一、背景

大家都知道gevent的机制是单线程+协程机制，当遇到可能会阻塞的操作时，就切换到可运行的协程中继续运行，以此来实现提交系统运行效率的目标，但是具体是怎么实现的呢？让我们直接从代码中看一下吧。

二、切换机制

让我们从socket的send、recv方法入手：

def recv(self, *args):
  while 1:
    try:
      return self._sock.recv(*args)
    except error as ex:
      if ex.args[0] != EWOULDBLOCK or self.timeout == 0.0:
        raise
      # QQQ without clearing exc_info test__refcount.test_clean_exit fails
      sys.exc_clear()
    self._wait(self._read_event)

这里会开启一个死循环，在循环中调用self._sock.recv()方法，并捕获异常，当错误是EWOULDBLOCK时，则调用self._wait(self._read_event)方法，该方法其实是：_wait = _wait_on_socket，_wait_on_socket方法的定义在文件：_hub_primitives.py中，如下：

# Suitable to be bound as an instance method
def wait_on_socket(socket, watcher, timeout_exc=None):
  if socket is None or watcher is None:
    # test__hub TestCloseSocketWhilePolling, on Python 2; Python 3
    # catches the EBADF differently.
    raise ConcurrentObjectUseError("The socket has already been closed by another greenlet")
  _primitive_wait(watcher, socket.timeout,
          timeout_exc if timeout_exc is not None else _NONE,
          socket.hub)

该方法其实是调用了函数：_primitive_wait()，其仍然在文件：_hub_primitives.py中定义，如下：

def _primitive_wait(watcher, timeout, timeout_exc, hub):
  if watcher.callback is not None:
    raise ConcurrentObjectUseError('This socket is already used by another greenlet: %r'
                    % (watcher.callback, ))

  if hub is None:
    hub = get_hub()

  if timeout is None:
    hub.wait(watcher)
    return

  timeout = Timeout._start_new_or_dummy(
    timeout,
    (timeout_exc
     if timeout_exc is not _NONE or timeout is None
     else _timeout_error('timed out')))

  with timeout:
    hub.wait(watcher)

这里其实是调用了hub.wait()函数，该函数的定义在文件_hub.py中，如下：

class WaitOperationsGreenlet(SwitchOutGreenletWithLoop): # pylint:disable=undefined-variable

  def wait(self, watcher):
    """
    Wait until the *watcher* (which must not be started) is ready.

    The current greenlet will be unscheduled during this time.
    """
    waiter = Waiter(self) # pylint:disable=undefined-variable
    watcher.start(waiter.switch, waiter)
    try:
      result = waiter.get()
      if result is not waiter:
        raise InvalidSwitchError(
          'Invalid switch into %s: got %r (expected %r; waiting on %r with %r)' % (
            getcurrent(), # pylint:disable=undefined-variable
            result,
            waiter,
            self,
            watcher
          )
        )
    finally:
      watcher.stop()

watcher.stop()

该类WaitOperationsGreenlet是Hub的基类，其方法wait中的逻辑是：生成一个Waiter对象，并调用watcher.start(waiter.switch, waiter)方法，watcher是最开始recv方法中使用的self._read_event，watcher是gevent的底层事件框架libev中的概念；同时还有一个waiter对象，它类似与python中的future概念，该对象有一个switch()方法以及get()方法，当没有得到结果没有准备好时，调用waiter.get()方法回导致协程被挂起；get()函数的定义如下：

def get(self):
  """If a value/an exception is stored, return/raise it. Otherwise until switch() or throw() is called."""
  if self._exception is not _NONE:
    if self._exception is None:
      return self.value
    getcurrent().throw(*self._exception) # pylint:disable=undefined-variable
  else:
    if self.greenlet is not None:
      raise ConcurrentObjectUseError('This Waiter is already used by %r' % (self.greenlet, ))
    self.greenlet = getcurrent() # pylint:disable=undefined-variable
    try:
      return self.hub.switch()
    finally:
      self.greenlet = None

在get()中最关键的是self.hub.switch()函数，该函数将执行权转移到hub，并继续运行，至此已经分析完了当在worker协程中从网络获取数据遇到阻塞时，如何避免阻塞并切换到hub中的实现，至于何时再切换会worker协程，我们后续再继续分析。

总结

要记得gevent中一个重要的概念，协程切换不是调用而是执行权的转移，从可能会阻塞的协程切换到hub，并由hub在合适的时机切换到另一个可以继续运行的协程继续执行；gevent通过这种形式实现了提高io密集型应用吞吐率的目标。

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