SubscribeVersion from the Relay will be blocked by new parachains

[mrshiposha]

The Relay chain uses the WithUniqueTopic router, which adds a SetTopic instruction to every XCM message the Relay sends if the message doesn't end with such an instruction already.

This could break the XCM version subscriptions for new parachains that didn't receive the SubscribeVersion in the past.

The SubscribeVersion message will also be augmented with the SetTopic instruction. Currently, the AllowSubscriptionsFrom barrier used by the parachains forbids any message that contains anything but the SubscribeVersion/UnsubscribeVersion instruction. So, any SubscribeVersion or UnsubscribeVersion from the Relay chain will be blocked.

[bkchr]

CC @franciscoaguirre

[franciscoaguirre]

That sounds bad, I'll look into it

[bkontur]

@mrshiposha Yes, you're right. AllowSubscriptionsFrom accepts just SubscribeVersion / UnsubscribeVersion, but there is a TrailingSetTopicAsId barrier, which is exactly designed to handle this SetTopic added by WithUniqueTopic router. Therefore, the parachain should wrap its barrier with TrailingSetTopicAsId, for example: https://github.com/paritytech/polkadot-sdk/blob/master/cumulus/parachains/runtimes/assets/asset-hub-rococo/src/xcm_config.rs#L485-L506. Anyway, the parachain still has the possibility to write custom barriers according to its needs.

And it is not a specific problem of the RelayChain, this can happen between any chains when using the WithUniqueTopic wrapper for SendXcm between them.

I've just added more tests for these barriers here: #3955.

[mrshiposha]

Oh, I forgot I filed this issue :)
Thanks for the response, @bkontur! I used the TrailingSetTopicAsId barrier in the Unique Network chain as a fix but forgot to comment here.

It's great to have tests to check that version subscriptions work. IIRC, I filed this issue because the subscription mechanism was unexpectedly broken in our chain when Relay suddenly changed its behavior.

[bkontur]

I filed this issue because the subscription mechanism was unexpectedly broken in our chain when Relay suddenly changed its behavior.

@mrshiposha Thank you for reporting! Yeah, got it. I hope that similar issues will be avoided with our improved release notes, including this kind of breaking change.