What happens is a node receives a message while it's in the middle of a callback?

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I'm thinking of having a node send a message to another node as part of a callback function. It should receive a message from another node at about this time as well, before it finishes the callback function. What will ROS do in this case? Will the node pause the first callback function in order to execute the second, or will it finish executing the first callback before executing the callback function for the message it just received?

Thanks!