consensus/beacon: check ttd reached on pos blocks (ethereum#25552)

* consensus/beacon: check ttd reached on pos blocks

* consensus/beacon: check ttd reached on pos blocks

* consensus/beacon: check ttd reached on pos blocks

consensus/beacon/consensus.go

@@ -114,8 +114,16 @@ func (beacon *Beacon) VerifyHeaders(chain consensus.ChainHeaderReader, headers [
 		}
 	}
 
-	// All the headers have passed the transition point, use new rules.
 	if len(preHeaders) == 0 {
+		// All the headers are pos headers. Verify that the parent block reached total terminal difficulty.
+		if reached, _ := IsTTDReached(chain, headers[0].ParentHash, headers[0].Number.Uint64()-1); !reached {
+			// TTD not reached for the first block, mark subsequent with invalid terminal block
+			results := make(chan error, len(headers))
+			for i := 0; i < len(headers); i++ {
+				results <- consensus.ErrInvalidTerminalBlock
+			}
+			return make(chan struct{}), results
+		}
 		return beacon.verifyHeaders(chain, headers, nil)
 	}