gevent: AssertionError: Impossible to call blocking function in the event loop callback

今天在用爬虫时gevent报了AssertionError: Impossible to call blocking function in the event loop callback

异常，很奇怪，难道是patch_socket惹的货，因为之前没有使用patch_socket是正常的，代码简化如下

import urllib
import gevent
from gevent.monkey import patch_socket
from gevent.hub import  get_hub

def f():
    r = urllib.urlopen("http://www.baidu.com/").read()
    print r[:10]

def timer(after, repeat, f):
    t = get_hub().loop.timer(after, repeat)
    t.start(f)
    return t

def run():
    patch_socket()
    timer(1, 5, f)
    gevent.sleep(100)

run()

这段代码就是每5秒调用一次f，f也就是很简单的打压百度首页前10个字符，各位看官在揭开答案请先想想为什么为这样？

我把异常栈也贴在下面，有助有分析

  File "C:\Python27\lib\httplib.py", line 772, in connect
    self.timeout, self.source_address)
  File "C:\Python27\lib\site-packages\gevent\socket.py", line 570, in create_connection
    for res in getaddrinfo(host, port, 0 if has_ipv6 else AF_INET, SOCK_STREAM):
  File "C:\Python27\lib\site-packages\gevent\socket.py", line 621, in getaddrinfo
    return get_hub().resolver.getaddrinfo(host, port, family, socktype, proto, flags)
  File "C:\Python27\lib\site-packages\gevent\resolver_thread.py", line 34, in getaddrinfo
    return self.pool.apply_e(self.expected_errors, _socket.getaddrinfo, args, kwargs)
  File "C:\Python27\lib\site-packages\gevent\threadpool.py", line 222, in apply_e
    success, result = self.spawn(wrap_errors, expected_errors, function, args, kwargs).get()
  File "C:\Python27\lib\site-packages\gevent\event.py", line 226, in get
    result = self.hub.switch()
  File "C:\Python27\lib\site-packages\gevent\hub.py", line 330, in switch
    switch_out()
  File "C:\Python27\lib\site-packages\gevent\hub.py", line 334, in switch_out
    raise AssertionError(‘Impossible to call blocking function in the event loop callback‘)
AssertionError: Impossible to call blocking function in the event loop callback
<timer at 0x2652ed0 callback=<function f at 0x026B0070> args=()> failed with AssertionError

刚开始我百思不得其解，就这么简单为什么会有问题？

看异常栈是调用hub的switch_out出的问题，

    def switch(self):
        switch_out = getattr(getcurrent(), ‘switch_out‘, None)
        if switch_out is not None:
            switch_out()
        return greenlet.switch(self)

    def switch_out(self):
        raise AssertionError(‘Impossible to call blocking function in the event loop callback‘)

以前文章提过，gevent提供了switch_out方法用于当前greenlet换出时调用，咦，可为什么调用的hub的

switch_out？按理说应该调用其它greenlet的switch_out，怪不得有问题，hub都被换出了，谁去做调度呢？

问题就出在这里？你有没有发现，在上面的代码中只有hub，压根没有其它的greenlet。

我们走一遍代码逻辑，首先给系统注册一定时器f，当调用f时由于socket阻塞，所以会切换到hub,此时会调用之前greenlet的switch_out方法，可不幸的是之前的greenlet就是hub，所以出问题了。

知道了问题所在就好解决了，也就是用一个greenlet包装一下f，代码如下：

import urllib
import gevent
from gevent.monkey import patch_socket
from gevent.hub import  get_hub

def patch_greenlet(f):
    def inner(*args, **kwargs):
         return gevent.spawn(f, *args, **kwargs)
    return inner

@patch_greenlet
def f():
    r = urllib.urlopen("http://www.baidu.com/").read()
    print r[:10]

def timer(after, repeat, f):
    t = get_hub().loop.timer(after, repeat)
    t.start(f)
    return t

def run():
    patch_socket()
    timer(1, 0, f)
    gevent.sleep(100)

run()

不得不说使用gevent会碰到很多问题，这也许就是协成让人痴迷的一个原因吧，享受"找虐"的兴趣，越享受，越能驾驭它。