eth/gasprice: feeHistory improvements (#23422)

* eth/gasprice: cache feeHistory results

* eth/gasprice: changed feeHistory block count limitation

* eth/gasprice: do not use embedded struct in blockFees

* eth/gasprice: fee processing logic cleanup

* eth/gasprice: purge feeHistory cache at chain reorgs

eth/gasprice/gasprice.go

@@ -23,10 +23,13 @@ import (
 	"sync"
 
 	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/core"
 	"github.com/ethereum/go-ethereum/core/types"
+	"github.com/ethereum/go-ethereum/event"
 	"github.com/ethereum/go-ethereum/log"
 	"github.com/ethereum/go-ethereum/params"
 	"github.com/ethereum/go-ethereum/rpc"
+	lru "github.com/hashicorp/golang-lru"
 )
 
 const sampleNumber = 3 // Number of transactions sampled in a block
@@ -53,6 +56,7 @@ type OracleBackend interface {
 	GetReceipts(ctx context.Context, hash common.Hash) (types.Receipts, error)
 	PendingBlockAndReceipts() (*types.Block, types.Receipts)
 	ChainConfig() *params.ChainConfig
+	SubscribeChainHeadEvent(ch chan<- core.ChainHeadEvent) event.Subscription
 }
 
 // Oracle recommends gas prices based on the content of recent
@@ -68,6 +72,7 @@ type Oracle struct {
 
 	checkBlocks, percentile           int
 	maxHeaderHistory, maxBlockHistory int
+	historyCache                      *lru.Cache
 }
 
 // NewOracle returns a new gasprice oracle which can recommend suitable
@@ -99,6 +104,20 @@ func NewOracle(backend OracleBackend, params Config) *Oracle {
 	} else if ignorePrice.Int64() > 0 {
 		log.Info("Gasprice oracle is ignoring threshold set", "threshold", ignorePrice)
 	}
+
+	cache, _ := lru.New(2048)
+	headEvent := make(chan core.ChainHeadEvent, 1)
+	backend.SubscribeChainHeadEvent(headEvent)
+	go func() {
+		var lastHead common.Hash
+		for ev := range headEvent {
+			if ev.Block.ParentHash() != lastHead {
+				cache.Purge()
+			}
+			lastHead = ev.Block.Hash()
+		}
+	}()
+
 	return &Oracle{
 		backend:          backend,
 		lastPrice:        params.Default,
@@ -108,6 +127,7 @@ func NewOracle(backend OracleBackend, params Config) *Oracle {
 		percentile:       percent,
 		maxHeaderHistory: params.MaxHeaderHistory,
 		maxBlockHistory:  params.MaxBlockHistory,
+		historyCache:     cache,
 	}
 }