Fix an off-by-one: revert rather than revert-to (#3991)

* fix off-by-one in disputes reversion code

* bump Rococo spec version

roadmap/implementers-guide/src/runtime/disputes.md

@@ -109,4 +109,5 @@ Frozen: Option<BlockNumber>,
 
 * `revert_and_freeze(BlockNumber)`:
   1. If `is_frozen()` return.
-  1. Set `Frozen` to `Some(BlockNumber)` to indicate a rollback to the given block number is necessary.
+  1. Set `Frozen` to `Some(BlockNumber)` to indicate a rollback to the block number.
+  1. Issue a `Revert(BlockNumber + 1)` log to indicate a rollback of the block's child in the header chain, which is the same as a rollback to the block number.

runtime/parachains/src/disputes.rs

@@ -304,8 +304,8 @@ pub mod pallet {
 		DisputeTimedOut(CandidateHash),
 		/// A dispute has concluded with supermajority against a candidate.
 		/// Block authors should no longer build on top of this head and should
-		/// instead revert to the block at the given height which is the last
-		/// known valid block in this chain.
+		/// instead revert the block at the given height. This should be the
+		/// number of the child of the last known valid block in the chain.
 		Revert(T::BlockNumber),
 	}
 
@@ -1127,9 +1127,14 @@ impl<T: Config> Pallet<T> {
 	pub(crate) fn revert_and_freeze(revert_to: T::BlockNumber) {
 		if Self::last_valid_block().map_or(true, |last| last > revert_to) {
 			Frozen::<T>::set(Some(revert_to));
-			Self::deposit_event(Event::Revert(revert_to));
+
+			// The `Revert` log is about reverting a block, not reverting to a block.
+			// If we want to revert to block X in the current chain, we need to revert
+			// block X+1.
+			let revert = revert_to + One::one();
+			Self::deposit_event(Event::Revert(revert));
 			frame_system::Pallet::<T>::deposit_log(
-				ConsensusLog::Revert(revert_to.saturated_into()).into(),
+				ConsensusLog::Revert(revert.saturated_into()).into(),
 			);
 		}
 	}
@@ -2284,8 +2289,8 @@ mod tests {
 			Pallet::<Test>::revert_and_freeze(0);
 
 			assert_eq!(Frozen::<Test>::get(), Some(0));
-			assert_eq!(System::digest().logs[0], ConsensusLog::Revert(0).into());
-			System::assert_has_event(Event::Revert(0).into());
+			assert_eq!(System::digest().logs[0], ConsensusLog::Revert(1).into());
+			System::assert_has_event(Event::Revert(1).into());
 		})
 	}
 

runtime/rococo/src/lib.rs

@@ -105,7 +105,7 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	spec_name: create_runtime_str!("rococo"),
 	impl_name: create_runtime_str!("parity-rococo-v1.8"),
 	authoring_version: 0,
-	spec_version: 9103,
+	spec_version: 9104,
 	impl_version: 0,
 	#[cfg(not(feature = "disable-runtime-api"))]
 	apis: RUNTIME_API_VERSIONS,