target: Run Target.disconnect() upon process termination

Use atexit handler to run Target.disconnect() when the process is about
to exit. This avoids running it with a half torn down namespace, with
ensuing exceptions and non-clean disconnect.

devlib/target.py

@@ -13,6 +13,7 @@
 # limitations under the License.
 #
 
+import atexit
 import asyncio
 from contextlib import contextmanager
 import io
@@ -40,6 +41,7 @@ from past.builtins import long
 from past.types import basestring
 from numbers import Number
 from shlex import quote
+from weakref import WeakMethod
 try:
     from collections.abc import Mapping
 except ImportError:
@@ -413,6 +415,10 @@ class Target(object):
         ))
         self._modules = modules
 
+        atexit.register(
+            WeakMethod(self.disconnect, atexit.unregister)
+        )
+
         self._update_modules('early')
         if connect:
             self.connect(max_async=max_async)
@@ -530,8 +536,11 @@ class Target(object):
 
         for conn in itertools.chain(connections, self._unused_conns):
             conn.close()
-        if self._async_pool is not None:
-            self._async_pool.__exit__(None, None, None)
+
+        pool = self._async_pool
+        self._async_pool = None
+        if pool is not None:
+            pool.__exit__(None, None, None)
 
     def __enter__(self):
         return self

devlib/utils/ssh.py

@@ -24,14 +24,12 @@ import tempfile
 import socket
 import sys
 import time
-import atexit
 import contextlib
 import select
 import copy
 import functools
 import shutil
 from shlex import quote
-from weakref import WeakMethod
 
 from paramiko.client import SSHClient, AutoAddPolicy, RejectPolicy
 import paramiko.ssh_exception
@@ -372,8 +370,6 @@ class SshConnection(SshConnectionBase):
         self.client = None
         try:
             self.client = self._make_client()
-            weak_close = WeakMethod(self.close, atexit.unregister)
-            atexit.register(weak_close)
 
             # Use a marker in the output so that we will be able to differentiate
             # target connection issues with "password needed".
@@ -818,9 +814,6 @@ class TelnetConnection(SshConnectionBase):
 
         self.conn = telnet_get_shell(host, username, password, port, timeout, original_prompt)
 
-        weak_close = WeakMethod(self.close, atexit.unregister)
-        atexit.register(weak_close)
-
     def fmt_remote_path(self, path):
         return '{}@{}:{}'.format(self.username, self.host, path)