core: use sync.Once for SenderCacher initialization (#31029)

This changes the SenderCacher so its goroutines will only be started on first use.
Avoids starting them when package core is just imported but core.BlockChain isn't used.

core/blockchain.go

@@ -1619,7 +1619,7 @@ func (bc *BlockChain) insertChain(chain types.Blocks, setHead bool, makeWitness
 		return nil, 0, nil
 	}
 	// Start a parallel signature recovery (signer will fluke on fork transition, minimal perf loss)
-	SenderCacher.RecoverFromBlocks(types.MakeSigner(bc.chainConfig, chain[0].Number(), chain[0].Time()), chain)
+	SenderCacher().RecoverFromBlocks(types.MakeSigner(bc.chainConfig, chain[0].Number(), chain[0].Time()), chain)
 
 	var (
 		stats     = insertStats{startTime: mclock.Now()}

core/sender_cacher.go

@@ -18,12 +18,21 @@ package core
 
 import (
 	"runtime"
+	"sync"
 
 	"github.com/ethereum/go-ethereum/core/types"
 )
 
-// SenderCacher is a concurrent transaction sender recoverer and cacher.
+// senderCacherOnce is used to ensure that the SenderCacher is initialized only once.
-var SenderCacher = newTxSenderCacher(runtime.NumCPU())
+var senderCacherOnce = sync.OnceValue(func() *txSenderCacher {
+	return newTxSenderCacher(runtime.NumCPU())
+})
+
+// SenderCacher returns the singleton instance of SenderCacher, initializing it if called for the first time.
+// This function is thread-safe and ensures that initialization happens only once.
+func SenderCacher() *txSenderCacher {
+	return senderCacherOnce()
+}
 
 // txSenderCacherRequest is a request for recovering transaction senders with a
 // specific signature scheme and caching it into the transactions themselves.

core/txpool/legacypool/legacypool.go

@@ -1440,7 +1440,7 @@ func (pool *LegacyPool) reset(oldHead, newHead *types.Header) {
 
 	// Inject any transactions discarded due to reorgs
 	log.Debug("Reinjecting stale transactions", "count", len(reinject))
-	core.SenderCacher.Recover(pool.signer, reinject)
+	core.SenderCacher().Recover(pool.signer, reinject)
 	pool.addTxsLocked(reinject, false)
 }