def __call__(self, req):
'keystonecontext : Make a request context from keystone headers.'
ctx = context.RequestContext(user_id,
                                     project_id,
                                     project_name=project_name,
                                     roles=roles,
                                     auth_token=auth_token,
                                     remote_address=remote_address,
                                     service_catalog=service_catalog,
                                     request_id=req_id)

req.environ['cinder.context'] = ctx #添加 cinder.context
return self.application //If you are not modifying the output, you can just return the app.

def __call__(self, req):
     if req.content_length > CONF.osapi_max_request_body_size: #判断 req.content_length，大于 CONF.osapi_max_request_body_size 则直接失败
            msg = _("Request is too large.")
            raise webob.exc.HTTPRequestEntityTooLarge(explanation=msg)
        if req.content_length is None and req.is_body_readable:
            limiter = LimitingReader(req.body_file, CONF.osapi_max_request_body_size)
            req.body_file = limiter
        return self.application 

def __call__(self, req):
       try:
           return req.get_response(self.application)
       except Exception as ex:
           return self._error(ex, req) //return wsgi.Fault(outer) 如果application返回response过程出错，则返回Traceback (most recent call last)

def __call__(self, req):
        req_id = context.generate_request_id()  //req-<UUID>
        req.environ[ENV_REQUEST_ID] = req_id //添加 'openstack.request_id'
        response = req.get_response(self.application)
        if HTTP_RESP_HEADER_REQUEST_ID not in response.headers: // 'x-openstack-request-id'
            response.headers.add(HTTP_RESP_HEADER_REQUEST_ID, req_id) #在response上添加x-openstack-request-id，比如 x-openstack-request-id: 123567890
        return response //return 修改后的 response

def __call__(self, environ, start_response):
        """Resolves the URL in PATH_INFO, and uses wsgi.routing_args to pass on URL resolver results."""
                
        # Run the actual route matching
        # -- Assignment of environ to config triggers route matching
        if self.singleton: #singleton 默认值为 True，所以执行该分支
            config = request_config()
            config.mapper = self.mapper
            config.environ = environ
            match = config.mapper_dict #获取match，（{'action': u'detail', 'controller': <cinder.api.openstack.wsgi.Resource object at 0x7fa137be8950>, 'project_id': u'fa2046aaead44a698de8268f94759fc1'})
            route = config.route #获取route
        ...                
        url = URLGenerator(self.mapper, environ) #比如 <routes.util.URLGenerator object at 0x7fa137a15590>
        environ['wsgiorg.routing_args'] = ((url), match) 
        environ['routes.route'] = route #比如 'routes.route': <routes.route.Route object at 0x7fa137bef4d0>
        environ['routes.url'] = url #比如 'routes.url': <routes.util.URLGenerator object at 0x7fa137a15590>
        ...
       
        response = self.app(environ, start_response) #调用 _dispatch WSGI App
        ...
        return response

match = req.environ['wsgiorg.routing_args'][1] #match：{'action': u'detail', 'controller': <cinder.api.openstack.wsgi.Resource object at 0x7fa137be8950>, 'project_id': u'fa2046aaead44a698de8268f94759fc1'}
if not match:
     return webob.exc.HTTPNotFound()
app = match['controller'] #<cinder.api.openstack.wsgi.Resource object at 0x7fa137be8950>
return app #调用该 App 的 __call__ 方法

# Identify the action, its arguments, and the requested content type
action_args = self.get_action_args(request.environ) #从environ 获取 match
action = action_args.pop('action', None) #得到 action 'detail'
content_type, body = self.get_body(request) #得到request 的 content_type 和 body。body 为空。
accept = request.best_match_content_type() #如果 environ 中有 'cinder.best_content_type' 的话，直接返回，比如 'application/json'；没有的话，则找个最合适的content type，并设置到environ。
return self._process_stack(request, action, action_args, content_type, body, accept) #调用 _process_stack

def _process_stack(self, request, action, action_args,content_type, body, accept):

