fix app hash in state rollback (#7837)

When testing rollback feature in the Cosmos SDK, we found that the app hash
in Tendermint after rollback was the value after the latest block, rather than 
before it.

Co-authored-by: Callum Waters <cmwaters19@gmail.com>

CHANGELOG_PENDING.md

@@ -74,3 +74,4 @@ Special thanks to external contributors on this release:
 - fix: assignment copies lock value in `BitArray.UnmarshalJSON()` (@lklimek)
 - [light] \#7640 Light Client: fix absence proof verification (@ashcherbakov)
 - [light] \#7641 Light Client: fix querying against the latest height (@ashcherbakov)
+- [cli] [#7837](https://github.com/tendermint/tendermint/pull/7837) fix app hash in state rollback. (@yihuang)

internal/state/rollback.go

@@ -41,6 +41,11 @@ func Rollback(bs BlockStore, ss Store) (int64, []byte, error) {
 	if rollbackBlock == nil {
 		return -1, nil, fmt.Errorf("block at height %d not found", rollbackHeight)
 	}
+	// we also need to retrieve the latest block because the app hash and last results hash is only agreed upon in the following block
+	latestBlock := bs.LoadBlockMeta(invalidState.LastBlockHeight)
+	if latestBlock == nil {
+		return -1, nil, fmt.Errorf("block at height %d not found", invalidState.LastBlockHeight)
+	}
 
 	previousLastValidatorSet, err := ss.LoadValidators(rollbackHeight)
 	if err != nil {
@@ -89,8 +94,8 @@ func Rollback(bs BlockStore, ss Store) (int64, []byte, error) {
 		ConsensusParams:                  previousParams,
 		LastHeightConsensusParamsChanged: paramsChangeHeight,
 
-		LastResultsHash: rollbackBlock.Header.LastResultsHash,
-		AppHash:         rollbackBlock.Header.AppHash,
+		LastResultsHash: latestBlock.Header.LastResultsHash,
+		AppHash:         latestBlock.Header.AppHash,
 	}
 
 	// persist the new state. This overrides the invalid one. NOTE: this will also

internal/state/rollback_test.go

@@ -48,12 +48,22 @@ func TestRollback(t *testing.T) {
 		BlockID: initialState.LastBlockID,
 		Header: types.Header{
 			Height:          initialState.LastBlockHeight,
-			AppHash:         initialState.AppHash,
+			AppHash:         factory.RandomHash(),
 			LastBlockID:     factory.MakeBlockID(),
 			LastResultsHash: initialState.LastResultsHash,
 		},
 	}
-	blockStore.On("LoadBlockMeta", initialState.LastBlockHeight).Return(block)
+	nextBlock := &types.BlockMeta{
+		BlockID: initialState.LastBlockID,
+		Header: types.Header{
+			Height:          nextState.LastBlockHeight,
+			AppHash:         initialState.AppHash,
+			LastBlockID:     block.BlockID,
+			LastResultsHash: nextState.LastResultsHash,
+		},
+	}
+	blockStore.On("LoadBlockMeta", height).Return(block)
+	blockStore.On("LoadBlockMeta", nextHeight).Return(nextBlock)
 	blockStore.On("Height").Return(nextHeight)
 
 	// rollback the state
@@ -84,6 +94,7 @@ func TestRollbackNoBlocks(t *testing.T) {
 	stateStore := setupStateStore(t, height)
 	blockStore := &mocks.BlockStore{}
 	blockStore.On("Height").Return(height)
+	blockStore.On("LoadBlockMeta", height).Return(nil)
 	blockStore.On("LoadBlockMeta", height-1).Return(nil)
 
 	_, _, err := state.Rollback(blockStore, stateStore)