================================================================
ERROR: psutil.tests.test_process.TestProcess.test_long_name
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/vagrant/psutil/psutil/_psbsd.py", line 566, in wrapper
    return fun(self, *args, **kwargs)
  File "/vagrant/psutil/psutil/_psbsd.py", line 684, in cmdline
    return cext.proc_cmdline(self.pid)
ProcessLookupError: [Errno 3] No such process

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/vagrant/psutil/psutil/tests/test_process.py", line 751, in test_long_name
    self.assertEqual(p.name(), os.path.basename(testfn))
  File "/vagrant/psutil/psutil/__init__.py", line 628, in name
    cmdline = self.cmdline()
  File "/vagrant/psutil/psutil/__init__.py", line 681, in cmdline
    return self._proc.cmdline()
  File "/vagrant/psutil/psutil/_psbsd.py", line 569, in wrapper
    raise ZombieProcess(self.pid, self._name, self._ppid)
psutil.ZombieProcess: PID still exists but it's a zombie (pid=48379)
----------------------------------------------------------------------
```

The exception above occurs sporadically. It originates from `sysctl
(KERN_PROC_ARGV)`:
https://github.com/giampaolo/psutil/blob/0a81fa089fd4b25b4b7ee71ed39213b83f73c052/psutil/arch/openbsd/proc.c#L149
The error per se does not represent a bug in the OpenBSD `cmdline
()` implemention because the process **really** is a zombie at that point
(I'm not sure why it's a zombie - this seems only to occur only on OpenBSD for
this specific test case - but that's not the point).

The interesting thing is that the test calls process `name()` (which succeeds,
despite it's a zombie process), but since the process name is too long it gets
truncated to 15 chars (this is a UNIX thing) so psutil tries to guess the
remaining characters from the process `cmdline()`, which fails:
https://github.com/giampaolo/psutil/blob/0a81fa089fd4b25b4b7ee71ed39213b83f73c052/psutil/__init__.py#L623-L630

The problem to fix here is that, if `name()` succeeds but `cmdline()` fails, we
should not raise `ZombieProcess`: we should simply return the
(truncated) process `name()` instead, because that is better than nothing.
Not on OpenBSD but on all platforms.

Signed-off-by: Giampaolo Rodola <g.rodola@gmail.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/HISTORY.rst b/HISTORY.rst
index 97c2c20e..003dad23 100644
--- a/HISTORY.rst
+++ b/HISTORY.rst
@@ -19,6 +19,10 @@
   (e.g. directory no longer exists), in which case we returned either ``None``
   or an empty string. This was consolidated and we now return ``""`` on all
   platforms.
+- 2239_, [UNIX]: if process is a zombie, and we can only determine part of the
+  its truncated `Process.name()`_ (15 chars), don't fail with `ZombieProcess`_
+  when we try to guess the full name from the `Process.cmdline()`_. Just
+  return the truncated name.
 
 **Bug fixes**