Document that spin_once() should not be called from multiple threads

[AndyZe]

Fixes #1078

[fujitatomoya]

I think this depends on arguments, if user application calls rclpy::spin_once with the same nodes and two different executor, this would be problem since we have a node in 2 wait sets. but we can create multiple threads to call SingleThreadedExecutor::snip_once: for each node, that is usually what user would do.

[AndyZe]

Can you please suggest better wording for this PR?

but we can create multiple threads to call SingleThreadedExecutor::snip_once: for each node, that is usually what user would do.

This is what happened in my experience and it didn't work.

I'd actually suggest to deprecate spin_once() since it has these pitfalls that aren't well understood and recommend spin() only ¯_(ツ)_/¯

[fujitatomoya]

To be honest, i am not sure how to address this as documentation. How about adding node which is not already added to any other executors in the description of node argument? that is the situation that we need to avoid here. there has been discussion related about this with rclcpp, ros2/rclcpp#1953. probably node should be aware that it is already added to executor or not, or we can add the same node to different executor (wait sets).

I'd actually suggest to deprecate spin_once() since it has these pitfalls that aren't well understood and recommend spin() only ¯_(ツ)_/¯

i believe we can have the same problem with spin_once(), that is not gonna be the solution.

This is what happened in my experience and it didn't work.

i understand, for user experience, probably please do not use those with multi-thread would be reasonable for now. instead, we can recommend MultiThreadedExecutor. i will keep this open so that we can have more feedback.

thanks for iterating.

[sloretz]

@Mergifyio rebase

[mergify]

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[sloretz]

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