XCM: Tools for uniquely referencing messages

[gavofyork]

Cumulus Companion: paritytech/cumulus#2601

This introduces several tools to ensure XCM messages are uniquely referenceable, using the topic identifiers introduced in XCMv3.

• The SendXcm implementation WithUniqueTopic is introduced as a wrapping router and appends all XCMs sent through it with a SetTopic(t) where t is a universally unique XcmHash value which will be used to identify the message. This is returned by the fn deliver implementation for recording in events.
• The barrier TrailingSetTopicAsId is introduced which is a wrapping barrier (i.e. accepts another barrier as a parameter) and which elides a trailing SetTopic on the message if one exists before passing the rest into the inner barrier. It is designed to wrap around all other barriers at the top level.

To use this functionality, the top-level XcmRouter type should be altered from:

type XcmRouter = (
    ...
);

To:

type XcmRouter = WithUniqueTopic<(
    ...
)>;

Also, the barrier condition should be altered from:

type Barrier = (
    ...
);

To:

type Barrier = TrailingSetTopicAsId<(
    ...
)>;

Messages coming from any chains instituting this will always have a SetTopic instruction as the last instruction, which allows chain explorers to track messages.

This is further improved by integrating the topic into the barriers API, allowing the Topic barriers to pass the topic statically into the dispatch system and thus have the event include it rather than the now-defunct message hash.

Also, topics are linkable when XCM is sent over a bridge. This is done through having the bridge primitives in universal_exports.rs append the derivative (bridged) messages with SetTopic containing a derivative topic value. For this to work, WithUniqueTopic does not append a SetTopic to the message if it already ends with one.

Additionally this adds the message ID to all events which send a message and refactors the Event enum in pallet XCM to have named fields. Also, added Weight to xcm::latest::prelude and by extension xcm::prelude.

Breaking Changes

This contains a breaking change into the XCM Barrier API, trait ShouldExecute: weight_credit: &mut Weight becomes properties: &mut Properties.

This won't affect you unless you implement the ShouldExecute trait; if you do, you'll need to alter the prototype accordingly. If you use or mutate the weight_credit argument, then you can find it in the new API as properties.weight_credit.

TODO

• Repot the router into routing.rs
• Barrier API should include the possibility to change the message hash/ID
• *SetTopic should change message hash/ID accordingly
• Changes of message hash/ID should be respected in any events deposited
  • UMP arrival/dispatch (Polkadot)
  • DMP arrival/dispatch (Cumulus)
  • XCMP arrival/dispatch (Cumulus)
  • Departure (send_xcm usage)
  • Consider adding the ID into existing events
• WithUniqueTopic avoids setting topic again
• Universal exports (bridges) support topic propagation
• Integrate into relay-chains (Polkadot)
• Integrate into parachains (Cumulus)
• Tests (Polkadot)

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[KiChjang]

I'm guessing the code here should be migrated to v4 when we get to creating it.

[gavofyork]

Indeed.