tpool: exception in tpool-ed call leaked memory via backtrace

eventlet#469
eventlet#470
https://cosmicpercolator.com/2016/01/13/exception-leaks-in-python-2-and-3/

eventlet/tpool.py

@@ -85,6 +85,10 @@ def tworker():
             raise
         except EXC_CLASSES:
             rv = sys.exc_info()
+            if sys.version_info >= (3, 4):
+                traceback.clear_frames(rv[1].__traceback__)
+        if six.PY2:
+            sys.exc_clear()
         # test_leakage_from_tracebacks verifies that the use of
         # exc_info does not lead to memory leaks
         _rspq.put((e, rv))

tests/isolated/tpool_exception_leak.py

@@ -0,0 +1,43 @@
+__test__ = False
+
+if __name__ == '__main__':
+    import eventlet
+    import eventlet.tpool
+    import gc
+    import pprint
+
+    class RequiredException(Exception):
+        pass
+
+    class A(object):
+        def ok(self):
+            return 'ok'
+
+        def err(self):
+            raise RequiredException
+
+    a = A()
+
+    # case 1 no exception
+    assert eventlet.tpool.Proxy(a).ok() == 'ok'
+    # yield to tpool_trampoline(), otherwise e.send(rv) have a reference
+    eventlet.sleep(0.1)
+    gc.collect()
+    refs = gc.get_referrers(a)
+    assert len(refs) == 1, 'tpool.Proxy-ied object leaked: {}'.format(pprint.pformat(refs))
+
+    # case 2 with exception
+    def test_exception():
+        try:
+            eventlet.tpool.Proxy(a).err()
+            assert False, 'expected exception'
+        except RequiredException:
+            pass
+    test_exception()
+    # yield to tpool_trampoline(), otherwise e.send(rv) have a reference
+    eventlet.sleep(0.1)
+    gc.collect()
+    refs = gc.get_referrers(a)
+    assert len(refs) == 1, 'tpool.Proxy-ied object leaked: {}'.format(pprint.pformat(refs))
+
+    print('pass')

tests/tpool_test.py

@@ -367,3 +367,7 @@ def test_leakage_from_tracebacks(self):
 
 def test_isolate_from_socket_default_timeout():
     tests.run_isolated('tpool_isolate_socket_default_timeout.py', timeout=1)
+
+
+def test_exception_leak():
+    tests.run_isolated('tpool_exception_leak.py')