Topic handler gets paralleled

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I noticed a very strange behaviour in Rospy topic subscriber.

If it receives messages from one node, the messages get queued, and this is expected behaviour.

But, if there are several nodes, that put messages to the topic, the topic subscriber callback gets paralleled, which is very unexpected and confusing. There is no information about thread-safety requirements to subscriber callbacks.

Is it intended behaviour? Is there some setting to make callback calls queued, even from multiple nodes?