Openstack Cinder中建立volume过程的源码解析（6）----以及taskflow相关解析

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在博客 Openstack Cinder中建立volume过程的源码解析（4）----以及taskflow相关解析中我们曾经说过，在方法/cinder/volume/api.py----class API----def create中，主要通过4步实现了应用taskflow进行卷的建立操作，也就是：

1.构建字典create_what，实现整合建立卷的具体参数；

2.构建并返回用于建立卷的flow；

3.执行构建的用于建立卷的flow；

4.从flow中获取建立卷的反馈信息；

其中前2个步骤的解析已经完成，这里来看第3个步骤的实现，即如何来运行已经构建好的flow；

我们再来看方法/cinder/volume/api.py----class API----def create的源码实现：

    def create(self, context, size, name, description, snapshot=None,  
               image_id=None, volume_type=None, metadata=None,  
               availability_zone=None, source_volume=None,  
               scheduler_hints=None, backup_source_volume=None):  
        """ 
        实现建立卷的操作； 
        """  
      
        def check_volume_az_zone(availability_zone):  
            """ 
            验证availability_zone是否是可用的（即是否包含在可用zone的列表中）； 
            """  
            try:  
                # _valid_availabilty_zone：验证availability_zone是否是可用的（即是否包含在可用zone的列表中）；  
                return self._valid_availabilty_zone(availability_zone)  
            except exception.CinderException:  
                LOG.exception(_("Unable to query if %s is in the "  
                                "availability zone set"), availability_zone)  
                return False  
      
        # 所要建立卷的规格数据信息；  
        create_what = {  
            'size': size,  
            'name': name,  
            'description': description,  
            'snapshot': snapshot,  
            'image_id': image_id,  
            'volume_type': volume_type,  
            'metadata': metadata,  
            'availability_zone': availability_zone,  
            'source_volume': source_volume,  
            'scheduler_hints': scheduler_hints,  
            'key_manager': self.key_manager,  
            'backup_source_volume': backup_source_volume,  
        }  
          
        # 构建并返回用于建立卷的flow；  
        # self.scheduler_rpcapi = scheduler_rpcapi.SchedulerAPI()；  
        # self.volume_rpcapi = volume_rpcapi.VolumeAPI()；  
        # self.image_service = (image_service or glance.get_default_image_service())  
        # check_volume_az_zone：验证availability_zone是否是可用的（即是否包含在可用zone的列表中）；  
        # create_what：所要建立卷的规格数据信息；  
        (flow, uuid) = create_volume.get_api_flow(self.scheduler_rpcapi,  
                                                  self.volume_rpcapi,  
                                                  self.db,  
                                                  self.image_service,  
                                                  check_volume_az_zone,  
                                                  create_what)  
      
        # 应用assert关键字来声明flow是真的；  
        assert flow, _('Create volume flow not retrieved')  
          
          
        # 运行用于建立卷的flow；  
        flow.run(context)  
          
        # 如果flow的运行状态不为states.SUCCESS，则引发异常；  
        if flow.state != states.SUCCESS:  
            raise exception.CinderException(_("Failed to successfully complete"  
                                              " create volume workflow"))  
      
        # Extract the volume information from the task uuid that was specified  
        # to produce said information.  
        # 通过task的uuid值获取建立卷的信息；  
        volume = None  
        try:  
            volume = flow.results[uuid]['volume']  
        except KeyError:  
            pass  
      
        # Raise an error, nobody provided it??  
        # 应用assert关键字来声明volume是真的；  
        assert volume, _('Expected volume result not found')  
          
        return volume  

我们来看语句：

flow.run(context)

就实现了构建的flow的执行操作；

我们具体来看方法run的源码实现：

def run(self, context, *args, **kwargs):
      """
      工作流（workflow）的执行操作；
      context = <cinder.context.RequestContext object at 0x382fd50> //从cinder请求中获取上下文环境信息；
      args = ()
      kwargs = {}
      """
        super(Flow, self).run(context, *args, **kwargs)

        def resume_it():
            # self._leftoff_at = None
            if self._leftoff_at is not None:
                return ([], self._leftoff_at)
            
