Graceful shutdown when stopping with Ctrl+C

[theova]

When running a mininet without a cli (net.disableCli()) and hitting Ctrl-C, I get the following trace:

Traceback (most recent call last):
  File "/usr/local/lib/python3.8/dist-packages/psutil/_pslinux.py", line 1653, in wrapper
    return fun(self, *args, **kwargs)
  File "/usr/local/lib/python3.8/dist-packages/psutil/_common.py", line 480, in wrapper
    raise raise_from(err, None)
  File "<string>", line 3, in raise_from
  File "/usr/local/lib/python3.8/dist-packages/psutil/_common.py", line 478, in wrapper
    return fun(self)
  File "/usr/local/lib/python3.8/dist-packages/psutil/_pslinux.py", line 1695, in _parse_stat_file
    data = bcat("%s/%s/stat" % (self._procfs_path, self.pid))
  File "/usr/local/lib/python3.8/dist-packages/psutil/_common.py", line 813, in bcat
    return cat(fname, fallback=fallback, _open=open_binary)
  File "/usr/local/lib/python3.8/dist-packages/psutil/_common.py", line 801, in cat
    with _open(fname) as f:
  File "/usr/local/lib/python3.8/dist-packages/psutil/_common.py", line 765, in open_binary
    return open(fname, "rb", buffering=FILE_READ_BUFFER_SIZE)
FileNotFoundError: [Errno 2] No such file or directory: '/proc/2442/stat'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/lib/python3.8/dist-packages/psutil/__init__.py", line 361, in _init
    self.create_time()
  File "/usr/local/lib/python3.8/dist-packages/psutil/__init__.py", line 719, in create_time
    self._create_time = self._proc.create_time()
  File "/usr/local/lib/python3.8/dist-packages/psutil/_pslinux.py", line 1653, in wrapper
    return fun(self, *args, **kwargs)
  File "/usr/local/lib/python3.8/dist-packages/psutil/_pslinux.py", line 1863, in create_time
    ctime = float(self._parse_stat_file()['create_time'])
  File "/usr/local/lib/python3.8/dist-packages/psutil/_pslinux.py", line 1660, in wrapper
    raise NoSuchProcess(self.pid, self._name)
psutil.NoSuchProcess: process no longer exists (pid=2442)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "network.py", line 187, in <module>
    net.stopNetwork()
  File "/home/p4/p4-tools/p4-utils/p4utils/mininetlib/network_API.py", line 1160, in stopNetwork
    self.stop_exec_scripts()
  File "/home/p4/p4-tools/p4-utils/p4utils/mininetlib/network_API.py", line 1154, in stop_exec_scripts
    kill_proc_tree(pid)
  File "/home/p4/p4-tools/p4-utils/p4utils/utils/helper.py", line 481, in kill_proc_tree
    parent = psutil.Process(pid)
  File "/usr/local/lib/python3.8/dist-packages/psutil/__init__.py", line 332, in __init__
    self._init(pid)
  File "/usr/local/lib/python3.8/dist-packages/psutil/__init__.py", line 373, in _init
    raise NoSuchProcess(pid, msg='process PID not found')
psutil.NoSuchProcess: process PID not found (pid=2442)

The error comes from the following lines:

p4-utils/p4utils/utils/helper.py

Lines 466 to 483 in e02f06f

	def kill_proc_tree(pid, sig=signal.SIGKILL, include_parent=True,
	timeout=None, on_terminate=None):
	"""Kills a process tree (including children).
	Args:
	pid (int) : PID of the parent process
	sig (int) : signal used to kill the tree
	include_parent (bool) : whether to kill the parent process or not
	timeout (int or float) : time to wait for a process to terminate
	on_terminate (types.FunctionType): callback function executed as soon as a child terminates.
	Returns:
	tuple: ``(gone, still_alive)``.
	"""
	assert pid != os.getpid(), "won't kill myself"
	parent = psutil.Process(pid)
	children = parent.children(recursive=True)
	if include_parent:

An approach to fix this problem is to check whether the process actually exists:

    try:
        parent = psutil.Process(pid)
    except psutil.NoSuchProcess:
        return (None, None)

However, I'm not sure what should be returned in case that there is no process; returning (None, None) is most probably not the best way to do it.

[theova]

The problem originated from

    subprocess.call(["pkill", "mx"])
    net.stopNetwork()
```.

Swapping the two lines fixes it.