[pypy-issue] Issue #2683: Infinite traceback with greenlets (pypy/pypy)

[Antonio Cuni]

New issue 2683: Infinite traceback with greenlets
https://bitbucket.org/pypy/pypy/issues/2683/infinite-traceback-with-greenlets

Antonio Cuni:

Consider the following minimal example:
```
import traceback
import greenlet

def run_in_b():
    print 2
    A.switch()
    print 4

def run_in_a():
    print 1
    B.switch()
    print 3

A = greenlet.getcurrent()
B = greenlet.greenlet(run_in_b)

run_in_a()
traceback.print_stack(B.gr_frame, 10)
```

On CPython, it prints simply this:
```
$ python hangs.py 
1
2
3
  File "hangs.py", line 6, in run_in_b
    A.switch()
```

On PyPy, there seems to be an endless cycle of frames:
```
$ pypy hangs.py 
1
2
3
  File "/home/antocuni/pypy/default/lib_pypy/greenlet.py", line 190, in _greenlet_start
    res = greenlet.run(*args, **kwds)
  File "hangs.py", line 6, in run_in_b
    A.switch()
  File "/home/antocuni/pypy/default/lib_pypy/greenlet.py", line 53, in switch
    return self.__switch('switch', (args, kwds))
  File "/home/antocuni/pypy/default/lib_pypy/greenlet.py", line 92, in __switch
    args, kwds = unbound_method(current, *baseargs, to=target)
  File "/home/antocuni/pypy/default/lib_pypy/greenlet.py", line 190, in _greenlet_start
    res = greenlet.run(*args, **kwds)
  File "hangs.py", line 6, in run_in_b
    A.switch()
  File "/home/antocuni/pypy/default/lib_pypy/greenlet.py", line 53, in switch
    return self.__switch('switch', (args, kwds))
  File "/home/antocuni/pypy/default/lib_pypy/greenlet.py", line 92, in __switch
    args, kwds = unbound_method(current, *baseargs, to=target)
  File "/home/antocuni/pypy/default/lib_pypy/greenlet.py", line 190, in _greenlet_start
    res = greenlet.run(*args, **kwds)
  File "hangs.py", line 6, in run_in_b
    A.switch()
```

This is a minimal example which shows the issue; however, the bug causes issues also with eventlet, in which `get_hub().run` plays the role of `A` and an arbitrary `spawn()`ed function plays the role of `B`.

Tested with pypy 5.9 and nightly.