Do not save traceback object in Hub.switch() as that can leak it; only save an exception object

gevent/core.pyx

@@ -584,7 +584,8 @@ def get_header_version():
     return levent._EVENT_VERSION
 
 
-def set_exc_info(object typ, object value, object tb):
+def set_exc_info(object value):
+    cdef object typ
     cdef PyThreadState* tstate = PyThreadState_GET()
     if tstate.exc_type != NULL:
         Py_DECREF(<object>tstate.exc_type)
@@ -592,13 +593,16 @@ def set_exc_info(object typ, object value, object tb):
         Py_DECREF(<object>tstate.exc_value)
     if tstate.exc_traceback != NULL:
         Py_DECREF(<object>tstate.exc_traceback)
-    Py_INCREF(typ)
-    Py_INCREF(value)
-    Py_INCREF(tb)
-    tstate.exc_type = <void*>typ
-    tstate.exc_value = <void *>value
-    tstate.exc_traceback = <void *>tb
-
+    if value is None:
+        tstate.exc_type = NULL
+        tstate.exc_value = NULL
+    else:
+        typ = type(value)
+        Py_INCREF(typ)
+        Py_INCREF(value)
+        tstate.exc_type = <void*>typ
+        tstate.exc_value = <void *>value
+    tstate.exc_traceback = NULL
 
 #include "evdns.pxi"
 include "evbuffer.pxi"

gevent/hub.py

@@ -169,7 +169,7 @@ class Hub(greenlet):
     def switch(self):
         cur = getcurrent()
         assert cur is not self, 'Impossible to call blocking function in the event loop callback'
-        exc_info = sys.exc_info()
+        exception = sys.exc_info()[1]
         try:
             switch_out = getattr(cur, 'switch_out', None)
             if switch_out is not None:
@@ -180,7 +180,7 @@ class Hub(greenlet):
             sys.exc_clear()
             return greenlet.switch(self)
         finally:
-            core.set_exc_info(*exc_info)
+            core.set_exc_info(exception)
 
     def run(self):
         global _threadlocal