Multiprocess: parallel testing

Overview

The multiprocess plugin enables you to distribute your test run among a set of worker processes that run tests in parallel. This can speed up CPU-bound test runs (as long as the number of work processeses is around the number of processors or cores available), but is mainly useful for IO-bound tests that spend most of their time waiting for data to arrive from someplace else.

Note

See ../doc_tests/test_multiprocess/multiprocess for additional documentation and examples. Use of this plugin on python 2.5 or earlier requires the multiprocessing module, also available from PyPI.

How tests are distributed

The ideal case would be to dispatch each test to a worker process separately. This ideal is not attainable in all cases, however, because many test suites depend on context (class, module or package) fixtures.

The plugin can’t know (unless you tell it – see below!) if a context fixture can be called many times concurrently (is re-entrant), or if it can be shared among tests running in different processes. Therefore, if a context has fixtures, the default behavior is to dispatch the entire suite to a worker as a unit.

Controlling distribution

There are two context-level variables that you can use to control this default behavior.

If a context’s fixtures are re-entrant, set _multiprocess_can_split_ = True in the context, and the plugin will dispatch tests in suites bound to that context as if the context had no fixtures. This means that the fixtures will execute concurrently and multiple times, typically once per test.

If a context’s fixtures can be shared by tests running in different processes – such as a package-level fixture that starts an external http server or initializes a shared database – then set _multiprocess_shared_ = True in the context. These fixtures will then execute in the primary nose process, and tests in those contexts will be individually dispatched to run in parallel.

How results are collected and reported

As each test or suite executes in a worker process, results (failures, errors, and specially handled exceptions like SkipTest) are collected in that process. When the worker process finishes, it returns results to the main nose process. There, any progress output is printed (dots!), and the results from the test run are combined into a consolidated result set. When results have been received for all dispatched tests, or all workers have died, the result summary is output as normal.

Beware!

Not all test suites will benefit from, or even operate correctly using, this plugin. For example, CPU-bound tests will run more slowly if you don’t have multiple processors. There are also some differences in plugin interactions and behaviors due to the way in which tests are dispatched and loaded. In general, test loading under this plugin operates as if it were always in directed mode instead of discovered mode. For instance, doctests in test modules will always be found when using this plugin with the doctest plugin.

But the biggest issue you will face is probably concurrency. Unless you have kept your tests as religiously pure unit tests, with no side-effects, no ordering issues, and no external dependencies, chances are you will experience odd, intermittent and unexplainable failures and errors when using this plugin. This doesn’t necessarily mean the plugin is broken; it may mean that your test suite is not safe for concurrency.

New Features in 1.1.0

  • functions generated by test generators are now added to the worker queue making them multi-threaded.

  • fixed timeout functionality, now functions will be terminated with a TimedOutException exception when they exceed their execution time. The worker processes are not terminated.

  • added --process-restartworker option to restart workers once they are done, this helps control memory usage. Sometimes memory leaks can accumulate making long runs very difficult.

  • added global _instantiate_plugins to configure which plugins are started on the worker processes.

Options

--processes=NUM

Spread test run among this many processes. Set a number equal to the number of processors or cores in your machine for best results. Pass a negative number to have the number of processes automatically set to the number of cores. Passing 0 means to disable parallel testing. Default is 0 unless NOSE_PROCESSES is set. [NOSE_PROCESSES]

--process-timeout=SECONDS

Set timeout for return of results from each test runner process. Default is 10. [NOSE_PROCESS_TIMEOUT]

--process-restartworker

If set, will restart each worker process once their tests are done, this helps control memory leaks from killing the system. [NOSE_PROCESS_RESTARTWORKER]

Plugin

class nose.plugins.multiprocess.MultiProcess

Bases: nose.plugins.base.Plugin

Run tests in multiple processes. Requires processing module.

configure(options, config)

Configure plugin.

options(parser, env)

Register command-line options.

prepareTestLoader(loader)

Remember loader class so MultiProcessTestRunner can instantiate the right loader.

prepareTestRunner(runner)

Replace test runner with MultiProcessTestRunner.

Source

"""
Overview
========

The multiprocess plugin enables you to distribute your test run among a set of
worker processes that run tests in parallel. This can speed up CPU-bound test
runs (as long as the number of work processeses is around the number of
processors or cores available), but is mainly useful for IO-bound tests that
spend most of their time waiting for data to arrive from someplace else.

.. note ::

   See :doc:`../doc_tests/test_multiprocess/multiprocess` for
   additional documentation and examples. Use of this plugin on python
   2.5 or earlier requires the multiprocessing_ module, also available
   from PyPI.

.. _multiprocessing : http://code.google.com/p/python-multiprocessing/

How tests are distributed
=========================

The ideal case would be to dispatch each test to a worker process
separately. This ideal is not attainable in all cases, however, because many
test suites depend on context (class, module or package) fixtures.

The plugin can't know (unless you tell it -- see below!) if a context fixture
can be called many times concurrently (is re-entrant), or if it can be shared
among tests running in different processes. Therefore, if a context has
fixtures, the default behavior is to dispatch the entire suite to a worker as
a unit.

Controlling distribution
^^^^^^^^^^^^^^^^^^^^^^^^

There are two context-level variables that you can use to control this default
behavior.

If a context's fixtures are re-entrant, set ``_multiprocess_can_split_ = True``
in the context, and the plugin will dispatch tests in suites bound to that
context as if the context had no fixtures. This means that the fixtures will
execute concurrently and multiple times, typically once per test.

If a context's fixtures can be shared by tests running in different processes
-- such as a package-level fixture that starts an external http server or
initializes a shared database -- then set ``_multiprocess_shared_ = True`` in
the context. These fixtures will then execute in the primary nose process, and
tests in those contexts will be individually dispatched to run in parallel.

How results are collected and reported
======================================

As each test or suite executes in a worker process, results (failures, errors,
and specially handled exceptions like SkipTest) are collected in that
process. When the worker process finishes, it returns results to the main
nose process. There, any progress output is printed (dots!), and the
results from the test run are combined into a consolidated result
set. When results have been received for all dispatched tests, or all
workers have died, the result summary is output as normal.

Beware!
=======

Not all test suites will benefit from, or even operate correctly using, this
plugin. For example, CPU-bound tests will run more slowly if you don't have
multiple processors. There are also some differences in plugin
interactions and behaviors due to the way in which tests are dispatched and
loaded. In general, test loading under this plugin operates as if it were
always in directed mode instead of discovered mode. For instance, doctests
in test modules will always be found when using this plugin with the doctest
plugin.

But the biggest issue you will face is probably concurrency. Unless you
have kept your tests as religiously pure unit tests, with no side-effects, no
ordering issues, and no external dependencies, chances are you will experience
odd, intermittent and unexplainable failures and errors when using this
plugin. This doesn't necessarily mean the plugin is broken; it may mean that
your test suite is not safe for concurrency.

New Features in 1.1.0
=====================

* functions generated by test generators are now added to the worker queue
  making them multi-threaded.
* fixed timeout functionality, now functions will be terminated with a
  TimedOutException exception when they exceed their execution time. The
  worker processes are not terminated.
* added ``--process-restartworker`` option to restart workers once they are
  done, this helps control memory usage. Sometimes memory leaks can accumulate
  making long runs very difficult.
* added global _instantiate_plugins to configure which plugins are started
  on the worker processes.

