rpc: add GetFinalizedBlock to simplify using the fast finality feature

consensus/parlia/parlia.go

@@ -1758,10 +1758,12 @@ func (p *Parlia) GetFinalizedHeader(chain consensus.ChainHeaderReader, header *t
 		return nil
 	}
 
-	if snap.Attestation != nil {
-		return chain.GetHeader(snap.Attestation.SourceHash, snap.Attestation.SourceNumber)
+	// snap.Attestation is nil after plato upgrade, only can happen in local testnet
+	if snap.Attestation == nil {
+		return chain.GetHeaderByNumber(0) // keep consistent with GetJustifiedNumberAndHash
 	}
-	return nil
+
+	return chain.GetHeader(snap.Attestation.SourceHash, snap.Attestation.SourceNumber)
 }
 
 // ===========================     utility function        ==========================

internal/ethapi/api.go

@@ -54,6 +54,14 @@ import (
 
 const UnHealthyTimeout = 5 * time.Second
 
+// max is a helper function which returns the larger of the two given integers.
+func max(a, b int64) int64 {
+	if a > b {
+		return a
+	}
+	return b
+}
+
 // PublicEthereumAPI provides an API to access Ethereum related information.
 // It offers only methods that operate on public data that is freely available to anyone.
 type PublicEthereumAPI struct {
@@ -773,6 +781,27 @@ func (s *PublicBlockChainAPI) Health() bool {
 	return true
 }
 
+// GetFinalizedBlock returns the requested finalized block.
+// * probabilisticFinalized should be in range [2,21], then the block with number `max(voteFinalized, latest-probabilisticFinalized)` is returned
+func (s *PublicBlockChainAPI) GetFinalizedBlock(ctx context.Context, probabilisticFinalized int64, fullTx bool) (map[string]interface{}, error) {
+	if probabilisticFinalized < 2 || probabilisticFinalized > 21 {
+		return nil, fmt.Errorf("%d out of range [2,21]", probabilisticFinalized)
+	}
+
+	var err error
+	voteFinalizedHeader, err := s.b.HeaderByNumber(ctx, rpc.FinalizedBlockNumber)
+	if err != nil { // impossible
+		return nil, err
+	}
+	latestHeader, err := s.b.HeaderByNumber(ctx, rpc.LatestBlockNumber)
+	if err != nil { // impossible
+		return nil, err
+	}
+	finalizedBlockNumber := max(voteFinalizedHeader.Number.Int64(), latestHeader.Number.Int64()-probabilisticFinalized)
+
+	return s.GetBlockByNumber(ctx, rpc.BlockNumber(finalizedBlockNumber), fullTx)
+}
+
 // GetUncleByBlockNumberAndIndex returns the uncle block for the given block hash and index.
 func (s *PublicBlockChainAPI) GetUncleByBlockNumberAndIndex(ctx context.Context, blockNr rpc.BlockNumber, index hexutil.Uint) (map[string]interface{}, error) {
 	block, err := s.b.BlockByNumber(ctx, blockNr)