            # self.resumer = None
            # 注：这里还没有应用恢复策略，因为类的初始化过程中赋值为None；
            if self.resumer:
                # self._ordering()：获取迭代器包装的任务运行列表；
                (finished, leftover) = self.resumer.resume(self, self._ordering())
            else:
                finished = []
                # self._ordering()：获取迭代器包装的任务运行列表；
                leftover = self._ordering()
            
            # leftover：获取迭代器包装的任务运行列表；
            # finished = []
            # leftover = <listiterator object at 0x441fa50>
            return (finished, leftover)

        # 改变目前的flow状态为新的状态STARTED，并执行通知操作；
        # flow状态标志为STARTED，表示任务开始运行操作；
        self._change_state(context, states.STARTED)
        try:
            # leftover：获取迭代器包装的任务运行列表；
            # those_finished = []
            # leftover = <listiterator object at 0x40c1990>
            those_finished, leftover = resume_it()
        except Exception:
            with excutils.save_and_reraise_exception():
                self._change_state(context, states.FAILURE)

        def run_it(runner, failed=False, result=None, simulate_run=False):
           
            try:
                # Add the task to be rolled back *immediately* so that even if
                # the task fails while producing results it will be given a
                # chance to rollback.
                
                # RollbackTask：实现调用任务对应的可用的逆转回滚方法；
                # runner.task = cinder.volume.flows.create_volume.VolumeCastTask;volume:create==1.0
                rb = utils.RollbackTask(context, runner.task, result=None)
                
                # 在回滚方法累加器中添加逆转回滚方法任务；
                self._accumulator.add(rb)

                self.task_notifier.notify(states.STARTED, details={
                    'context': context,
                    'flow': self,
                    'runner': runner,
                })
                
                # simulate_run = False
                if not simulate_run:
                    result = runner(context, *args, **kwargs)

                else:
                    if failed:
                        if not result:
                            result = "%s failed running." % (runner.task)
                        if isinstance(result, basestring):
                            result = exc.InvalidStateException(result)
                        if not isinstance(result, Exception):
                            LOG.warn("Can not raise a non-exception"
                                     " object: %s", result)
                            result = exc.InvalidStateException()
                        raise result
                rb.result = result
                runner.result = result
                self.results[runner.uuid] = result
                
                self.task_notifier.notify(states.SUCCESS, details={
                    'context': context,
                    'flow': self,
                    'runner': runner,
                })
            except Exception as e:
                runner.result = e
                cause = utils.FlowFailure(runner, self, e)
                with excutils.save_and_reraise_exception():
                    self.task_notifier.notify(states.FAILURE, details={
                        'context': context,
                        'flow': self,
                        'runner': runner,
                    })
                    self.rollback(context, cause)

        # those_finished = []
        if len(those_finished):
            self._change_state(context, states.RESUMING)
            for (r, details) in those_finished:
                failed = states.FAILURE in details.get('states', [])
                result = details.get('result')
                run_it(r, failed=failed, result=result, simulate_run=True)

        # leftover：获取迭代器包装的任务运行列表；
        # leftover = <listiterator object at 0x40c1990>
        self._leftoff_at = leftover
        # 改变目前的flow状态为新的状态RUNNING，并执行通知操作；
        self._change_state(context, states.RUNNING)
               
        # 如果状态为中断，则返回；
        if self.state == states.INTERRUPTED:
            return

        # 标志任务运行状态不为states.INTERRUPTED；
        was_interrupted = False
        
        # leftover：获取迭代器包装的任务运行列表；
        # leftover = <listiterator object at 0x40c1990>
        for r in leftover:
            r.reset()
            run_it(r)
            if self.state == states.INTERRUPTED:
                was_interrupted = True
                break

        if not was_interrupted:
            # Only gets here if everything went successfully.
            self._change_state(context, states.SUCCESS)
            self._leftoff_at = None

我们来分析这个方法的实现过程；

首先来看语句：

try:
    # leftover：获取迭代器包装的任务运行列表；
    # those_finished = []
    # leftover = <listiterator object at 0x40c1990>
    those_finished, leftover = resume_it()

这部分语句的功能是实现获取迭代器包装的task列表，以及已经完成的task列表；

我们具体来看方法resume_it的实现过程：

        def resume_it():
            # self._leftoff_at = None
            if self._leftoff_at is not None:
                return ([], self._leftoff_at)
            
            # self.resumer = None
            # 注：这里还没有应用恢复策略，因为类的初始化过程中赋值为None；
            if self.resumer:
                # self._ordering()：获取迭代器包装的任务运行列表；
                (finished, leftover) = self.resumer.resume(self, self._ordering())
            else:
                finished = []
                # self._ordering()：获取迭代器包装的任务运行列表；
                leftover = self._ordering()
            
            # leftover：获取迭代器包装的任务运行列表；
            # finished = []
            # leftover = <listiterator object at 0x441fa50>
            return (finished, leftover)

我们再来看方法_ordering的源码实现：

    def _ordering(self):        
        # 返回迭代器包装的任务运行列表；
        return iter(self._connect())

    def _connect(self):
        # self._runners：所有要运行的任务集合；
        # self._connected = False
        if self._connected:
            return self._runners
        
        for r in self._runners:
            r.providers = {}
            
        for r in reversed(self._runners):
            self._associate_providers(r)
        self._connected = True
        
        return self._runners

我们可以看到在方法_connect中，由类Flow的初始化方法中可以知道，变量self._runners表示的是所有要运行的任务的集合；我们在语句for r in reversed(self._runners)下面添加了一条输出调试语句，获得以下的输出实例：

r = Runner: cinder.volume.flows.create_volume.VolumeCastTask;volume:create==1.0;
    r-a41753ed-a9b4-4999-8aab-979d244425d1; 1.0
r = Runner: cinder.volume.flows.create_volume.OnFailureChangeStatusTask;volume:create==1.0;
    r-c689085f-df0a-4459-a8a9-c2518c6e471c; 1.0
r = Runner: cinder.volume.flows.create_volume.QuotaCommitTask;volume:create==1.0;
    r-adea0991-6fc9-4a74-9c2f-33848c77b29c; 1.0
r = Runner: cinder.volume.flows.create_volume.EntryCreateTask;volume:create==1.0;
    r-c05d7892-7cb0-4e80-b17b-5777ca0ae167; 1.0
r = Runner: cinder.volume.flows.create_volume.QuotaReserveTask;volume:create==1.0;
    r-4d1c0c90-b6ac-4406-a3ae-50943fb22cd3; 1.0
r = Runner: cinder.volume.flows.create_volume.ExtractVolumeRequestTask;volume:create==1.0;
    r-d7175a90-de39-40c2-853f-462a4c566d8f; 1.0
r = Runner: cinder.volume.flows.base.InjectTask;volume:create==1.0;
    r-77f89843-25d6-4dc1-b8c5-4d13816b62fa; 1.0

我们可以看到此时，self._runners中集合了所有要运行的task，当然这里暂时调用了reversed方法进行了逆序排序的操作，是为了下面的验证操作做准备的，而self._runners中task的排序还是顺序的；输出实例：

self._runners = [<cinder.taskflow.utils.Runner object at 0x2ca86d0>,
                 <cinder.taskflow.utils.Runner object at 0x2ca8750>,
                 <cinder.taskflow.utils.Runner object at 0x2ca87d0>,
                 <cinder.taskflow.utils.Runner object at 0x2ca8850>,
                 <cinder.taskflow.utils.Runner object at 0x2ca88d0>,
                 <cinder.taskflow.utils.Runner object at 0x2ca8950>,
                 <cinder.taskflow.utils.Runner object at 0x28e6490>]

所以获取了task的集合的变量self._runners，再经过_ordering方法的迭代器包装，最后返回给变量leftover；

下面我们再来看语句：

for r in leftover:

    r.reset()

    run_it(r)

