Introduce System version to RuntimeVersion object

text/0042-extrinsics-state-version.md

@@ -0,0 +1,102 @@
+# RFC-0042: Add Extrinsic State version to `RuntimeVersion`
+
+|                 |                                               |
+|-----------------|-----------------------------------------------|
+| **Start Date**  | 25th October 2023                             |
+| **Description** | Add extrinsic state version to RuntimeVersion |
+| **Authors**     | Vedhavyas Singareddi                          |
+
+## Summary
+
+At the moment, extrinsics root is derived using `StateVersion::V0` without the possibility to use different version
+unlike storage state version.
+It would be useful for projects like `Subspace` to make extrinsics state version be part of the `RuntimeVersion`.
+
+## Motivation
+
+Since the extrinsic state version is always `StateVersion::V0`, deriving extrinsic root requires full extrinsic data.
+For `Subspace` project, we have an enshrined rollups called `Domain` with optimistic verification and Fraud proofs are
+used to detect malicious behavior.
+One of the `Fraud proof` variant is to derive `Domain` block extrinsic root on `Subspace`'s consensus chain.
+Since `StateVersion::V0` requires full extrinsic data, we are forced to pass all the extrinsics through the Fraud proof.
+One of the main challenge here is some extrinsics could be big enough that this variant of Fraud proof may not be
+included in the Consensus block due to Block's weight restriction.
+If the extrinsic root is derived using `StateVersion::V1`, then we do not need to pass the full extrinsic data but
+rather at maximum, 32 byte of extrinsic data.
+
+## Stakeholders
+
+- Technical Fellowship, in its role of maintaining system runtimes.
+
+## Explanation
+
+In order to use project specific StateVersion for extrinsic roots, we proposed
+an [implementation](https://github.com/paritytech/polkadot-sdk/pull/1691) that introduced
+parameter to `frame_system::Config` but that unfortunately did not feel correct.
+So we would like to [propose](https://github.com/paritytech/polkadot-sdk/pull/1968) adding this change to
+the `RuntimeVersion`
+object. The state version, if introduced, will fallback
+to using `StateVersion::V0` for runtimes with `CoreApi` version <=4
+and projects using `CoreApi` >= 5 will have the ability to pick State version they would like to use.
+
+If implemented, the new `RuntimeVersion` definition would look something similar to
+
+```rust
+/// Runtime version (Rococo).
+#[sp_version::runtime_version]
+pub const VERSION: RuntimeVersion = RuntimeVersion {
+		spec_name: create_runtime_str!("rococo"),
+		impl_name: create_runtime_str!("parity-rococo-v2.0"),
+		authoring_version: 0,
+		spec_version: 10020,
+		impl_version: 0,
+		apis: RUNTIME_API_VERSIONS,
+		transaction_version: 22,
+		state_version: 1,
+		extrinsic_state_version: 0,
+	};
+```
+
+## Drawbacks
+
+Since the `CoreApi` version would need to be bumped, there would require a Runtime upgrade for existing projects
+with `extrinsic_state_version` set to `0` so that it wont brick their chain. New projects can choose between `0` or `1`
+depending on their use-case.
+
+## Testing, Security, and Privacy
+
+Care should be taken when Decoding `RuntimeVersion` with `CoreApi` <=4 since this field is not available and
+documentation clear enough to indicate existing projects to continue using `StateVersion::V0` as before. This change,
+AFAIK, should not have any impact on the security or privacy.
+
+## Performance, Ergonomics, and Compatibility
+
+Since this change introduces a new field to `RuntimeVersion`, it will be unavailable in previous Runtimes and as result,
+should always fallback to `StateVersion::V0`.
+
+### Performance
+
+I do not believe there is any performance hit with this change.
+
+### Ergonomics
+
+This does not break any exposed Apis.
+
+### Compatibility
+
+It does break compatibility with older runtimes and, as a result, Decoding of embed runtime version should consider
+using `CoreApi` version to decode this field or fallback to `StateVersion::V0`.
+
+## Prior Art and References
+
+We [proposed](https://github.com/paritytech/polkadot-sdk/pull/1691) introducing a similar change by introducing a
+parameter to `frame_system::Config` but did not feel that
+is the correct way of introducing this change.
+
+## Unresolved Questions
+
+I do not have any specific questions about this change at the moment.
+
+## Future Directions and Related Material
+
+IMO, this change is pretty self-contained and there won't be any future work necessary.