"""

import logging
import os
import sys
import time
import traceback
import unittest
import pickle
import signal
import nose.case
from nose.core import TextTestRunner
from nose import failure
from nose import loader
from nose.plugins.base import Plugin
from nose.pyversion import bytes_
from nose.result import TextTestResult
from nose.suite import ContextSuite
from nose.util import test_address
try:
    # 2.7+
    from unittest.runner import _WritelnDecorator
except ImportError:
    from unittest import _WritelnDecorator
from queue import Empty
from warnings import warn
try:
    from io import StringIO
except ImportError:
    import io

# this is a list of plugin classes that will be checked for and created inside 
# each worker process
_instantiate_plugins = None

log = logging.getLogger(__name__)

Process = Queue = Pool = Event = Value = Array = None

# have to inherit KeyboardInterrupt to it will interrupt process properly
class TimedOutException(KeyboardInterrupt):
    def __init__(self, value = "Timed Out"):
        self.value = value
    def __str__(self):
        return repr(self.value)

def _import_mp():
    global Process, Queue, Pool, Event, Value, Array
    try:
        from multiprocessing import Manager, Process
        #prevent the server process created in the manager which holds Python 
        #objects and allows other processes to manipulate them using proxies
        #to interrupt on SIGINT (keyboardinterrupt) so that the communication
        #channel between subprocesses and main process is still usable after
        #ctrl+C is received in the main process.
        old=signal.signal(signal.SIGINT, signal.SIG_IGN)
        m = Manager()
        #reset it back so main process will receive a KeyboardInterrupt
        #exception on ctrl+c
        signal.signal(signal.SIGINT, old)
        Queue, Pool, Event, Value, Array = (
                m.Queue, m.Pool, m.Event, m.Value, m.Array
        )
    except ImportError:
        warn("multiprocessing module is not available, multiprocess plugin "
             "cannot be used", RuntimeWarning)


class TestLet:
    def __init__(self, case):
        try:
            self._id = case.id()
        except AttributeError:
            pass
        self._short_description = case.shortDescription()
        self._str = str(case)

    def id(self):
        return self._id

    def shortDescription(self):
        return self._short_description

    def __str__(self):
        return self._str

class MultiProcess(Plugin):
    """
    Run tests in multiple processes. Requires processing module.
    """
    score = 1000
    status = {}

    def options(self, parser, env):
        """
        Register command-line options.
        """
        parser.add_option("--processes", action="store",
                          default=env.get('NOSE_PROCESSES', 0),
                          dest="multiprocess_workers",
                          metavar="NUM",
                          help="Spread test run among this many processes. "
                          "Set a number equal to the number of processors "
                          "or cores in your machine for best results. "
                          "Pass a negative number to have the number of "
                          "processes automatically set to the number of "
                          "cores. Passing 0 means to disable parallel "
                          "testing. Default is 0 unless NOSE_PROCESSES is "
                          "set. "
                          "[NOSE_PROCESSES]")
        parser.add_option("--process-timeout", action="store",
                          default=env.get('NOSE_PROCESS_TIMEOUT', 10),
                          dest="multiprocess_timeout",
                          metavar="SECONDS",
                          help="Set timeout for return of results from each "
                          "test runner process. Default is 10. "
                          "[NOSE_PROCESS_TIMEOUT]")
        parser.add_option("--process-restartworker", action="store_true",
                          default=env.get('NOSE_PROCESS_RESTARTWORKER', False),
                          dest="multiprocess_restartworker",
                          help="If set, will restart each worker process once"
                          " their tests are done, this helps control memory "
                          "leaks from killing the system. "
                          "[NOSE_PROCESS_RESTARTWORKER]")

    def configure(self, options, config):
        """
        Configure plugin.
        """
        try:
            self.status.pop('active')
        except KeyError:
            pass
        if not hasattr(options, 'multiprocess_workers'):
            self.enabled = False
            return
        # don't start inside of a worker process
        if config.worker:
            return
        self.config = config
        try:
            workers = int(options.multiprocess_workers)
        except (TypeError, ValueError):
            workers = 0
        if workers:
            _import_mp()
            if Process is None:
                self.enabled = False
                return
            # Negative number of workers will cause multiprocessing to hang.
            # Set the number of workers to the CPU count to avoid this.
            if workers < 0:
                try:
                    import multiprocessing
                    workers = multiprocessing.cpu_count()
                except NotImplementedError:
                    self.enabled = False
                    return
            self.enabled = True
            self.config.multiprocess_workers = workers
            t = float(options.multiprocess_timeout)
            self.config.multiprocess_timeout = t
            r = int(options.multiprocess_restartworker)
            self.config.multiprocess_restartworker = r
            self.status['active'] = True

