event: make sure callback passed to rawlink() is always called in hub's greenlet

gevent/event.py

@@ -10,6 +10,7 @@ class Event(object):
     def __init__(self):
         self._links = set()
         self._value = _NONE
+        self._notifier = None
 
     def ready(self):
         return self._value is not _NONE
@@ -20,12 +21,11 @@ class Event(object):
             return self._value
 
     def rawlink(self, callback):
-        if self._value is _NONE:
+        if not callable(callback):
+            raise TypeError('Expected callable: %r' % (callback, ))
         self._links.add(callback)
-        else:
-            # QQQ switch won't work as a callback!
-            # QQQ should I schedule the callback here too, like Greenlet.rawlink does?
-            callback(self)
+        if self.ready() and self._notifier is None:
+            self._notifier = core.active_event(self._notify_links)
 
     def unlink(self, callback):
         self._links.discard(callback)
@@ -38,10 +38,11 @@ class Event(object):
         oldvalue = self._value
         self._value = value
         if oldvalue is _NONE:
-            if self._links:
-                core.active_event(self._notify_links)
+            if self._links and self._notifier is None:
+                self._notifier = core.active_event(self._notify_links)
 
     def _notify_links(self):
+        try:
             assert getcurrent() is get_hub()
             while self._links:
                 link = self._links.pop()
@@ -54,6 +55,8 @@ class Event(object):
                     except:
                         pass
                 # even if g is left unscheduled, it will be deallocated because there are no more references to it
+        finally:
+            self._notifier = None
 
     def get(self, block=True, timeout=None):
         if self._value is not _NONE: