The close method of AbstractSSHHost cleans the ssh master connection and after that tries to execute a 'pkill -f ...' that creates an additional master connection that is never closed.

Steps To Reproduce:

import common
from autotest_lib.server import hosts
from server.cros.network import wifi_client

h = hosts.create_host('ip')
w = wifi_client.WiFiClient(h, './debug', False)
w.close()

Verify there is still one connection open:

ps aux | grep 'ip' 

Expected Result:

0 connections open from ps aux output

Actual Result:

1 connection open

How frequently does this problem reproduce? (Always, sometimes, hard to
reproduce?)

Always

What is the impact to the user, and is there a workaround? If so, what is
it?

Ssh master connections will accumulate until the system become unstable