    def prepareTestLoader(self, loader):
        """Remember loader class so MultiProcessTestRunner can instantiate
        the right loader.
        """
        self.loaderClass = loader.__class__

    def prepareTestRunner(self, runner):
        """Replace test runner with MultiProcessTestRunner.
        """
        # replace with our runner class
        return MultiProcessTestRunner(stream=runner.stream,
                                      verbosity=self.config.verbosity,
                                      config=self.config,
                                      loaderClass=self.loaderClass)

def signalhandler(sig, frame):
    raise TimedOutException()

class MultiProcessTestRunner(TextTestRunner):
    waitkilltime = 5.0 # max time to wait to terminate a process that does not
                       # respond to SIGILL
    def __init__(self, **kw):
        self.loaderClass = kw.pop('loaderClass', loader.defaultTestLoader)
        super(MultiProcessTestRunner, self).__init__(**kw)

    def collect(self, test, testQueue, tasks, to_teardown, result):
        # dispatch and collect results
        # put indexes only on queue because tests aren't picklable
        for case in self.nextBatch(test):
            log.debug("Next batch %s (%s)", case, type(case))
            if (isinstance(case, nose.case.Test) and
                isinstance(case.test, failure.Failure)):
                log.debug("Case is a Failure")
                case(result) # run here to capture the failure
                continue
            # handle shared fixtures
            if isinstance(case, ContextSuite) and case.context is failure.Failure:
                log.debug("Case is a Failure")
                case(result) # run here to capture the failure
                continue
            elif isinstance(case, ContextSuite) and self.sharedFixtures(case):
                log.debug("%s has shared fixtures", case)
                try:
                    case.setUp()
                except (KeyboardInterrupt, SystemExit):
                    raise
                except:
                    log.debug("%s setup failed", sys.exc_info())
                    result.addError(case, sys.exc_info())
                else:
                    to_teardown.append(case)
                    if case.factory:
                        ancestors=case.factory.context.get(case, [])
                        for an in ancestors[:2]:
                            #log.debug('reset ancestor %s', an)
                            if getattr(an, '_multiprocess_shared_', False):
                                an._multiprocess_can_split_=True
                            #an._multiprocess_shared_=False
                    self.collect(case, testQueue, tasks, to_teardown, result)

            else:
                test_addr = self.addtask(testQueue,tasks,case)
                log.debug("Queued test %s (%s) to %s",
                          len(tasks), test_addr, testQueue)

    def startProcess(self, iworker, testQueue, resultQueue, shouldStop, result):
        currentaddr = Value('c',bytes_(''))
        currentstart = Value('d',time.time())
        keyboardCaught = Event()
        p = Process(target=runner,
                   args=(iworker, testQueue,
                         resultQueue,
                         currentaddr,
                         currentstart,
                         keyboardCaught,
                         shouldStop,
                         self.loaderClass,
                         result.__class__,
                         pickle.dumps(self.config)))
        p.currentaddr = currentaddr
        p.currentstart = currentstart
        p.keyboardCaught = keyboardCaught
        old = signal.signal(signal.SIGILL, signalhandler)
        p.start()
        signal.signal(signal.SIGILL, old)
        return p

    def run(self, test):
        """
        Execute the test (which may be a test suite). If the test is a suite,
        distribute it out among as many processes as have been configured, at
        as fine a level as is possible given the context fixtures defined in
        the suite or any sub-suites.

