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Fix a concurrency problem in the multithreaded executor #703

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Apr 22, 2019
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Fix a concurrency problem in the multithreaded executor #703

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3 changes: 3 additions & 0 deletions rclcpp/src/rclcpp/executors/multi_threaded_executor.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -101,5 +101,8 @@ MultiThreadedExecutor::run(size_t)
scheduled_timers_.erase(it);
}
}
// Clear the callback_group to prevent the AnyExecutable destructor from
// resetting the callback group `can_be_taken_from`
any_exec.callback_group.reset();
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I'm concerned this isn't the right fix, @guillaumeautran why did you not change the destructor of AnyExecutable instead?

Just changing the callback group to nullptr is fragile. There's nothing in the destructor of AnyExecutable which implies the callback group might be reset to affect behavior.

Instead, I think, at the very least, AnyExecutable should have had a boolean like should_reset_can_be_taken_from_automatically which defaulted to true, and the multi threaded executor could change to false. Then it's clearer from both the multi threaded executor's code and any executable's code what's happening.

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I was not sure if there was a specific reason why the destructor was setup the way it was and did not want to run the risk of leaving the group "locked". I'll make the requested changes then and push another PR.

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It's possible that since this exists:

https://github.com/ros2/rclcpp/blob/master/rclcpp/src/rclcpp/executor.cpp#L288

We should just remove the code in the destructor of AnyExecutable that changes the can_be_taken_from of the callback group.

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@wjwwood Do you think I can add an argument to the constructor of the AnyExecutable class?

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I would be hesitant from removing the if statement. There is a comment that makes a lot of sense:

rclcpp/rclcpp/src/rclcpp/any_executable.cpp

Line 34 in 97ed34a

if (callback_group) {

}
}