fix: use the correct revision height when querying trusted validator …

…set in testingpkg (cosmos#4483)

testing/chain.go

@@ -373,17 +373,24 @@ func (chain *TestChain) GetConsensusState(clientID string, height exported.Heigh
 	return chain.App.GetIBCKeeper().ClientKeeper.GetClientConsensusState(chain.GetContext(), clientID, height)
 }
 
-// GetValsAtHeight will return the validator set of the chain at a given height. It will return
+// GetValsAtHeight will return the trusted validator set of the chain for the given trusted height. It will return
 // a success boolean depending on if the validator set exists or not at that height.
-func (chain *TestChain) GetValsAtHeight(height int64) (*cmttypes.ValidatorSet, bool) {
-	// if the current uncommitted header equals the requested height, then we can return
-	// the current validator set as this validator set will be stored in the historical info
-	// when the block height is executed
-	if height == chain.CurrentHeader.Height {
-		return chain.Vals, true
-	}
-
-	histInfo, ok := chain.App.GetStakingKeeper().GetHistoricalInfo(chain.GetContext(), height)
+func (chain *TestChain) GetValsAtHeight(trustedHeight int64) (*cmttypes.ValidatorSet, bool) {
+	// historical information does not store the validator set which committed the header at
+	// height h. During BeginBlock, it stores the last updated validator set. This is equivalent to
+	// the next validator set at height h. This is because cometbft processes the validator set
+	// as follows:
+	//
+	// valSetChanges := endBlock()
+	// chain.Vals = chain.NextVals
+	// chain.NextVals = applyValSetChanges(chain.NextVals, valSetChanges)
+	//
+	// At height h, the validators in the historical information are the:
+	// validators used to sign height h + 1 (next validator set)
+	//
+	// Since we want to return the trusted validator set, which is the next validator set
+	// for height h, we can simply query using the trusted height.
+	histInfo, ok := chain.App.GetStakingKeeper().GetHistoricalInfo(chain.GetContext(), trustedHeight)
 	if !ok {
 		return nil, false
 	}
@@ -430,13 +437,11 @@ func (chain *TestChain) ConstructUpdateTMClientHeaderWithTrustedHeight(counterpa
 		ok            bool
 	)
 
-	// NOTE: We need to get validators from counterparty at height: trustedHeight+1
-	// since the last trusted validators for a header at height h
-	// is the NextValidators at h+1 committed to in header h via the NextValidatorsHash field
-	tmTrustedVals, ok = counterparty.GetValsAtHeight(int64(trustedHeight.RevisionHeight + 1))
+	tmTrustedVals, ok = counterparty.GetValsAtHeight(int64(trustedHeight.RevisionHeight))
 	if !ok {
 		return nil, errorsmod.Wrapf(ibctm.ErrInvalidHeaderHeight, "could not retrieve trusted validators at trustedHeight: %d", trustedHeight)
 	}
+
 	// inject trusted fields into last header
 	// for now assume revision number is 0
 	header.TrustedHeight = trustedHeight