# Buggy code
def check_operand_can_be_freed(self, succ_op_keys):
    """
    Check if the data of an operand can be freed.

    :param succ_op_keys: keys of successor operands
    :return: True if can be freed, False if cannot. None when the result
             is not determinant and we need to test later.
    """
    operand_infos = self._operand_infos
    for k in succ_op_keys:
        op_info = operand_infos[k]
        op_state = op_info.get("state")
        if op_state not in OperandState.SUCCESSFUL_STATES:
            return False
        failover_state = op_info.get("failover_state")
        if failover_state and failover_state not in OperandState.SUCCESSFUL_STATES:
            return False
    # if can be freed but blocked by an ongoing fail-over step,
    # we try later.
    if self._operand_free_paused:
        return None
    return True


# Traceback (SystemError)
In [1]: from mars.deploy.local import new_cluster

In [2]: import mars.tensor as mt

In [3]: cluster = new_cluster(scheduler_n_process=2, worker_n_process=2)
/Users/travis/miniconda3/conda-bld/arrow-cpp_1540410566532/work/cpp/src/plasma/store.cc:971: Allowing the Plasma store to use up to 3.43597GB of memory.
/Users/travis/miniconda3/conda-bld/arrow-cpp_1540410566532/work/cpp/src/plasma/store.cc:1001: Starting object store with directory /tmp and huge page support disabled
/Users/travis/miniconda3/conda-bld/arrow-cpp_1540410566532/work/cpp/src/plasma/eviction_policy.cc:85: There is not enough space to create this object, so evicting 0 objects to free up 0 bytes. The number of bytes in use (before this eviction) is 2130706432.

In [4]: a = mt.ones((3, 3), chunk_size=2)

In [5]: cluster.session.run(a)
Out[5]:
array([[1., 1., 1.],
[1., 1., 1.],
[1., 1., 1.]])

