consensus/beacon: check that only the latest pow block is valid ttd block

consensus/beacon/consensus.go

@@ -112,10 +112,12 @@ func (beacon *Beacon) VerifyHeaders(chain consensus.ChainHeaderReader, headers [
 			break
 		}
 	}
+
 	// All the headers have passed the transition point, use new rules.
 	if len(preHeaders) == 0 {
 		return beacon.verifyHeaders(chain, headers, nil)
 	}
+
 	// The transition point exists in the middle, separate the headers
 	// into two batches and apply different verification rules for them.
 	var (
@@ -130,6 +132,14 @@ func (beacon *Beacon) VerifyHeaders(chain consensus.ChainHeaderReader, headers [
 			oldDone, oldResult = beacon.ethone.VerifyHeaders(chain, preHeaders, preSeals)
 			newDone, newResult = beacon.verifyHeaders(chain, postHeaders, preHeaders[len(preHeaders)-1])
 		)
+		// verify that the headers are valid wrt. the terminal block.
+		index, err := beacon.verifyTerminalPoWBlock(chain, preHeaders)
+		if err != nil {
+			// Mark all subsequent headers with the error.
+			for i := index; i < len(preHeaders); i++ {
+				errors[i], done[i] = err, true
+			}
+		}
 		for {
 			for ; done[out]; out++ {
 				results <- errors[out]
@@ -154,6 +164,40 @@ func (beacon *Beacon) VerifyHeaders(chain consensus.ChainHeaderReader, headers [
 	return abort, results
 }
 
+// verifyTerminalPoWBlock verifies that the preHeaders confirm to the specification
+// wrt. their total difficulty.
+// It expects:
+// - preHeaders to be at least 1 element
+// - the parent of the header element to be stored in the chain correctly
+// - the preHeaders to have a set difficulty
+// - the last element to be the terminal block
+func (beacon *Beacon) verifyTerminalPoWBlock(chain consensus.ChainHeaderReader, preHeaders []*types.Header) (int, error) {
+	var (
+		first = preHeaders[0]
+		last  = preHeaders[len(preHeaders)-1]
+	)
+
+	td := chain.GetTd(first.ParentHash, first.Number.Uint64()-1)
+	if td == nil {
+		return 0, consensus.ErrUnknownAncestor
+	}
+	if len(preHeaders) != 1 {
+		for i, head := range preHeaders[:len(preHeaders)-2] {
+			td.Add(td, head.Difficulty)
+			// Check if the parent was already the terminal block
+			if td.Cmp(chain.Config().TerminalTotalDifficulty) >= 0 {
+				return i, consensus.ErrInvalidTerminalBlock
+			}
+		}
+	}
+
+	// Check that the last block is the terminal block
+	if td.Add(td, last.Difficulty).Cmp(chain.Config().TerminalTotalDifficulty) < 0 {
+		return len(preHeaders) - 1, consensus.ErrInvalidTerminalBlock
+	}
+	return 0, nil
+}
+
 // VerifyUncles verifies that the given block's uncles conform to the consensus
 // rules of the Ethereum consensus engine.
 func (beacon *Beacon) VerifyUncles(chain consensus.ChainReader, block *types.Block) error {

consensus/errors.go

@@ -34,4 +34,8 @@ var (
 	// ErrInvalidNumber is returned if a block's number doesn't equal its parent's
 	// plus one.
 	ErrInvalidNumber = errors.New("invalid block number")
+
+	// ErrInvalidTerminalBlock is returned if a block is invalid wrt. the terminal
+	// total difficulty.
+	ErrInvalidTerminalBlock = errors.New("invalid terminal block")
 )