host: Add kill_children utility method

This method is useful for killing the children spawned by
Popen() calls with shell=True. For instance:

proc = Popen('sleep 100', shell=True)
proc.kill()

This would spawn a shell task and that shell would spawn the sleep task.
Issuing a kill to the Popen handle will only kill the shell task,
and the sleep task will keep running.

Using host.kill_children(proc.pid) will ensure all child tasks are
killed.

devlib/host.py

@@ -14,6 +14,7 @@
 #
 from glob import iglob
 import os
+import signal
 import shutil
 import subprocess
 import logging
@@ -24,6 +25,11 @@ from devlib.utils.misc import check_output
 
 PACKAGE_BIN_DIRECTORY = os.path.join(os.path.dirname(__file__), 'bin')
 
+def kill_children(pid, signal=signal.SIGKILL):
+    with open('/proc/{0}/task/{0}/children'.format(pid), 'r') as fd:
+        for cpid in map(int, fd.read().strip().split()):
+            kill_children(cpid, signal)
+            os.kill(cpid, signal)
 
 class LocalConnection(object):