Fix race condition when getting cmdline

Even though we check for the existence of the process before, it may
still terminate before we get to read its command line, so catch the
error that can occur here.

Backport: Amended test using tools.OpenFixture() as the move of this
code to neutron-lib only happened later

Change-Id: I3e89aca8bedfd2912effe2490718223f7d03133e
Closes-Bug: 1844500
(cherry picked from commit 3330bc01cb)

neutron/agent/linux/utils.py

@@ -323,8 +323,13 @@ def process_is_running(pid):
 def get_cmdline_from_pid(pid):
     if not process_is_running(pid):
         return []
+    # NOTE(jh): Even after the above check, the process may terminate
+    # before the open below happens
+    try:
         with open('/proc/%s/cmdline' % pid, 'r') as f:
             cmdline = f.readline().split('\0')[:-1]
+    except IOError:
+        return []
 
     # NOTE(slaweq): sometimes it may happen that values in
     # /proc/{pid}/cmdline are separated by space instead of NUL char,

neutron/tests/unit/agent/linux/test_utils.py

@@ -336,6 +336,16 @@ class TestGetCmdlineFromPid(base.BaseTestCase):
         mock_open.assert_not_called()
         self.assertEqual([], cmdline)
 
+    def test_cmdline_process_disappearing(self):
+        self.process_is_running_mock.return_value = True
+        mock_open = self.useFixture(
+            tools.OpenFixture('/proc/%s/cmdline' % self.pid, 'process')
+        ).mock_open
+        mock_open.side_effect = IOError()
+        cmdline = utils.get_cmdline_from_pid(self.pid)
+        mock_open.assert_called_once_with('/proc/%s/cmdline' % self.pid, 'r')
+        self.assertEqual([], cmdline)
+
 
 class TestFindChildPids(base.BaseTestCase):