Branislav Kontur
575b8f8d15
## Summary This pull request proposes a solution for improved control of the versioned XCM flow over the bridge (across different consensus chains) and resolves the situation where the sending chain/consensus has already migrated to a higher XCM version than the receiving chain/consensus. ## Problem/Motivation The current flow over the bridge involves a transfer from AssetHubRococo (AHR) to BridgeHubRococo (BHR) to BridgeHubWestend (BHW) and finally to AssetHubWestend (AHW), beginning with a reserve-backed transfer on AHR. In this process: 1. AHR sends XCM `ExportMessage` through `XcmpQueue`, incorporating XCM version checks using the `WrapVersion` feature, influenced by `pallet_xcm::SupportedVersion` (managed by `pallet_xcm::force_xcm_version` or version discovery). 2. BHR handles the `ExportMessage` instruction, utilizing the latest XCM version. The `HaulBlobExporter` converts the inner XCM to [`VersionedXcm::from`](https://github.com/paritytech/polkadot-sdk/blob/63ac2471aa0210f0ac9903bdd7d8f9351f9a635f/polkadot/xcm/xcm-builder/src/universal_exports.rs#L465-L467), also using the latest XCM version. However, challenges arise: - Incompatibility when BHW uses a different version than BHR. For instance, if BHR migrates to **XCMv4** while BHW remains on **XCMv3**, BHR's `VersionedXcm::from` uses `VersionedXcm::V4` variant, causing encoding issues for BHW. ``` /// Just a simulation of possible error, which could happen on BHW /// (this code is based on actual master without XCMv4) let encoded = hex_literal::hex!("0400"); println!("{:?}", VersionedXcm::<()>::decode(&mut &encoded[..])); Err(Error { cause: None, desc: "Could not decode `VersionedXcm`, variant doesn't exist" }) ``` - Similar compatibility issues exist between AHR and AHW. ## Solution This pull request introduces the following solutions: 1. **New trait `CheckVersion`** - added to the `xcm` module and exposing `pallet_xcm::SupportedVersion`. This enhancement allows checking the actual XCM version for desired destinations outside of the `pallet_xcm` module. 2. **Version Check in `HaulBlobExporter`** uses `CheckVersion` to check known/configured destination versions, ensuring compatibility. For example, in the scenario mentioned, BHR can store the version `3` for BHW. If BHR is on XCMv4, it will attempt to downgrade the message to version `3` instead of using the latest version `4`. 3. **Version Check in `pallet-xcm-bridge-hub-router`** - this check ensures compatibility with the real destination's XCM version, preventing the unnecessary sending of messages to the local bridge hub if versions are incompatible. These additions aim to improve the control and compatibility of XCM flows over the bridge and addressing issues related to version mismatches. ## Possible alternative solution _(More investigation is needed, and at the very least, it should extend to XCMv4/5. If this proves to be a viable option, I can open an RFC for XCM.)._ Add the `XcmVersion` attribute to the `ExportMessage` so that the sending chain can determine, based on what is stored in `pallet_xcm::SupportedVersion`, the version the destination is using. This way, we may not need to handle the version in `HaulBlobExporter`. ``` ExportMessage { network: NetworkId, destination: InteriorMultiLocation, xcm: Xcm<()> destination_xcm_version: Version, // <- new attritbute }, ``` ``` pub trait ExportXcm { fn validate( network: NetworkId, channel: u32, universal_source: &mut Option<InteriorMultiLocation>, destination: &mut Option<InteriorMultiLocation>, message: &mut Option<Xcm<()>>, destination_xcm_version: Version, , // <- new attritbute ) -> SendResult<Self::Ticket>; ``` ## Future Directions This PR does not fix version discovery over bridge, further investigation will be conducted here: https://github.com/paritytech/polkadot-sdk/issues/2417. ## TODO - [x] `pallet_xcm` mock for tests uses hard-coded XCM version `2` - change to 3 or lastest? - [x] fix `pallet-xcm-bridge-hub-router` - [x] fix HaulBlobExporter with version determination [here](https://github.com/paritytech/polkadot-sdk/blob/2183669d05f9b510f979a0cc3c7847707bacba2e/polkadot/xcm/xcm-builder/src/universal_exports.rs#L465) - [x] add unit-tests to the runtimes - [x] run benchmarks for `ExportMessage` - [x] extend local run scripts about `force_xcm_version(dest, version)` - [ ] when merged, prepare governance calls for Rococo/Westend - [ ] add PRDoc Part of: https://github.com/paritytech/parity-bridges-common/issues/2719 --------- Co-authored-by: command-bot <>
NOTE: We have recently made significant changes to our repository structure. In order to streamline our development process and foster better contributions, we have merged three separate repositories Cumulus, Substrate and Polkadot into this repository. Read more about the changes here.
Polkadot SDK
The Polkadot SDK repository provides all the resources needed to start building on the Polkadot network, a multi-chain blockchain platform that enables different blockchains to interoperate and share information in a secure and scalable way. The Polkadot SDK comprises three main pieces of software:
Polkadot
Implementation of a node for the https://polkadot.network in Rust, using the Substrate framework. This directory
currently contains runtimes for the Polkadot, Kusama, Westend, and Rococo networks. In the future, these will be
relocated to the runtimes repository.
Substrate
Substrate is the primary blockchain SDK used by developers to create the parachains that make up the Polkadot network. Additionally, it allows for the development of self-sovereign blockchains that operate completely independently of Polkadot.
Cumulus
Cumulus is a set of tools for writing Substrate-based Polkadot parachains.
Upstream Dependencies
Below are the primary upstream dependencies utilized in this project:
Security
The security policy and procedures can be found in docs/contributor/SECURITY.md.
Contributing & Code of Conduct
Ensure you follow our contribution guidelines. In every interaction and contribution, this project adheres to the Contributor Covenant Code of Conduct.
Additional Resources
- For monitoring upcoming changes and current proposals related to the technical implementation of the Polkadot network,
visit the
Requests for Comment (RFC)repository. While it's maintained by the Polkadot Fellowship, the RFC process welcomes contributions from everyone.