proc: add Waiter class - a lightweight helper class for wait() function (replacement for event). fix Source to use Waiter

eventlet/proc.py

@@ -425,11 +425,7 @@ class Source(object):
             EXC = True
         try:
             try:
-                # QQQ for successive send() calls, event can be notified more than once here
-                # QQQ forbid to call send()/send_exception() more than once
-                # QQQ use smart_switch() (i.e. switch that can be cancelled in finally) here,
-                #     it won't have this problem
-                event = coros.event()
+                event = Waiter()
                 self.link(event)
                 try:
                     return event.wait()
@@ -444,6 +440,41 @@ class Source(object):
                 timer.__exit__(None, None, None)
 
 
+class Waiter(object):
+
+    def __init__(self):
+        self.greenlet = None
+
+    def send(self, value):
+        """Make greenlet calling wait() wake up (if there is a wait()).
+        Can only be called from get_hub().greenlet.
+        """
+        assert api.getcurrent() is api.get_hub().greenlet
+        if self.greenlet is not None:
+            self.greenlet.switch(value)
+        
+    def send_exception(self, *throw_args):
+        """Make greenlet calling wait() wake up (if there is a wait()).
+        Can only be called from get_hub().greenlet.
+        """
+        assert api.getcurrent() is api.get_hub().greenlet
+        if self.greenlet is not None:
+            self.greenlet.throw(*throw_args)
+
+    def wait(self):
+        """Wait until send or send_exception is called. Return value passed
+        into send() or raise exception passed into send_exception().
+        """
+        assert self.greenlet is None
+        current = api.getcurrent()
+        assert current is not api.get_hub().greenlet
+        self.greenlet = current
+        try:
+            return api.get_hub().switch()
+        finally:
+            self.greenlet = None
+
+
 class Proc(Source):
     """A linkable coroutine based on Source.
     Upon completion, delivers coroutine's result to the listeners.