显然，这里完成的就是按照task集合中的顺序，逐个执行所要执行的task，从而成功实现卷的建立；

从上面的调试输出我们可以知道，self._runners为cinder.taskflow.utils.Runner对象的集合，即为要运行的任务的集合，所以这里我们来看一下类Runner的初始化方法：

class Runner(object):
    def __init__(self, task, uuid=None):
        assert isinstance(task, collections.Callable)
        self.task = task
        self.providers = {}
        self.runs_before = []
        self.result = None
        if not uuid:
            self._id = uuidutils.generate_uuid()
        else:
            self._id = str(uuid)

所以我们在这里可以做以下的调试输出，具体看一下各个变量的作用，这有利于后续代码的理解：

========================================================================================
r = Runner: cinder.volume.flows.base.InjectTask;

    volume:create==1.0;
    r-dfc38a9a-a67f-44b9-8297-48dc586806ce; 1.0
r.task = cinder.volume.flows.base.InjectTask;volume:create==1.0 # 表示当前任务的类名和任务描述；
r.providers = {}
r.runs_before = [] # 表示当前任务之前的所有任务集合；
r.result = None
r._id = dfc38a9a-a67f-44b9-8297-48dc586806ce
========================================================================================
r = Runner: cinder.volume.flows.create_volume.ExtractVolumeRequestTask;

    volume:create==1.0;
    r-920b4ad9-edd8-436b-bfce-18f940d714a7; 1.0
r.task = cinder.volume.flows.create_volume.ExtractVolumeRequestTask;volume:create==1.0
r.providers = {'backup_source_volume': <cinder.taskflow.utils.Runner object at 0x3a28650>,
               'availability_zone': <cinder.taskflow.utils.Runner object at 0x3a28650>,
               'source_volume': <cinder.taskflow.utils.Runner object at 0x3a28650>,
               'volume_type': <cinder.taskflow.utils.Runner object at 0x3a28650>,
               'image_id': <cinder.taskflow.utils.Runner object at 0x3a28650>,
               'snapshot': <cinder.taskflow.utils.Runner object at 0x3a28650>,
               'size': <cinder.taskflow.utils.Runner object at 0x3a28650>,
               'key_manager': <cinder.taskflow.utils.Runner object at 0x3a28650>,
               'metadata': <cinder.taskflow.utils.Runner object at 0x3a28650>}
r.runs_before = [<cinder.taskflow.utils.Runner object at 0x3a28650>]
r.result = None
r._id = 920b4ad9-edd8-436b-bfce-18f940d714a7
========================================================================================
r = Runner: cinder.volume.flows.create_volume.QuotaReserveTask;

    volume:create==1.0;
    r-49eb28ff-4da0-4e17-bf89-c9d82abc4eb8; 1.0
r.task = cinder.volume.flows.create_volume.QuotaReserveTask;volume:create==1.0
r.providers = {'volume_type_id': <cinder.taskflow.utils.Runner object at 0x3a286d0>,
               'size': <cinder.taskflow.utils.Runner object at 0x3a286d0>}
r.runs_before = [<cinder.taskflow.utils.Runner object at 0x3a286d0>,
                 <cinder.taskflow.utils.Runner object at 0x3a28650>]
r.result = None
r._id = 49eb28ff-4da0-4e17-bf89-c9d82abc4eb8
========================================================================================
r = Runner: cinder.volume.flows.create_volume.EntryCreateTask;

    volume:create==1.0;
    r-b338cd2d-282b-4da2-9f10-7cd79f9de18d; 1.0
r.task = cinder.volume.flows.create_volume.EntryCreateTask;volume:create==1.0
r.providers = {'size': <cinder.taskflow.utils.Runner object at 0x3a286d0>,
               'volume_type_id': <cinder.taskflow.utils.Runner object at 0x3a286d0>,
               'description': <cinder.taskflow.utils.Runner object at 0x3a28650>,
               'availability_zone': <cinder.taskflow.utils.Runner object at 0x3a286d0>,
               'reservations': <cinder.taskflow.utils.Runner object at 0x3a28750>,
               'source_volid': <cinder.taskflow.utils.Runner object at 0x3a286d0>,
               'encryption_key_id': <cinder.taskflow.utils.Runner object at 0x3a286d0>,
               'snapshot_id': <cinder.taskflow.utils.Runner object at 0x3a286d0>,
               'metadata': <cinder.taskflow.utils.Runner object at 0x3a28650>,
               'name': <cinder.taskflow.utils.Runner object at 0x3a28650>}
r.runs_before = [<cinder.taskflow.utils.Runner object at 0x3a28750>,
                 <cinder.taskflow.utils.Runner object at 0x3a286d0>,
                 <cinder.taskflow.utils.Runner object at 0x3a28650>]
r.result = None
r._id = b338cd2d-282b-4da2-9f10-7cd79f9de18d
========================================================================================
r = Runner: cinder.volume.flows.create_volume.QuotaCommitTask;

