text: import contextlib
  import sys
  import logging
  import time
  import itertools
  
  from teuthology.config import config
  from teuthology.exceptions import MaxWhileTries
  
  
  log = logging.getLogger(__name__)
  
  @contextlib.contextmanager
  def nested(*managers):
      """
      Like contextlib.nested but takes callables returning context
      managers, to avoid the major reason why contextlib.nested was
      deprecated.
  
      This version also logs any exceptions early, much like run_tasks,
      to ease debugging. TODO combine nested and run_tasks.
      """
      exits = []
      vars = []
      exc = (None, None, None)
      try:
          for mgr_fn in managers:
              mgr = mgr_fn()
              exit = mgr.__exit__
              enter = mgr.__enter__
              vars.append(enter())
              exits.append(exit)
          yield vars
      except Exception:
          log.exception('Saw exception from nested tasks')
          exc = sys.exc_info()
          # FIXME this needs to be more generic
          if config.ctx and config.ctx.config.get('interactive-on-error'):
              config.ctx.config['interactive-on-error'] = False
              from teuthology.task import interactive
              log.warning('Saw failure, going into interactive mode...')
              interactive.task(ctx=config.ctx, config=None)
      finally:
          while exits:
              exit = exits.pop()
              try:
                  if exit(*exc):
                      exc = (None, None, None)
              except Exception:
                  exc = sys.exc_info()
          if exc != (None, None, None):
              # Don't rely on sys.exc_info() still containing
              # the right information. Another exception may
              # have been raised and caught by an exit method
              raise exc[1]
  
  
  class safe_while(object):
      """
      A context manager to remove boiler plate code that deals with `while` loops
      that need a given number of tries and some seconds to sleep between each
      one of those tries.
  
      The most simple example possible will try 10 times sleeping for 6 seconds:
  
          >>> from teuthology.contexutil import safe_while
          >>> with safe_while() as proceed:
          ...    while proceed():
          ...        # repetitive code here
          ...        print("hello world")
          ...
          Traceback (most recent call last):
          ...
          MaxWhileTries: reached maximum tries (5) after waiting for 75 seconds
  
      Yes, this adds yet another level of indentation but it allows you to
      implement while loops exactly the same as before with just 1 more
      indentation level and one extra call. Everything else stays the same,
      code-wise. So adding this helper to existing code is simpler.
  
      :param sleep:     The amount of time to sleep between tries. Default 6
      :param increment: The amount to add to the sleep value on each try.
                        Default 0.
      :param tries:     The amount of tries before giving up. Default 10.
      :param action:    The name of the action being attempted. Default none.
      :param _raise:    Whether to raise an exception (or log a warning).
                        Default True.
      :param _sleeper:  The function to use to sleep. Only used for testing.
                        Default time.sleep
      """
  
      def __init__(self, sleep=6, increment=0, tries=10, action=None,
                   _raise=True, _sleeper=None):
          self.sleep = sleep
          self.increment = increment
          self.tries = tries
          self.counter = 0
          self.sleep_current = sleep
          self.action = action
          self._raise = _raise
          self.sleeper = _sleeper or time.sleep
  
      def _make_error_msg(self):
          """
          Sum the total number of seconds we waited while providing the number
          of tries we attempted
          """
          total_seconds_waiting = sum(
              itertools.islice(
                  itertools.count(self.sleep, self.increment),
                  self.tries
              )
          )
          msg = 'reached maximum tries ({tries})' + \
              ' after waiting for {total} seconds'
          if self.action:
              msg = "'{action}' " + msg
  
          msg = msg.format(
              action=self.action,
              tries=self.tries,
              total=total_seconds_waiting,
          )
          return msg
  
      def __call__(self):
          self.counter += 1
          if self.counter == 1:
              return True
          if self.counter > self.tries:
              error_msg = self._make_error_msg()
              if self._raise:
                  raise MaxWhileTries(error_msg)
              else:
                  log.warning(error_msg)
                  return False
          self.sleeper(self.sleep_current)
          self.sleep_current += self.increment
          return True
  
      def __enter__(self):
          return self
  
      def __exit__(self, exc_type, exc_val, exc_tb):
          return False
  