RecursionError after exception in the psutil.Popen.__init__()

[Cykooz]

Look at this code from constructor of psutil.Popen:

psutil/psutil/__init__.py

Line 1384 in fe0799f

self.__subproc = subprocess.Popen(*args, **kwargs)

If it raises exception, then self.__subproc will be absent in the instance of psutil.Popen. In this case follow code will causes an exception RecursionError.

psutil/psutil/__init__.py

Line 1416 in fe0799f

return object.__getattribute__(self.__subproc, name)

This bug can be fixed very simple - you will need to add a default value of __subproc at class level:

class Popen(Process):
    __subproc = None

    def __init__(self, *args, **kwargs):
        self.__subproc = subprocess.Popen(*args, **kwargs)
        self._init(self.__subproc.pid, _ignore_nsp=True)

[giampaolo]

How can I reproduce this?

[Cykooz]

from raven.base import DummyClient
import psutil

client = DummyClient()
client.is_enabled = lambda: True

try:
    proc = psutil.Popen('program_not_found')
except Exception as e:
    client.captureException()

[giampaolo]

Can't reproduce it. I get:

Traceback (most recent call last):
  File "foo.py", line 6, in <module>
    proc = psutil.Popen('program_not_found')
  File "/home/giampaolo/svn/psutil/psutil/__init__.py", line 1384, in __init__
    self.__subproc = subprocess.Popen(*args, **kwargs)
  File "/usr/lib/python2.7/subprocess.py", line 711, in __init__
    errread, errwrite)
  File "/usr/lib/python2.7/subprocess.py", line 1343, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory

[Cykooz]

I uses Python 3.6. Maybe the error only appears in Python 3.

[giampaolo]

I can't reproduce it. Please show me the full traceback. Actually can you do that (reproduce the issue) from the interactive python shell itself and paste the output?

[Cykooz]

https://gist.github.com/Cykooz/5a5a7d1c3d78017fcefb6769fde811b1

[giampaolo]

There's something weird. Before RecursionError I see 'Popen' object has no attribute '_pid' but the traceback is truncated (perhaps due to client.captureException()?). Please just do:

>>> import psutil
>>> psutil.Popen('program_not_found')

Do you get an error like that?

[Cykooz]

The whole point is in using raven (it is a client for Sentry). Without it, the RecursionError error does not occur, because default traceback from Python does not look at the contents of objects inside the frame with an error.

[giampaolo]

So it does not occur if you remove client.captureException()?

[Cykooz]

Traceback is not truncated - it ends with RecursionError, because raven tries to get some information about the Popen instance (it calls method __repr__() for it), in which an exception occurs.

[Cykooz]

Yes, RecursionError does not occur if used default Python traceback.

[giampaolo]

If you define __subproc as a class attribute what error do you get? Can you see AttributeError: 'Popen' object has no attribute '_pid'? Actually I'm more interested in understanding where that comes from rather than RecursionError.

[Cykooz]

• raven gets Popen instance as self variable from code frame of Popen.__init__() .
• Next, raven calls something like str(variable). This leads to calling this code:

psutil/psutil/__init__.py

Lines 441 to 454 in fe0799f

	def __str__(self):
	try:
	pid = self.pid
	name = repr(self.name())
	except ZombieProcess:
	details = "(pid=%s (zombie))" % self.pid
	except NoSuchProcess:
	details = "(pid=%s (terminated))" % self.pid
	except AccessDenied:
	details = "(pid=%s)" % (self.pid)
	else:
	details = "(pid=%s, name=%s)" % (pid, name)
	return "%s.%s%s" % (self.__class__.__module__,
	self.__class__.__name__, details)

• pid = self.pid leads to calling this code:
  

  psutil/psutil/__init__.py

  Lines 1411 to 1419 in fe0799f

  	def __getattribute__(self, name):
  	try:
  	return object.__getattribute__(self, name)
  	except AttributeError:
  	try:
  	return object.__getattribute__(self.__subproc, name)
  	except AttributeError:
  	raise AttributeError("%s instance has no attribute '%s'"
  	% (self.__class__.__name__, name))

• The first exception that will occur here is AttributeError: 'Popen' object has no attribute '_pid'
• Then the code return object.__getattribute__(self.__subproc, name) will be called. This again causes the call to Popen.__getattribute__() but already in order to get the __subproc.

