lite2: modified sequence method to match bisection (#4403)

Currently the sequence function always starts from the trustedHeader and trustedNextVals stored in the lite client. Whereas the bisection one allows the method to be started from any combination of header and validator set. I opened up the sequence verification method to do the same

lite2/client.go

@@ -617,7 +617,7 @@ func (c *Client) VerifyHeader(newHeader *types.SignedHeader, newVals *types.Vali
 	var err error
 	switch c.verificationMode {
 	case sequential:
-		err = c.sequence(newHeader, newVals, now)
+		err = c.sequence(c.trustedHeader, c.trustedNextVals, newHeader, newVals, now)
 	case skipping:
 		err = c.bisection(c.trustedHeader, c.trustedNextVals, newHeader, newVals, now)
 	default:
@@ -697,25 +697,30 @@ func (c *Client) cleanup(stopHeight int64) error {
 }
 
 // see VerifyHeader
-func (c *Client) sequence(newHeader *types.SignedHeader, newVals *types.ValidatorSet, now time.Time) error {
+func (c *Client) sequence(
+	trustedHeader *types.SignedHeader,
+	trustedNextVals *types.ValidatorSet,
+	newHeader *types.SignedHeader,
+	newVals *types.ValidatorSet,
+	now time.Time) error {
 	// 1) Verify any intermediate headers.
 	var (
-		interimHeader *types.SignedHeader
-		nextVals      *types.ValidatorSet
-		err           error
+		interimHeader   *types.SignedHeader
+		interimNextVals *types.ValidatorSet
+		err             error
 	)
-	for height := c.trustedHeader.Height + 1; height < newHeader.Height; height++ {
+	for height := trustedHeader.Height + 1; height < newHeader.Height; height++ {
 		interimHeader, err = c.signedHeaderFromPrimary(height)
 		if err != nil {
 			return errors.Wrapf(err, "failed to obtain the header #%d", height)
 		}
 
-		c.logger.Debug("Verify newHeader against lastHeader",
+		c.logger.Debug("Verify newHeader against trustedHeader",
 			"lastHeight", c.trustedHeader.Height,
 			"lastHash", c.trustedHeader.Hash(),
 			"newHeight", interimHeader.Height,
 			"newHash", interimHeader.Hash())
-		err = Verify(c.chainID, c.trustedHeader, c.trustedNextVals, interimHeader, c.trustedNextVals,
+		err = Verify(c.chainID, trustedHeader, trustedNextVals, interimHeader, trustedNextVals,
 			c.trustingPeriod, now, c.trustLevel)
 		if err != nil {
 			return errors.Wrapf(err, "failed to verify the header #%d", height)
@@ -723,17 +728,18 @@ func (c *Client) sequence(newHeader *types.SignedHeader, newVals *types.Validato
 
 		// Update trusted header and vals.
 		if height == newHeader.Height-1 {
-			nextVals = newVals
+			interimNextVals = newVals
 		} else {
-			nextVals, err = c.validatorSetFromPrimary(height + 1)
+			interimNextVals, err = c.validatorSetFromPrimary(height + 1)
 			if err != nil {
 				return errors.Wrapf(err, "failed to obtain the vals #%d", height+1)
 			}
 		}
-		err = c.updateTrustedHeaderAndVals(interimHeader, nextVals)
+		err = c.updateTrustedHeaderAndVals(interimHeader, interimNextVals)
 		if err != nil {
 			return errors.Wrapf(err, "failed to update trusted state #%d", height)
 		}
+		trustedHeader, trustedNextVals = interimHeader, interimNextVals
 	}
 
 	// 2) Verify the new header.