python-memcached并发调用get/set时出现RunTimeError：Second simultaneous read on fileno 8 detected.
出现异常如下 RuntimeError: Second simultaneous read on fileno 8 detected. Unless you really know what you're doing, make sure that only one greenthread can read any particular socket. Consider using a pools.Pool. If you do know what you're doing and want to disable this error, call eventlet.debug.hub_multiple_reader_prevention(False)
2013-03-08 18:27:27 17870 TRACE glance.store.chunkcache.fake File "/usr/lib/python2.7/dist-packages/memcache.py", line 862, in get
2013-03-08 18:27:27 17870 TRACE glance.store.chunkcache.fake return self._get('get', key)
2013-03-08 18:27:27 17870 TRACE glance.store.chunkcache.fake File "/usr/lib/python2.7/dist-packages/memcache.py", line 846, in _get
2013-03-08 18:27:27 17870 TRACE glance.store.chunkcache.fake return _unsafe_get()
2013-03-08 18:27:27 17870 TRACE glance.store.chunkcache.fake File "/usr/lib/python2.7/dist-packages/memcache.py", line 830, in _unsafe_get
2013-03-08 18:27:27 17870 TRACE glance.store.chunkcache.fake rkey, flags, rlen, = self._expectvalue(server)
2013-03-08 18:27:27 17870 TRACE glance.store.chunkcache.fake File "/usr/lib/python2.7/dist-packages/memcache.py", line 955, in _expectvalue
2013-03-08 18:27:27 17870 TRACE glance.store.chunkcache.fake line = server.readline()
2013-03-08 18:27:27 17870 TRACE glance.store.chunkcache.fake File "/usr/lib/python2.7/dist-packages/memcache.py", line 1125, in readline
2013-03-08 18:27:27 17870 TRACE glance.store.chunkcache.fake data = recv(4096)
2013-03-08 18:27:27 17870 TRACE glance.store.chunkcache.fake File "/usr/local/lib/python2.7/dist-packages/eventlet/greenio.py", line 249, in recv
2013-03-08 18:27:27 17870 TRACE glance.store.chunkcache.fake timeout_exc=socket.timeout("timed out"))
2013-03-08 18:27:27 17870 TRACE glance.store.chunkcache.fake File "/usr/local/lib/python2.7/dist-packages/eventlet/hubs/__init__.py", line 117, in trampoline
2013-03-08 18:27:27 17870 TRACE glance.store.chunkcache.fake listener = hub.add(hub.READ, fileno, current.switch)
2013-03-08 18:27:27 17870 TRACE glance.store.chunkcache.fake File "/usr/local/lib/python2.7/dist-packages/eventlet/hubs/poll.py", line 27, in add
2013-03-08 18:27:27 17870 TRACE glance.store.chunkcache.fake listener = super(Hub, self).add(evtype, fileno, cb)
2013-03-08 18:27:27 17870 TRACE glance.store.chunkcache.fake File "/usr/local/lib/python2.7/dist-packages/eventlet/hubs/hub.py", line 126, in add
2013-03-08 18:27:27 17870 TRACE glance.store.chunkcache.fake evtype, fileno, evtype))
2013-03-08 18:27:27 17870 TRACE glance.store.chunkcache.fake RuntimeError: Second simultaneous read on fileno 8 detected. Unless you really know what you're doing, make sure that only one greenthread can read any particular socket. Consider using a pools.Pool. If you do know what you're doing and want to disable this error, call eventlet.debug.hub_multiple_reader_prevention(False)
出现问题时的大致模型如下
class CacheBackend(CacheBackendBase):
'''
Cache in memcached servers.
'''
def __init__(self):
value_length = CONF.chunkcache_memcached_value_length
servers = CONF.chunkcache_memcached_servers
self.memcache = memcache.Client(servers, debug=1,
server_max_value_length=value_length)
@staticmethod
def instance():
global CACHE_BACKEND
if CACHE_BACKEND is None:
CACHE_BACKEND = CacheBackend()
return CACHE_BACKEND
def get(self, key, default=None):
result = self.memcache.get(str(key))
return result or default
def exist(self, key):
return self.get(str(key)) != None
def set(self, key, value):
result = self.memcache.set(str(key), value)
if result == 0:
raise exception.MemcacheSetError()
return result
def delete(self, key):
if self.exist(key):
result = self.memcache.delete(str(key))
if result == 0:
raise exception.MemcacheDeleteError()
return True
def clear(self):
# No need for clearing memcached.
return True
经过同事交流，推测可能是因为fork之前有socket连接在或者是多个进程重用了一个socket导致，但是感觉都不太能讲得通，对这块确实不太了解，先记录下来解决方案，有谁了解的求解释~
下面是解决的模型，大致改动就是在每个set，get，delete处都new一个memcache client，
可以新建一个socket？不过看memcache模块，好像也是有socket缓存的啊，会去缓存中拿同个socket，
下面的disconnect_all加与不加都一样，不会出现问题，而且在上面的模块中加上disconnect_all再重试也不起作用，
实在无法理解，但是确实解决了问题，多次测试都未复现上述问题。
class CacheBackend(CacheBackendBase):
'''
Cache in memcached servers.
'''
def __init__(self):
self.value_length = CONF.chunkcache_memcached_value_length
self.servers = CONF.chunkcache_memcached_servers
@staticmethod
def instance():
global CACHE_BACKEND
if CACHE_BACKEND is None:
CACHE_BACKEND = CacheBackend()
return CACHE_BACKEND
def get(self, key, default=None):
client = _connect(self.servers, self.value_length)
result = client.get(str(key))
client.disconnect_all()
return result or default
def exist(self, key):
return self.get(str(key)) != None
def set(self, key, value):
client = _connect(self.servers, self.value_length)
result = client.set(str(key), value)
client.disconnect_all()
if result == 0:
raise exception.MemcacheSetError()
return result
def delete(self, key):
client = _connect(self.servers, self.value_length)
if self.exist(key):
result = client.delete(str(key))
if result == 0:
raise exception.MemcacheDeleteError()
client.disconnect_all()
return True
def clear(self):
# No need for clearing memcached.
return True
def _connect(servers, value_length=chunkstore.Store.CHUNKSIZE):
return memcache.Client(servers, debug=1,
server_max_value_length=value_length)
另外想解释下，并不是单例或者维护一个client对象导致，去掉单例，问题一样，而且并发是多进程并发的，也不应该存在什么单例之类的问题。