#action_args：{'project_id': u'43f66bb82e684bbe9eb9ef6892bd7fd6'}， action: detail， body:， 


meth, extensions = self.get_method(request, action, content_type, body) 
#找到 action 对应的 Controller 的 method 和 这个 Core Resource 的所有带这个 aciton 的 Resource  extensions（如果 action 是 ‘action’ 的话，根据 request body 中的信息，会把 action 转化为具体的 Controller 的 method）。 
#返回值 meth： <bound method VolumeController.detail of <cinder.api.v2.volumes.VolumeController object at 0x7f2fd96863d0>>。
#返回值 extensions: [<bound method VolumeTenantAttributeController.detail of <cinder.api.contrib.volume_tenant_attribute.VolumeTenantAttributeController object at 0x7f2fd885ac50>>, <bound method VolumeReplicationController.detail of <cinder.api.contrib.volume_replication.VolumeReplicationController object at 0x7f2fd86eacd0>>, <bound method VolumeHostAttributeController.detail of <cinder.api.contrib.volume_host_attribute.VolumeHostAttributeController object at 0x7f2fd87088d0>>, <bound method VolumeMigStatusAttributeController.detail of <cinder.api.contrib.volume_mig_status_attribute.VolumeMigStatusAttributeController object at 0x7f2fd870e090>>, <bound method VolumeImageMetadataController.detail of <cinder.api.contrib.volume_image_metadata.VolumeImageMetadataController object at 0x7f2fd870e790>>]


contents = self.deserialize(meth, content_type, body) #根据 content_type 反序列化 request body

response, post = self.pre_process_extensions(extensions, request, action_args) # 找出 extensions 的 _call__ 方法，分别调用，得到 response

def pre_process_extensions(self, extensions, request, action_args)

{

for ext in extensions:
#遍历所有的Resource extension，比如 <bound method VolumeTenantAttributeController.detail of <cinder.api.contrib.volume_tenant_attribute.VolumeTenantAttributeController object at 0x7f2fd885ac50>>

    if inspect.isgeneratorfunction(ext): #如果object是一个生成器函数(任何包含yield表达式的函数即为生成器方法), 则返回True。现在还没有这样的方法。这里会返回false
        response = None

# If it's a generator function, the part before the yield is the preprocessing stage
try:
with ResourceExceptionHandler():
gen = ext(req=request, **action_args)
response = gen.next()
except Fault as ex:
response = ex

# We had a response...
if response:
return response, []

# No response, queue up generator for post-processing
post.append(gen)

else:
# Regular functions only perform post-processing
post.append(ext) #等待执行post-processing

# Run post-processing in the reverse order

#将所有不是generatorfunciton 的 ext 放进 post， 比如 [<bound method VolumeTenantAttributeController.detail of <cinder.api.contrib.volume_tenant_attribute.VolumeTenantAttributeController object at 0x7f2fd885ac50>>, <bound method VolumeReplicationController.detail of <cinder.api.contrib.volume_replication.VolumeReplicationController object at 0x7f2fd86eacd0>>, <bound method VolumeHostAttributeController.detail of <cinder.api.contrib.volume_host_attribute.VolumeHostAttributeController object at 0x7f2fd87088d0>>, <bound method VolumeMigStatusAttributeController.detail of <cinder.api.contrib.volume_mig_status_attribute.VolumeMigStatusAttributeController object at 0x7f2fd870e090>>, <bound method VolumeImageMetadataController.detail of <cinder.api.contrib.volume_image_metadata.VolumeImageMetadataController object at 0x7f2fd870e790>>]
return None, reversed(post)

}

if not response: #response 为 null，调用 method 方法，比如 <bound method VolumeController.detail of <cinder.api.v1.volumes.VolumeController object at 0x7fa137be8650>>
    action_result = self.dispatch(meth, request, action_args) #执行Resource Controller 的基本方法，得到其返回值    

def dispatch(self, method, request, action_args):
        """Dispatch a call to the action-specific method."""
        #method: <bound method VolumeController.detail of <cinder.api.v2.volumes.VolumeController object at 0x7f2fd96863d0>>