        """
        log.debug("%s.run(%s) (%s)", self, test, os.getpid())
        wrapper = self.config.plugins.prepareTest(test)
        if wrapper is not None:
            test = wrapper

        # plugins can decorate or capture the output stream
        wrapped = self.config.plugins.setOutputStream(self.stream)
        if wrapped is not None:
            self.stream = wrapped

        testQueue = Queue()
        resultQueue = Queue()
        tasks = []
        completed = []
        workers = []
        to_teardown = []
        shouldStop = Event()

        result = self._makeResult()
        start = time.time()

        self.collect(test, testQueue, tasks, to_teardown, result)

        log.debug("Starting %s workers", self.config.multiprocess_workers)
        for i in range(self.config.multiprocess_workers):
            p = self.startProcess(i, testQueue, resultQueue, shouldStop, result)
            workers.append(p)
            log.debug("Started worker process %s", i+1)

        total_tasks = len(tasks)
        # need to keep track of the next time to check for timeouts in case
        # more than one process times out at the same time.
        nexttimeout=self.config.multiprocess_timeout
        thrownError = None

        try:
            while tasks:
                log.debug("Waiting for results (%s/%s tasks), next timeout=%.3fs",
                          len(completed), total_tasks,nexttimeout)
                try:
                    iworker, addr, newtask_addrs, batch_result = resultQueue.get(
                                                            timeout=nexttimeout)
                    log.debug('Results received for worker %d, %s, new tasks: %d',
                              iworker,addr,len(newtask_addrs))
                    try:
                        try:
                            tasks.remove(addr)
                        except ValueError:
                            log.warn('worker %s failed to remove from tasks: %s',
                                     iworker,addr)
                        total_tasks += len(newtask_addrs)
                        tasks.extend(newtask_addrs)
                    except KeyError:
                        log.debug("Got result for unknown task? %s", addr)
                        log.debug("current: %s",str(list(tasks)[0]))
                    else:
                        completed.append([addr,batch_result])
                    self.consolidate(result, batch_result)
                    if (self.config.stopOnError
                        and not result.wasSuccessful()):
                        # set the stop condition
                        shouldStop.set()
                        break
                    if self.config.multiprocess_restartworker:
                        log.debug('joining worker %s',iworker)
                        # wait for working, but not that important if worker
                        # cannot be joined in fact, for workers that add to
                        # testQueue, they will not terminate until all their
                        # items are read
                        workers[iworker].join(timeout=1)
                        if not shouldStop.is_set() and not testQueue.empty():
                            log.debug('starting new process on worker %s',iworker)
                            workers[iworker] = self.startProcess(iworker, testQueue, resultQueue, shouldStop, result)
                except Empty:
                    log.debug("Timed out with %s tasks pending "
                              "(empty testQueue=%r): %s",
                              len(tasks),testQueue.empty(),str(tasks))
                    any_alive = False
                    for iworker, w in enumerate(workers):
                        if w.is_alive():
                            worker_addr = bytes_(w.currentaddr.value,'ascii')
                            timeprocessing = time.time() - w.currentstart.value
                            if ( len(worker_addr) == 0
                                    and timeprocessing > self.config.multiprocess_timeout-0.1):
                                log.debug('worker %d has finished its work item, '
                                          'but is not exiting? do we wait for it?',
                                          iworker)
                            else:
                                any_alive = True
                            if (len(worker_addr) > 0
                                and timeprocessing > self.config.multiprocess_timeout-0.1):
                                log.debug('timed out worker %s: %s',
                                          iworker,worker_addr)
                                w.currentaddr.value = bytes_('')
                                # If the process is in C++ code, sending a SIGILL
                                # might not send a python KeybordInterrupt exception
                                # therefore, send multiple signals until an
                                # exception is caught. If this takes too long, then
                                # terminate the process
                                w.keyboardCaught.clear()
                                startkilltime = time.time()
                                while not w.keyboardCaught.is_set() and w.is_alive():
                                    if time.time()-startkilltime > self.waitkilltime:
                                        # have to terminate...
                                        log.error("terminating worker %s",iworker)
                                        w.terminate()
                                        # there is a small probability that the
                                        # terminated process might send a result,
                                        # which has to be specially handled or
                                        # else processes might get orphaned.
                                        workers[iworker] = w = self.startProcess(iworker, testQueue, resultQueue, shouldStop, result)
                                        break
                                    os.kill(w.pid, signal.SIGILL)
                                    time.sleep(0.1)
                    if not any_alive and testQueue.empty():
                        log.debug("All workers dead")
                        break
                nexttimeout=self.config.multiprocess_timeout
                for w in workers:
                    if w.is_alive() and len(w.currentaddr.value) > 0:
                        timeprocessing = time.time()-w.currentstart.value
                        if timeprocessing <= self.config.multiprocess_timeout:
                            nexttimeout = min(nexttimeout,
                                self.config.multiprocess_timeout-timeprocessing)
            log.debug("Completed %s tasks (%s remain)", len(completed), len(tasks))