[giampaolo]

OK, good. What traceback do you get by defining __subproc as a class attribute?

[Cykooz]

https://gist.github.com/Cykooz/72ffb05e65112c06ea9b7e718d0d387f

As you can see, in this case python-shell did not fall and did not exit to the console.

[giampaolo]

Mmmm... probably better but still incorrect as the right exception should be FileNotFoundError not AttributeError. I assume raven is doing something wrong here. I am tempted to assume it's a raven's bug, not psutil's.

[Cykooz]

Exception FileNotFoundError located at the start of the traceback. All other exception occur because raven calls return repr(value), where value is instance of Popen. These exceptions do not matter, raven handles them correctly. As a result, raven will not be able to get information about the Popen instance, but this is not critical, because the instance has no useful information.

[Cykooz]

This is not bug of raven. raven only calls repr(value), where value is instance of Popen. This results in the whole process of the python falling with the error RecursionError.

Your code expects that a Popen instance (or the Popen class itself) must contain the __subproc variable. But if you have an error in the constructor of Popen, then the instance is created without this variable. Therefore, further access to this instance results in a fatal error RecursionError.

[giampaolo]

I don't understand how can raven call repr against the Popen instance since the error occurs when you try to instantiate the instance (hence there's no real "instance" at that point). How did you figure out it's raven calling repr()? I see no repr reference in the traceback you pasted. Are you able to identify where in the raven code this is done?

[Cykooz]

In Python, instance is created by __new__ method of class, not by __init__ method. Therefore, instance already exists when exception occur in __init__ method.

You can see repr at the end of the last traceback.

Traceback (most recent call last):
  File "/home/cykooz/buildout-eggs/raven-6.1.0-py2.py3-none-any.ovo/raven/utils/serializer/manager.py", line 76, in transform
    return repr(value)
  File "/home/cykooz/buildout-eggs/psutil-5.2.2-py3.6-linux-x86_64.egg/psutil/__init__.py", line 446, in __repr__
    return "<%s at %s>" % (self.__str__(), id(self))
  File "/home/cykooz/buildout-eggs/psutil-5.2.2-py3.6-linux-x86_64.egg/psutil/__init__.py", line 432, in __str__
    pid = self.pid
  File "/home/cykooz/buildout-eggs/psutil-5.2.2-py3.6-linux-x86_64.egg/psutil/__init__.py", line 1405, in __getattribute__
    % (self.__class__.__name__, name))
AttributeError: Popen instance has no attribute 'pid'
'83a1e04e00654a99800c2e9b25f0eccc'

[Cykooz]

Ok, you may fix it by other way - by correct of getting self.__subproc in the Popen.__getattribute__().

    def __getattribute__(self, name):
        try:
            return object.__getattribute__(self, name)
        except AttributeError:
            try:
                subproc = object.__getattribute__(self, '__subproc')
                return object.__getattribute__(subproc, name)
            except AttributeError:
                raise AttributeError("%s instance has no attribute '%s'"
                                     % (self.__class__.__name__, name))

[Cykooz]

Or you may use __getattr__() instead of __getattribute__():

    def __getattr__(self, name):
        subproc = object.__getattribute__(self, '__subproc')
        return getattr(subproc, name)

[Cykooz]

Using access to the attributes of an instance through a dot (like self.__subproc) inside of __getattribute__() or __getattr__() methods is not a very good way.

[Cykooz]

Small fix for my last code example:

    def __getattr__(self, name):
        subproc = object.__getattribute__(self, '_Popen__subproc')
        return getattr(subproc, name)

An identifier of class-private member is textually replaced with _classname__varname (https://docs.python.org/3/tutorial/classes.html#private-variables)