    volume:create==1.0;
    r-11bfeedf-6cc7-46f6-b257-472114132cb5; 1.0
r.task = cinder.volume.flows.create_volume.QuotaCommitTask;volume:create==1.0
r.providers = {'reservations': <cinder.taskflow.utils.Runner object at 0x3a28750>,
               'volume_properties': <cinder.taskflow.utils.Runner object at 0x3a287d0>}
r.runs_before = [<cinder.taskflow.utils.Runner object at 0x3a287d0>,
                 <cinder.taskflow.utils.Runner object at 0x3a28750>,
                 <cinder.taskflow.utils.Runner object at 0x3a286d0>,
                 <cinder.taskflow.utils.Runner object at 0x3a28650>]
r.result = None
r._id = 11bfeedf-6cc7-46f6-b257-472114132cb5
========================================================================================
r = Runner: cinder.volume.flows.create_volume.OnFailureChangeStatusTask;

    volume:create==1.0;
    r-255b5031-abb0-4c02-9c1d-ddb59fe4066b; 1.0
r.task = cinder.volume.flows.create_volume.OnFailureChangeStatusTask;volume:create==1.0
r.providers = {'volume_id': <cinder.taskflow.utils.Runner object at 0x3a287d0>}
r.runs_before = [<cinder.taskflow.utils.Runner object at 0x3a28850>,
                 <cinder.taskflow.utils.Runner object at 0x3a287d0>,
                 <cinder.taskflow.utils.Runner object at 0x3a28750>,
                 <cinder.taskflow.utils.Runner object at 0x3a286d0>,
                 <cinder.taskflow.utils.Runner object at 0x3a28650>]
r.result = None
r._id = 255b5031-abb0-4c02-9c1d-ddb59fe4066b
========================================================================================
r = Runner: cinder.volume.flows.create_volume.VolumeCastTask;

    volume:create==1.0;
    r-68318d88-aecb-4ac4-b80d-26f6e91b9b95; 1.0
r.task = cinder.volume.flows.create_volume.VolumeCastTask;volume:create==1.0
r.providers = {'image_id': <cinder.taskflow.utils.Runner object at 0x3a28650>,
               'snapshot_id': <cinder.taskflow.utils.Runner object at 0x3a286d0>,
               'source_volid': <cinder.taskflow.utils.Runner object at 0x3a286d0>,
               'volume_id': <cinder.taskflow.utils.Runner object at 0x3a287d0>,
               'volume_properties': <cinder.taskflow.utils.Runner object at 0x3a287d0>,
               'volume_type': <cinder.taskflow.utils.Runner object at 0x3a286d0>,
               'scheduler_hints': <cinder.taskflow.utils.Runner object at 0x3a28650>}
r.runs_before = [<cinder.taskflow.utils.Runner object at 0x3a288d0>,
                 <cinder.taskflow.utils.Runner object at 0x3a28850>,
                 <cinder.taskflow.utils.Runner object at 0x3a287d0>,
                 <cinder.taskflow.utils.Runner object at 0x3a28750>,
                 <cinder.taskflow.utils.Runner object at 0x3a286d0>,
                 <cinder.taskflow.utils.Runner object at 0x3a28650>]
r.result = None
r._id = 68318d88-aecb-4ac4-b80d-26f6e91b9b95
========================================================================================

