Proper zombie process handling

Giampaolo RodolaJun 13, 20151 min readapi-designcompatibility

This is part of the psutil 3.0 release (see the full release notes).

Except on Linux and Windows (which does not have them), support for zombie processes was broken. The full story is in #428.

The problem

Say you create a zombie process and instantiate a Process for it:

import os, time

def create_zombie():
    pid = os.fork()  # the zombie
    if pid == 0:
        os._exit(0)  # child exits immediately
    else:
        time.sleep(1000)  # parent does NOT call wait()

pid = create_zombie()
p = psutil.Process(pid)

Up until psutil 2.X, every time you tried to query it you’d get a NoSuchProcess exception:

>>> p.name()
  File "psutil/__init__.py", line 374, in _init
    raise NoSuchProcess(pid, None, msg)
psutil.NoSuchProcess: no process found with pid 123

This was misleading, because the PID technically still existed:

>>> psutil.pid_exists(p.pid)
True

Depending on the platform, some process information could still be retrieved:

>>> p.cmdline()
['python']

Worst of all, psutil.process_iter() didn’t return zombies at all. That was a real problem, because identifying them is a legitimate use case: a zombie usually indicates a bug where a parent process spawns a child, kills it, but never calls wait() to reap it.

What changed

• A new ZombieProcess exception is raised whenever a process cannot be queried because it is a zombie.

• It replaces NoSuchProcess, which was incorrect and misleading.

• ZombieProcess inherits from NoSuchProcess, so existing code keeps working.

• psutil.process_iter() now correctly includes zombie processes, so you can reliably identify them:

import psutil

zombies = []
for p in psutil.process_iter():
    try:
        if p.status() == psutil.STATUS_ZOMBIE:
            zombies.append(p)
    except psutil.NoSuchProcess:
        pass