        except (KeyboardInterrupt, SystemExit) as e:
            log.info('parent received ctrl-c when waiting for test results')
            thrownError = e
            #resultQueue.get(False)
                
            result.addError(test, sys.exc_info())

        try:
            for case in to_teardown:
                log.debug("Tearing down shared fixtures for %s", case)
                try:
                    case.tearDown()
                except (KeyboardInterrupt, SystemExit):
                    raise
                except:
                    result.addError(case, sys.exc_info())

            stop = time.time()

            # first write since can freeze on shutting down processes
            result.printErrors()
            result.printSummary(start, stop)
            self.config.plugins.finalize(result)

            if thrownError is None:
                log.debug("Tell all workers to stop")
                for w in workers:
                    if w.is_alive():
                        testQueue.put('STOP', block=False)

            # wait for the workers to end
            for iworker,worker in enumerate(workers):
                if worker.is_alive():
                    log.debug('joining worker %s',iworker)
                    worker.join()
                    if worker.is_alive():
                        log.debug('failed to join worker %s',iworker)
        except (KeyboardInterrupt, SystemExit):
            log.info('parent received ctrl-c when shutting down: stop all processes')
            for worker in workers:
                if worker.is_alive():
                    worker.terminate()

            if thrownError: raise thrownError
            else: raise

        return result

    def addtask(testQueue,tasks,case):
        arg = None
        if isinstance(case,nose.case.Test) and hasattr(case.test,'arg'):
            # this removes the top level descriptor and allows real function
            # name to be returned
            case.test.descriptor = None
            arg = case.test.arg
        test_addr = MultiProcessTestRunner.address(case)
        testQueue.put((test_addr,arg), block=False)
        if arg is not None:
            test_addr += str(arg)
        if tasks is not None:
            tasks.append(test_addr)
        return test_addr
    addtask = staticmethod(addtask)

    def address(case):
        if hasattr(case, 'address'):
            file, mod, call = case.address()
        elif hasattr(case, 'context'):
            file, mod, call = test_address(case.context)
        else:
            raise Exception("Unable to convert %s to address" % case)
        parts = []
        if file is None:
            if mod is None:
                raise Exception("Unaddressable case %s" % case)
            else:
                parts.append(mod)
        else:
            # strip __init__.py(c) from end of file part
            # if present, having it there confuses loader
            dirname, basename = os.path.split(file)
            if basename.startswith('__init__'):
                file = dirname
            parts.append(file)
        if call is not None:
            parts.append(call)
        return ':'.join(map(str, parts))
    address = staticmethod(address)

    def nextBatch(self, test):
        # allows tests or suites to mark themselves as not safe
        # for multiprocess execution
        if hasattr(test, 'context'):
            if not getattr(test.context, '_multiprocess_', True):
                return

        if ((isinstance(test, ContextSuite)
             and test.hasFixtures(self.checkCanSplit))
            or not getattr(test, 'can_split', True)
            or not isinstance(test, unittest.TestSuite)):
            # regular test case, or a suite with context fixtures

            # special case: when run like nosetests path/to/module.py
            # the top-level suite has only one item, and it shares
            # the same context as that item. In that case, we want the
            # item, not the top-level suite
            if isinstance(test, ContextSuite):
                contained = list(test)
                if (len(contained) == 1
                    and getattr(contained[0],
                                'context', None) == test.context):
                    test = contained[0]
            yield test
        else:
            # Suite is without fixtures at this level; but it may have
            # fixtures at any deeper level, so we need to examine it all
            # the way down to the case level
            for case in test:
                for batch in self.nextBatch(case):
                    yield batch

    def checkCanSplit(context, fixt):
        """
        Callback that we use to check whether the fixtures found in a
        context or ancestor are ones we care about.