我们接着来看方法run_it的源码实现：

        def run_it(runner, failed=False, result=None, simulate_run=False):
         """
         以其中之一为例：
         runner = Runner: cinder.volume.flows.create_volume.VolumeCastTask;volume:create==1.0; 
                  r-68318d88-aecb-4ac4-b80d-26f6e91b9b95; 1.0
         runner.task = cinder.volume.flows.create_volume.VolumeCastTask;volume:create==1.0
         runner.providers = {'image_id': <cinder.taskflow.utils.Runner object at 0x3a28650>, 
                           'snapshot_id': <cinder.taskflow.utils.Runner object at 0x3a286d0>, 
                           'source_volid': <cinder.taskflow.utils.Runner object at 0x3a286d0>, 
                           'volume_id': <cinder.taskflow.utils.Runner object at 0x3a287d0>, 
                           'volume_properties': <cinder.taskflow.utils.Runner object at 0x3a287d0>, 
                           'volume_type': <cinder.taskflow.utils.Runner object at 0x3a286d0>, 
                           'scheduler_hints': <cinder.taskflow.utils.Runner object at 0x3a28650>}
         runner.runs_before = [<cinder.taskflow.utils.Runner object at 0x3a288d0>, 
                             <cinder.taskflow.utils.Runner object at 0x3a28850>, 
                             <cinder.taskflow.utils.Runner object at 0x3a287d0>, 
                             <cinder.taskflow.utils.Runner object at 0x3a28750>, 
                             <cinder.taskflow.utils.Runner object at 0x3a286d0>, 
                             <cinder.taskflow.utils.Runner object at 0x3a28650>]
         runner.result = None
         runner._id = 68318d88-aecb-4ac4-b80d-26f6e91b9b95

         runner = Runner: cinder.volume.flows.create_volume.VolumeCastTask;volume:create==1.0; 
                  r-6865ee0d-c5dc-4b62-972b-6ad3af46d147; 1.0
         failed = False
         result = None
         simulate_run = False
         """
            try:                
                # RollbackTask：实现调用任务对应的可用的逆转回滚方法；
                # runner.task = cinder.volume.flows.create_volume.VolumeCastTask;volume:create==1.0
                rb = utils.RollbackTask(context, runner.task, result=None)
                
                # 在回滚方法累加器中添加逆转回滚方法任务；
                self._accumulator.add(rb)
                
                self.task_notifier.notify(states.STARTED, details={
                    'context': context,
                    'flow': self,
                    'runner': runner,
                     })
                
                # simulate_run = False
                if not simulate_run:
                    result = runner(context, *args, **kwargs)
                else:
                    if failed:
                        if not result:
                            result = "%s failed running." % (runner.task)
                        if isinstance(result, basestring):
                            result = exc.InvalidStateException(result)
                        if not isinstance(result, Exception):
                            LOG.warn("Can not raise a non-exception"
                                     " object: %s", result)
                            result = exc.InvalidStateException()
                        raise result
                rb.result = result
                runner.result = result
                self.results[runner.uuid] = result
                
                self.task_notifier.notify(states.SUCCESS, details={
                    'context': context,
                    'flow': self,
                    'runner': runner,
                      })
            except Exception as e:
                runner.result = e
                cause = utils.FlowFailure(runner, self, e)
                with excutils.save_and_reraise_exception():
                    # Notify any listeners that the task has errored.
                    self.task_notifier.notify(states.FAILURE, details={
                        'context': context,
                        'flow': self,
                        'runner': runner,
                           })
                    self.rollback(context, cause)

首先来看语句：

rb = utils.RollbackTask(context, runner.task, result=None)

其中类RollbackTask实现调用任务对应的可用的逆转回滚方法；我们具体来看类RollbackTask的实现代码：

class RollbackTask(object):
    """
    A helper task that on being called will call the underlying callable
    tasks revert method (if said method exists).
    实现调用任务对应的可用的逆转回滚方法；
    """

    def __init__(self, context, task, result):
        # task = cinder.volume.flows.create_volume.VolumeCastTask;volume:create==1.0
        self.task = task
        self.result = result
        self.context = context

    def __str__(self):
        return str(self.task)

    def __call__(self, cause):
        """
        实现调用任务对应的可用的逆转回滚方法；
        """
        if ((hasattr(self.task, "revert") and
             isinstance(self.task.revert, collections.Callable))):
            #注：这里有几种不同的回滚方法；
            self.task.revert(self.context, self.result, cause)

