PicklingError using live_server fixture on Windows

首先，在使用 Python 2.7.12 的 Windows 上复现 live_server fixture 失败的问题，然后检查 pytest_flask/plugin.py 第 85 行附近以及 pytest_flask/fixtures.py 第 64 行和第 127 行附近的代码。跟踪引发 PicklingError 的 multiprocessing 启动过程，并确定预期结果是使 live_server fixture 正常工作，还是记录 Windows 支持方面的限制。

Not sure if Windows is supposed to be supported, but for me it fails on Windows 10 Python 2.7.12 with the following error when live_server fixture used:

request = <SubRequest '_push_request_context' for <Function 'test_get_url'>>                                                                                       
                                                                                                                                                                   
    @pytest.fixture(autouse=True)                                                                                                                                  
    def _push_request_context(request):                                                                                                                            
        """During tests execution request context has been pushed, e.g. `url_for`,                                                                                 
        `session`, etc. can be used in tests as is::                                                                                                               
                                                                                                                                                                   
            def test_app(app, client):                                                                                                                             
                assert client.get(url_for('myview')).status_code == 200                                                                                            
                                                                                                                                                                   
        """                                                                                                                                                        
        if 'app' not in request.fixturenames:                                                                                                                      
            return                                                                                                                                                 
                                                                                                                                                                   
        app = request.getfuncargvalue('app')                                                                                                                       
                                                                                                                                                                   
        # Get application bound to the live server if ``live_server`` fixture                                                                                      
        # is applyed. Live server application has an explicit ``SERVER_NAME``,                                                                                     
        # so ``url_for`` function generates a complete URL for endpoint which                                                                                      
        # includes application port as well.                                                                                                                       
        if 'live_server' in request.fixturenames:                                                                                                                  
>           app = request.getfuncargvalue('live_server').app                                                                                                       
                                                                                                                                                                   
..\..\..\..\python-virtualenv\shadow\Lib\site-packages\pytest_flask\plugin.py:85:                                                                                  
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _                                                                                    
..\..\..\..\python-virtualenv\shadow\Lib\site-packages\pytest_flask\fixtures.py:127: in live_server                                                                
    server.start()                                                                                                                                                 
..\..\..\..\python-virtualenv\shadow\Lib\site-packages\pytest_flask\fixtures.py:64: in start                                                                       
    self._process.start()                                                                                                                                          
c:\python27\Lib\multiprocessing\process.py:130: in start                                                                                                           
    self._popen = Popen(self)                                                                                                                                      
c:\python27\Lib\multiprocessing\forking.py:277: in __init__                                                                                                        
    dump(process_obj, to_child, HIGHEST_PROTOCOL)                                                                                                                  
c:\python27\Lib\multiprocessing\forking.py:199: in dump                                                                                                            
    ForkingPickler(file, protocol).dump(obj)                                                                                                                       
c:\python27\Lib\pickle.py:224: in dump                                                                                                                             
    self.save(obj)                                                                                                                                                 
c:\python27\Lib\pickle.py:331: in save                                                                                                                             
    self.save_reduce(obj=obj, *rv)                                                                                                                                 
c:\python27\Lib\pickle.py:425: in save_reduce                                                                                                                      
    save(state)                                                                                                                                                    
c:\python27\Lib\pickle.py:286: in save                                                                                                                             
    f(self, obj) # Call unbound method with explicit self                                                                                                          
c:\python27\Lib\pickle.py:655: in save_dict                                                                                                                        
    self._batch_setitems(obj.iteritems())                                                                                                                          
c:\python27\Lib\pickle.py:687: in _batch_setitems                                                                                                                  
    save(v)                                                                                                                                                        
c:\python27\Lib\pickle.py:286: in save                                                                                                                             
    f(self, obj) # Call unbound method with explicit self                                                                                                          
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _                                                                                    
                                                                                                                                                                   
self = <multiprocessing.forking.ForkingPickler instance at 0x0000000007EDBA08>                                                                                     
obj = <function <lambda> at 0x00000000081B9DD8>, name = '<lambda>'                                                                                                 
pack = <built-in function pack>                                                                                                                                    
                                                                                                                                                                   
    def save_global(self, obj, name=None, pack=struct.pack):                                                                                                       
        write = self.write                                                                                                                                         
        memo = self.memo                                                                                                                                           
                                                                                                                                                                   
        if name is None:                                                                                                                                           
            name = obj.__name__                                                                                                                                    
                                                                                                                                                                   
        module = getattr(obj, "__module__", None)                                                                                                                  
        if module is None:                                                                                                                                         
            module = whichmodule(obj, name)                                                                                                                        
                                                                                                                                                                   
        try:                                                                                                                                                       
            __import__(module)                                                                                                                                     
            mod = sys.modules[module]                                                                                                                              
            klass = getattr(mod, name)                                                                                                                             
        except (ImportError, KeyError, AttributeError):                                                                                                            
            raise PicklingError(                                                                                                                                   
                "Can't pickle %r: it's not found as %s.%s" %                                                                                                       
>               (obj, module, name))                                                                                                                               
E           PicklingError: Can't pickle <function <lambda> at 0x00000000081B9DD8>: it's not found as pytest_flask.fixtures.<lambda>