        Contexts can tell us that their fixtures are reentrant by setting
        _multiprocess_can_split_. So if we see that, we return False to
        disregard those fixtures.
        """
        if not fixt:
            return False
        if getattr(context, '_multiprocess_can_split_', False):
            return False
        return True
    checkCanSplit = staticmethod(checkCanSplit)

    def sharedFixtures(self, case):
        context = getattr(case, 'context', None)
        if not context:
            return False
        return getattr(context, '_multiprocess_shared_', False)

    def consolidate(self, result, batch_result):
        log.debug("batch result is %s" , batch_result)
        try:
            output, testsRun, failures, errors, errorClasses = batch_result
        except ValueError:
            log.debug("result in unexpected format %s", batch_result)
            failure.Failure(*sys.exc_info())(result)
            return
        self.stream.write(output)
        result.testsRun += testsRun
        result.failures.extend(failures)
        result.errors.extend(errors)
        for key, (storage, label, isfail) in list(errorClasses.items()):
            if key not in result.errorClasses:
                # Ordinarily storage is result attribute
                # but it's only processed through the errorClasses
                # dict, so it's ok to fake it here
                result.errorClasses[key] = ([], label, isfail)
            mystorage, _junk, _junk = result.errorClasses[key]
            mystorage.extend(storage)
        log.debug("Ran %s tests (total: %s)", testsRun, result.testsRun)


def runner(ix, testQueue, resultQueue, currentaddr, currentstart,
           keyboardCaught, shouldStop, loaderClass, resultClass, config):
    try:
        try:
            return __runner(ix, testQueue, resultQueue, currentaddr, currentstart,
                    keyboardCaught, shouldStop, loaderClass, resultClass, config)
        except KeyboardInterrupt:
            log.debug('Worker %s keyboard interrupt, stopping',ix)
    except Empty:
        log.debug("Worker %s timed out waiting for tasks", ix)

def __runner(ix, testQueue, resultQueue, currentaddr, currentstart,
           keyboardCaught, shouldStop, loaderClass, resultClass, config):

    config = pickle.loads(config)
    dummy_parser = config.parserClass()
    if _instantiate_plugins is not None:
        for pluginclass in _instantiate_plugins:
            plugin = pluginclass()
            plugin.addOptions(dummy_parser,{})
            config.plugins.addPlugin(plugin)
    config.plugins.configure(config.options,config)
    config.plugins.begin()
    log.debug("Worker %s executing, pid=%d", ix,os.getpid())
    loader = loaderClass(config=config)
    loader.suiteClass.suiteClass = NoSharedFixtureContextSuite

    def get():
        return testQueue.get(timeout=config.multiprocess_timeout)

    def makeResult():
        stream = _WritelnDecorator(StringIO())
        result = resultClass(stream, descriptions=1,
                             verbosity=config.verbosity,
                             config=config)
        plug_result = config.plugins.prepareTestResult(result)
        if plug_result:
            return plug_result
        return result

