Fix #202: periodic yield when running callbacks

Make sure run_callback() (and thus sleep(0)) does not starve CPU.
After some number of callbacks (1000), let the event loop run.

gevent/callbacks.h

@@ -38,3 +38,5 @@ static void gevent_call(struct PyGeventLoopObject* loop, struct PyGeventCallback
 #if defined(_WIN32)
 static void gevent_periodic_signal_check(struct ev_loop *, void *, int);
 #endif
+
+static void gevent_noop(struct ev_loop *_loop, void *watcher, int revents) { }

gevent/core.ppyx

@@ -31,6 +31,7 @@ cdef extern from "callbacks.h":
     void gevent_run_callbacks(libev.ev_loop, void*, int)
     void gevent_periodic_signal_check(libev.ev_loop, void*, int)
     void gevent_call(loop, callback)
+    void gevent_noop(libev.ev_loop, void*, int)
 
 cdef extern from *:
     int errno
@@ -228,6 +229,7 @@ cdef public class loop [object PyGeventLoopObject, type PyGeventLoop_Type]:
     cdef public object error_handler
     cdef libev.ev_prepare _signal_checker
     cdef public list _callbacks
+    cdef libev.ev_timer _timer0
 #ifdef _WIN32
     cdef libev.ev_timer _periodic_signal_checker
 #endif
@@ -239,6 +241,7 @@ cdef public class loop [object PyGeventLoopObject, type PyGeventLoop_Type]:
 #ifdef _WIN32
         libev.ev_timer_init(&self._periodic_signal_checker, <void*>gevent_periodic_signal_check, 0.3, 0.3)
 #endif
+        libev.ev_timer_init(&self._timer0, <void*>gevent_noop, 0.0, 0.0)
         if ptr:
             self._ptr = <libev.ev_loop*>ptr
         else:
@@ -270,12 +273,17 @@ cdef public class loop [object PyGeventLoopObject, type PyGeventLoop_Type]:
     cdef _run_callbacks(self):
         cdef callback cb
         cdef object callbacks
-        while self._callbacks:
+        cdef int count = 1000
+        libev.ev_timer_stop(self._ptr, &self._timer0)
+        while self._callbacks and count > 0:
             callbacks = self._callbacks
             self._callbacks = []
             for cb in callbacks:
                 libev.ev_unref(self._ptr)
                 gevent_call(self, cb)
+                count -= 1
+        if self._callbacks:
+            libev.ev_timer_start(self._ptr, &self._timer0)
 
     def _stop_watchers(self):
         if libev.ev_is_active(&self._signal_checker):

greentest/test__sleep0.py

+from __future__ import with_statement
+import gevent
+import time
+import sys
+import os
+import threading
+
+
+class alarm(threading.Thread):
+    # can't use signal.alarm because of Windows
+
+    def __init__(self, timeout):
+        threading.Thread.__init__(self)
+        self.setDaemon(True)
+        self.timeout = timeout
+        self.start()
+
+    def run(self):
+        time.sleep(self.timeout)
+        sys.stderr.write('Timeout.\n')
+        os._exit(5)
+
+
+alarm(1)
+
+
+with gevent.Timeout(0.01, False):
+    while True:
+        gevent.sleep(0)