How to cancel goals from within an action server?

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As @Javier mentioned in the comments the cancellation of a goal is done automatically. In order to handle multiple action servers at once you have to create a minimalistic SimpleActionClient for each action server (except for the one that cancels all of the other servers). Such a client will have only one task namely sending an empty goal of type X to the server which processes the respective action. By doing so you are exploiting (in a perfectly decent manner) the automatic preemption of the SimpleActionServer so each of the other action servers will receive it, do nothing but also cancel the goal each is currently executing. I would suggest (I'm also about to test it with my own code) that you trigger a cancel request using all the action clients and send to all the respective action servers. The triggering itself needs to be done inside the callback of the action server that needs to cancel all other servers.