类RollbackTask实现调用任务对应的可用的逆转回滚方法，调用task类中对应的revert方法都需要经过这个类中的__call__方法来实现，后面我们会解析到的。

我们再来看语句：

self._accumulator.add(rb)

这条语句实现了在回滚方法累加器中添加逆转回滚方法任务；在类Flow的初始化方法中可以看到，self._accumulator = utils.RollbackAccumulator()，所以我们先来看类RollbackAccumulator的源码实现：

class RollbackAccumulator(object):
    def __init__(self):
        self._rollbacks = []

    def add(self, *callables):
        self._rollbacks.extend(callables)

    def reset(self):
        self._rollbacks = []

    def __len__(self):
        return len(self._rollbacks)

    def __iter__(self):
        # Rollbacks happen in the reverse order that they were added.
        return reversed(self._rollbacks)

    def __enter__(self):
        return self

    def rollback(self, cause):
        LOG.warn("Activating %s rollbacks due to %s.", len(self), cause)
        
        for (i, f) in enumerate(self):
            LOG.debug("Calling rollback %s: %s", i + 1, f)
            try:
                f(cause)
            except Exception:
                LOG.exception(("Failed rolling back %s: %s due "
                               "to inner exception."), i + 1, f)

    def __exit__(self, type, value, tb):
        if any((value, type, tb)):
            self.rollback(value)

我们可以看到语句self._accumulator.add(rb)实现了把将要执行的回滚任务添加到变量self._rollbacks中；所以这里可以测试self._accumulator._rollbacks：

self._accumulator._rollbacks = [<cinder.taskflow.utils.RollbackTask object at 0x3ae6d10>]
self._accumulator._rollbacks = [<cinder.taskflow.utils.RollbackTask object at 0x3ae6d10>,
                                <cinder.taskflow.utils.RollbackTask object at 0x3ae6e50>]

......

self._accumulator._rollbacks = [<cinder.taskflow.utils.RollbackTask object at 0x3ae6d10>,
                                <cinder.taskflow.utils.RollbackTask object at 0x3ae6e50>,
                                <cinder.taskflow.utils.RollbackTask object at 0x3749150>,
                                <cinder.taskflow.utils.RollbackTask object at 0x3ae6dd0>,
                                <cinder.taskflow.utils.RollbackTask object at 0x3dac3d0>,
                                <cinder.taskflow.utils.RollbackTask object at 0x3df2f50>,
                                <cinder.taskflow.utils.RollbackTask object at 0x3df2610>]

我们可以看到，随着执行任务的增加，其对应的回滚任务也是对应增加的，并存储在变量self._accumulator._rollbacks中；

我们再来看语句：

# simulate_run = False
if not simulate_run:
    result = runner(context, *args, **kwargs)

这里实现的就是执行指定的task任务，并获取返回结果；

这里我们可以得到如下的输出实例：

========================================================================================

runner = Runner: cinder.volume.flows.base.InjectTask;

         volume:create==1.0;
         r-bf0f8b30-c2a0-4c22-ada2-22698adc71b9; 1.0
result = {'backup_source_volume': None,
          'description': None,
          'availability_zone': None,
          'source_volume': None,
          'volume_type': None,
          'name': u'shinian01',
          'image_id': None,
          'metadata': {},
          'snapshot': None,
          'size': 1,
          'key_manager': <cinder.keymgr.conf_key_mgr.ConfKeyManager object at 0x373f790>,
          'scheduler_hints': None}                   