In [6]: cluster.session.run(a.dot(a))
Failed to start graph execution.
Traceback (most recent call last):
File "/Users/hekaisheng/Documents/mars_dev/mars/mars/scheduler/graph.py", line 249, in execute_graph
self.create_operand_actors()
File "/Users/hekaisheng/Documents/mars_dev/mars/mars/scheduler/graph.py", line 679, in create_operand_actors
op_refs = dict((k, v.result()) for k, v in op_refs.items())
File "/Users/hekaisheng/Documents/mars_dev/mars/mars/scheduler/graph.py", line 679, in <genexpr>
op_refs = dict((k, v.result()) for k, v in op_refs.items())
File "src/gevent/event.py", line 457, in gevent._event.AsyncResult.result
File "src/gevent/event.py", line 381, in gevent._event.AsyncResult.get
File "src/gevent/event.py", line 409, in gevent._event.AsyncResult.get
File "src/gevent/event.py", line 399, in gevent._event.AsyncResult.get
File "src/gevent/event.py", line 379, in gevent._event.AsyncResult._raise_exception
File "/Users/hekaisheng/miniconda3/lib/python3.6/site-packages/gevent/_compat.py", line 47, in reraise
raise value.with_traceback(tb)
File "mars/actors/pool/gevent_pool.pyx", line 984, in mars.actors.pool.gevent_pool.Communicator._on_receive_create_actor
self.pool.create_actor(message.actor_cls, message.actor_ref.uid,
File "mars/actors/pool/gevent_pool.pyx", line 241, in mars.actors.pool.gevent_pool.LocalActorPool.create_actor
raise ActorAlreadyExist('Actor {0} already exist, cannot create'.format(actor.uid))
mars.actors.errors.ActorAlreadyExist: Actor s:operator$8a307641-78a8-43a9-a612-8edca02ca309$058707656a679b3a0ab03c10d40abb0e already exist, cannot create
Traceback (most recent call last):
File "src/gevent/greenlet.py", line 716, in gevent._greenlet.Greenlet.run
File "mars/actors/pool/gevent_pool.pyx", line 88, in mars.actors.pool.gevent_pool.ActorExecutionContext.fire_run
cpdef object fire_run(self):
File "mars/actors/pool/gevent_pool.pyx", line 91, in mars.actors.pool.gevent_pool.ActorExecutionContext.fire_run
with self.lock:
File "mars/actors/pool/gevent_pool.pyx", line 102, in mars.actors.pool.gevent_pool.ActorExecutionContext.fire_run
raise
File "mars/actors/pool/gevent_pool.pyx", line 96, in mars.actors.pool.gevent_pool.ActorExecutionContext.fire_run
res = actor.on_receive(message_ctx.message)
File "mars/actors/core.pyx", line 108, in mars.actors.core.FunctionActor.on_receive
File "mars/actors/core.pyx", line 110, in mars.actors.core.FunctionActor.on_receive
File "/Users/hekaisheng/Documents/mars_dev/mars/mars/scheduler/graph.py", line 249, in execute_graph
self.create_operand_actors()
File "/Users/hekaisheng/Documents/mars_dev/mars/mars/scheduler/graph.py", line 679, in create_operand_actors
op_refs = dict((k, v.result()) for k, v in op_refs.items())
File "/Users/hekaisheng/Documents/mars_dev/mars/mars/scheduler/graph.py", line 679, in <genexpr>
op_refs = dict((k, v.result()) for k, v in op_refs.items())
File "src/gevent/event.py", line 457, in gevent._event.AsyncResult.result
File "src/gevent/event.py", line 381, in gevent._event.AsyncResult.get
File "src/gevent/event.py", line 409, in gevent._event.AsyncResult.get
File "src/gevent/event.py", line 399, in gevent._event.AsyncResult.get
File "src/gevent/event.py", line 379, in gevent._event.AsyncResult._raise_exception
File "/Users/hekaisheng/miniconda3/lib/python3.6/site-packages/gevent/_compat.py", line 47, in reraise
raise value.with_traceback(tb)
File "mars/actors/pool/gevent_pool.pyx", line 984, in mars.actors.pool.gevent_pool.Communicator._on_receive_create_actor
self.pool.create_actor(message.actor_cls, message.actor_ref.uid,
File "mars/actors/pool/gevent_pool.pyx", line 241, in mars.actors.pool.gevent_pool.LocalActorPool.create_actor
raise ActorAlreadyExist('Actor {0} already exist, cannot create'.format(actor.uid))
mars.actors.errors.ActorAlreadyExist: Actor s:operator$8a307641-78a8-43a9-a612-8edca02ca309$058707656a679b3a0ab03c10d40abb0e already exist, cannot create
2019-01-24T12:05:09Z <Greenlet "Greenlet-0" at 0x119218448: <built-in method fire_run of mars.actors.pool.gevent_pool.ActorExecutionContext object at 0x319839688>> failed with ActorAlreadyExist

---------------------------------------------------------------------------
SystemError                               Traceback (most recent call last)
<ipython-input-6-d3363e4685e3> in <module>()
----> 1 cluster.session.run(a.dot(a))

~/Documents/mars_dev/mars/mars/session.py in run(self, *tensors, **kw)
115         # execute the non-executed tensors
116         if idx_to_run_tensors:
--> 117             execute_result = self._sess.run(*idx_to_run_tensors.values(), **kw)
118             if execute_result:
119                 # fetch is True

~/Documents/mars_dev/mars/mars/deploy/local/session.py in run(self, *tensors, **kw)
80             if graph_state == GraphState.FAILED:
81                 # TODO(qin): add traceback
---> 82                 raise SystemError('Graph execution failed with unknown reason')
83
84         if 0 < timeout < time.time() - exec_start_time:

SystemError: Graph execution failed with unknown reason

# Fixed code
def check_operand_can_be_freed(self, succ_op_keys):
    """
    Check if the data of an operand can be freed.

    :param succ_op_keys: keys of successor operands
    :return: True if can be freed, False if cannot. None when the result
             is not determinant and we need to test later.
    """
    operand_infos = self._operand_infos
    for k in succ_op_keys:
        if k not in operand_infos:
            continue
        op_info = operand_infos[k]
        op_state = op_info.get("state")
        if op_state not in OperandState.SUCCESSFUL_STATES:
            return False
        failover_state = op_info.get("failover_state")
        if failover_state and failover_state not in OperandState.SUCCESSFUL_STATES:
            return False
    # if can be freed but blocked by an ongoing fail-over step,
    # we try later.
    if self._operand_free_paused:
        return None
    return True