    def batch(result):
        failures = [(TestLet(c), err) for c, err in result.failures]
        errors = [(TestLet(c), err) for c, err in result.errors]
        errorClasses = {}
        for key, (storage, label, isfail) in list(result.errorClasses.items()):
            errorClasses[key] = ([(TestLet(c), err) for c, err in storage],
                                 label, isfail)
        return (
            result.stream.getvalue(),
            result.testsRun,
            failures,
            errors,
            errorClasses)
    for test_addr, arg in iter(get, 'STOP'):
        if shouldStop.is_set():
            log.exception('Worker %d STOPPED',ix)
            break
        result = makeResult()
        test = loader.loadTestsFromNames([test_addr])
        test.testQueue = testQueue
        test.tasks = []
        test.arg = arg
        log.debug("Worker %s Test is %s (%s)", ix, test_addr, test)
        try:
            if arg is not None:
                test_addr = test_addr + str(arg)
            currentaddr.value = bytes_(test_addr)
            currentstart.value = time.time()
            test(result)
            currentaddr.value = bytes_('')
            resultQueue.put((ix, test_addr, test.tasks, batch(result)))
        except KeyboardInterrupt as e: #TimedOutException:
            timeout = isinstance(e, TimedOutException)
            if timeout:
                keyboardCaught.set()
            if len(currentaddr.value):
                if timeout:
                    msg = 'Worker %s timed out, failing current test %s'
                else:
                    msg = 'Worker %s keyboard interrupt, failing current test %s'
                log.exception(msg,ix,test_addr)
                currentaddr.value = bytes_('')
                failure.Failure(*sys.exc_info())(result)
                resultQueue.put((ix, test_addr, test.tasks, batch(result)))
            else:
                if timeout:
                    msg = 'Worker %s test %s timed out'
                else:
                    msg = 'Worker %s test %s keyboard interrupt'
                log.debug(msg,ix,test_addr)
                resultQueue.put((ix, test_addr, test.tasks, batch(result)))
            if not timeout:
                raise
        except SystemExit:
            currentaddr.value = bytes_('')
            log.exception('Worker %s system exit',ix)
            raise
        except:
            currentaddr.value = bytes_('')
            log.exception("Worker %s error running test or returning "
                            "results",ix)
            failure.Failure(*sys.exc_info())(result)
            resultQueue.put((ix, test_addr, test.tasks, batch(result)))
        if config.multiprocess_restartworker:
            break
    log.debug("Worker %s ending", ix)


class NoSharedFixtureContextSuite(ContextSuite):
    """
    Context suite that never fires shared fixtures.

    When a context sets _multiprocess_shared_, fixtures in that context
    are executed by the main process. Using this suite class prevents them
    from executing in the runner process as well.

    """
    testQueue = None
    tasks = None
    arg = None
    def setupContext(self, context):
        if getattr(context, '_multiprocess_shared_', False):
            return
        super(NoSharedFixtureContextSuite, self).setupContext(context)

    def teardownContext(self, context):
        if getattr(context, '_multiprocess_shared_', False):
            return
        super(NoSharedFixtureContextSuite, self).teardownContext(context)
    def run(self, result):
        """Run tests in suite inside of suite fixtures.
        """
        # proxy the result for myself
        log.debug("suite %s (%s) run called, tests: %s",
                  id(self), self, self._tests)
        if self.resultProxy:
            result, orig = self.resultProxy(result, self), result
        else:
            result, orig = result, result
        try:
            #log.debug('setUp for %s', id(self));
            self.setUp()
        except KeyboardInterrupt:
            raise
        except:
            self.error_context = 'setup'
            result.addError(self, self._exc_info())
            return
        try:
            for test in self._tests:
                if (isinstance(test,nose.case.Test)
                    and self.arg is not None):
                    test.test.arg = self.arg
                else:
                    test.arg = self.arg
                test.testQueue = self.testQueue
                test.tasks = self.tasks
                if result.shouldStop:
                    log.debug("stopping")
                    break
                # each nose.case.Test will create its own result proxy
                # so the cases need the original result, to avoid proxy
                # chains
                #log.debug('running test %s in suite %s', test, self);
                try:
                    test(orig)
                except KeyboardInterrupt as e:
                    timeout = isinstance(e, TimedOutException)
                    if timeout:
                        msg = 'Timeout when running test %s in suite %s'
                    else:
                        msg = 'KeyboardInterrupt when running test %s in suite %s'
                    log.debug(msg, test, self)
                    err = (TimedOutException,TimedOutException(str(test)),
                           sys.exc_info()[2])
                    test.config.plugins.addError(test,err)
                    orig.addError(test,err)
                    if not timeout:
                        raise
        finally:
            self.has_run = True
            try:
                #log.debug('tearDown for %s', id(self));
                self.tearDown()
            except KeyboardInterrupt:
                raise
            except:
                self.error_context = 'teardown'
                result.addError(self, self._exc_info())