========================================================================================                            runner = Runner:cinder.volume.flows.create_volume.ExtractVolumeRequestTask;

         volume:create==1.0;
         r-c88edccb-d78b-4cfa-ae68-5d8b72bd173d;1.0
result = {'volume_type_id': None,
          'qos_specs': None,
          'encryption_key_id': None,
          'volume_type': {},
          'snapshot_id': None,
          'availability_zone': 'nova',
          'source_volid': None,
          'size': 1}
========================================================================================                    
runner = Runner: cinder.volume.flows.create_volume.QuotaReserveTask;

         volume:create==1.0;
         r-4b407b9a-c1a4-4778-8125-751ebe8e3c2b; 1.0
result = {'reservations': ['6fd57073-e9b6-49b9-bed4-a57b82fced51',
                           '365c158b-d291-4ac5-9d2d-5b7fb8a63dd6']}
========================================================================================                    
runner = Runner: cinder.volume.flows.create_volume.EntryCreateTask;

         volume:create==1.0;
         r-9ee48341-945d-4905-9b1d-31122cc9745e; 1.0
result = {'volume': <cinder.db.sqlalchemy.models.Volume object at 0x3ae6f10>,
          'volume_properties': {'status': 'creating',
                                'volume_type_id': None,
                                'user_id': u'ef073287176048bd861dcd9d9c4d9808',
                                'availability_zone': 'nova',
                                'reservations': ['6fd57073-e9b6-49b9-bed4-a57b82fced51',
                                                 '365c158b-d291-4ac5-9d2d-5b7fb8a63dd6'],
                                'volume_admin_metadata': [],
                                'attach_status': 'detached',
                                'display_description': None,
                                'volume_metadata': [],
                                'metadata': {},
                                'encryption_key_id': None,
                                'source_volid': None,
                                'snapshot_id': None,
                                'display_name': u'shinian01',
                                'project_id': u'6c3c74779a614d3b81dd75518824e25c',
                                'id': '431b1c64-4e43-4731-b052-ff10aefae850',
                                'size': 1},
          'volume_id': '431b1c64-4e43-4731-b052-ff10aefae850'}

========================================================================================        
runner = Runner: cinder.volume.flows.create_volume.QuotaCommitTask;

         volume:create==1.0;
         r-50d91e70-67fe-42f3-911f-1f59d4173b8b; 1.0
result = {'volume_properties': {'status': 'creating',
                                'volume_type_id': None,
                                'user_id': u'ef073287176048bd861dcd9d9c4d9808',
                                'availability_zone': 'nova',
                                'reservations': ['6fd57073-e9b6-49b9-bed4-a57b82fced51',
                                                 '365c158b-d291-4ac5-9d2d-5b7fb8a63dd6'],
                                'volume_admin_metadata': [],
                                'attach_status': 'detached',
                                'display_description': None,
                                'volume_metadata': [],
                                'metadata': {},
                                'encryption_key_id': None,
                                'source_volid': None,
                                'snapshot_id': None,
                                'display_name': u'shinian01',
                                'project_id': u'6c3c74779a614d3b81dd75518824e25c',
                                'id': '431b1c64-4e43-4731-b052-ff10aefae850',
                                'size': 1}}

========================================================================================      
runner = Runner:cinder.volume.flows.create_volume.OnFailureChangeStatusTask;

         volume:create==1.0;
         r-9cd4bfa9-3170-4713-a6ac-2daf984185b2;1.0
result = {'volume_spec': None,
          'volume_id': '431b1c64-4e43-4731-b052-ff10aefae850'}

========================================================================================      
runner = Runner: cinder.volume.flows.create_volume.VolumeCastTask;

         volume:create==1.0;
         r-6865ee0d-c5dc-4b62-972b-6ad3af46d147; 1.0
result = None

========================================================================================

可见这里runner指明了要执行的类的__call__方法，即运行先前添加的task任务；
result是运行task任务后获取的结果信息；
具体的task的实现过程需解析各个task任务类中的__call__方法，之前的博客中我们已经进行了简单的解析，这里不再进行赘述；

好了，到这里方法def run(self, context, *args, **kwargs)中关于flow中task执行的重要语句的实现基本解析完成，在下一篇博客中，我将重点解析在这个方法中，如果卷的建立出现异常，则如何执行相关的逆转回滚操作。