#action_args: {}

#method执行结果为： {'volumes': [{'status': u'error', 'user_id': u'1dc0db32a936496ebfc50be54924a7cc', 'attachments': [], 'links': [{'href': u'http://controller:8776/v2/43f66bb82e684bbe9eb9ef6892bd7fd6/volumes/42ddb30e-8a36-4301-99e4-9547ce4e860d', 'rel': 'self'}, {'href': u'http://controller:8776/43f66bb82e684bbe9eb9ef6892bd7fd6/volumes/42ddb30e-8a36-4301-99e4-9547ce4e860d', 'rel': 'bookmark'}], 'availability_zone': u'nova', 'bootable': 'false', 'encrypted': False, 'created_at': datetime.datetime(2015, 1, 3, 2, 47, 52), 'description': None, 'volume_type': None, 'name': None, 'replication_status': u'disabled', 'consistencygroup_id': None, 'source_volid': None, 'snapshot_id': None, 'metadata': {}, 'id': u'42ddb30e-8a36-4301-99e4-9547ce4e860d', 'size': 1L}]}
        return method(req=request, **action_args)

resp_obj = ResponseObject(action_result) #初始化其返回值为一个ResponseObject对象

_set_request_id_header(request, resp_obj) #设置 headers['x-compute-request-id'] = req.environ.get('cinder.context').request_id

serializers = getattr(meth, 'wsgi_serializers', {}) #获取Controller的 action 方法使用的serializers。detail 方法的serizlizer 是 {'xml': <class 'cinder.api.v2.volumes.VolumesTemplate'>}

resp_obj._bind_method_serializers(serializers) #设置 resp_obj 的序列化方法

resp_obj.preserialize(accept, self.default_serializers) # resp_obj 做序列化准备

  response = self.post_process_extensions(post, resp_obj, request, action_args) # 对每个 extension 执行操作，获取response：  response = ext(req=request, resp_obj=resp_obj, **action_args)     

def post_process_extensions(self, extensions, resp_obj, request,action_args):

    for ext in extensions:

             #遍历 Resource extension。以 <bound method VolumeImageMetadataController.detail of <cinder.api.contrib.volume_image_metadata.VolumeImageMetadataController object at 0x7f2fd870e790>> 为例。

            response = None

            if inspect.isgenerator(ext): #Return true if the object is a generator. 这里返回 false

                # If it's a generator, run the second half of
                # processing
                try:
                    with ResourceExceptionHandler():
                        response = ext.send(resp_obj)
                    except StopIteration:
                    # Normal exit of generator
                    continue
                except Fault as ex:
                    response = ex
            else:
                # Regular functions get post-processing...
                try:
                    with ResourceExceptionHandler():
                        response = ext(req=request, resp_obj=resp_obj, **action_args) #调用 extension method 的方法，它会把 reponse 插进 resp_obj。本例子中将返回none。

@wsgi.extends
    def detail(self, req, resp_obj):
        context = req.environ['cinder.context']
        if authorize(context):
            resp_obj.attach(xml=VolumesImageMetadataTemplate())
            all_meta = self._get_all_images_metadata(context)
            for vol in list(resp_obj.obj.get('volumes', [])):
                image_meta = all_meta.get(vol['id'], {})
                self._add_image_metadata(context, vol, image_meta)

                except Fault as ex:
                    response = ex

            # We had a response...
            if response:
                return response
        return None #返回none

  if resp_obj and not response:

      response = resp_obj.serialize(request, accept, self.default_serializers) #执行序列化，得到 response string

  return response #返回response，后续需要改response的middleware的__call__方法将得到继续执行，指导回到 WSGI Server，它会将 response 返回给cinderclient