avocado.utils.process: add get_parent_pid() utility method

That returns the parent PID for the given process.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

avocado/utils/process.py

@@ -135,6 +135,22 @@ def safe_kill(pid, signal):  # pylint: disable=W0621
         return False
 
 
+def get_parent_pid(pid):
+    """
+    Returns the parent PID for the given process
+
+    TODO: this is currently Linux specific, and needs to implement
+    similar features for other platforms.
+
+    :param pid: The PID of child process
+    :returns: The parent PID
+    :rtype: int
+    """
+    with open('/proc/%d/stat' % pid, 'rb') as proc_stat:
+        parent_pid = proc_stat.read().split(b' ')[-49]
+        return int(parent_pid)
+
+
 def kill_process_tree(pid, sig=signal.SIGKILL, send_sigcont=True):
     """
     Signal a process and all of its children.

selftests/unit/test_utils_process.py

@@ -10,6 +10,7 @@ except ImportError:
     import mock
 
 
+from .. import recent_mock
 from avocado.utils import astring
 from avocado.utils import gdb
 from avocado.utils import process
@@ -278,6 +279,14 @@ class MiscProcessTests(unittest.TestCase):
                          [u"avok\xe1do_test_runner",
                           u"arguments"])
 
+    @unittest.skipUnless(recent_mock(),
+                         "mock library version cannot (easily) patch open()")
+    def test_get_parent_pid(self):
+        stat = b'18405 (bash) S 24139 18405 18405 34818 8056 4210688 9792 170102 0 7 11 4 257 84 20 0 1 0 44336493 235409408 4281 18446744073709551615 94723230367744 94723231442728 140723100226000 0 0 0 65536 3670020 1266777851 0 0 0 17 1 0 0 0 0 0 94723233541456 94723233588580 94723248717824 140723100229613 140723100229623 140723100229623 140723100233710 0'
+        with mock.patch('avocado.utils.process.open',
+                        return_value=io.BytesIO(stat)):
+            self.assertTrue(process.get_parent_pid(0), 24139)
+
 
 class CmdResultTests(unittest.TestCase):