ost_utils: async abort of hanging connection

Use python's signal to abort a vmconsole connection that is hanging
forever.

Change-Id: I631f206b3f2fbf61dd0cd1f5337c4e58830fc18a
Signed-off-by: Eitan Raviv <eraviv@redhat.com>

ost_utils/vmconsole.py

@@ -57,6 +57,9 @@ def connect(self, vm_id):
     @contextlib.contextmanager
     def _connect(self, vm_id):
         try:
+            alarm = BlockingIOAlarm('timed out waiting to connect', 180)
+            signal.signal(signal.SIGALRM, alarm.handle)
+            signal.alarm(alarm.seconds)
             master, slave = pty.openpty()
             LOGGER.debug('vmconsole: opened pty')
             args = [
@@ -80,13 +83,15 @@ def _connect(self, vm_id):
                 stdout=subprocess.PIPE,
                 bufsize=0,
             )
+            signal.alarm(0)  # cancel
             LOGGER.debug(f'vmconsole: opened reader with args {args}')
             self._writer = os.fdopen(master, 'w')
             LOGGER.debug('vmconsole: opened writer')
             self._connected = True
             yield
         finally:
             self._disconnect()
+            signal.signal(signal.SIGALRM, signal.SIG_DFL)
 
     def _disconnect(self):
         LOGGER.debug('vmconsole: disconnecting...')