core, core/txpool, eth: move subscriptions to constructor (#35048)

Closes https://github.com/ethereum/go-ethereum/issues/20554
It makes it easier to reason about the lifecycle.

core/txindexer.go

@@ -23,6 +23,7 @@ import (
 	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/core/rawdb"
 	"github.com/ethereum/go-ethereum/ethdb"
+	"github.com/ethereum/go-ethereum/event"
 	"github.com/ethereum/go-ethereum/log"
 )
 
@@ -64,6 +65,9 @@ type txIndexer struct {
 	db     ethdb.Database
 	term   chan chan struct{}
 	closed chan struct{}
+
+	headCh  chan ChainHeadEvent
+	headSub event.Subscription
 }
 
 // newTxIndexer initializes the transaction indexer.
@@ -75,7 +79,9 @@ func newTxIndexer(limit uint64, chain *BlockChain) *txIndexer {
 		db:     chain.db,
 		term:   make(chan chan struct{}),
 		closed: make(chan struct{}),
+		headCh: make(chan ChainHeadEvent),
 	}
+	indexer.headSub = chain.SubscribeChainHeadEvent(indexer.headCh)
 	indexer.head.Store(indexer.resolveHead())
 	indexer.tail.Store(rawdb.ReadTxIndexTail(chain.db))
 
@@ -228,15 +234,14 @@ func (indexer *txIndexer) resolveHead() uint64 {
 // on the received chain event.
 func (indexer *txIndexer) loop(chain *BlockChain) {
 	defer close(indexer.closed)
+	defer indexer.headSub.Unsubscribe()
 
 	// Listening to chain events and manipulate the transaction indexes.
 	var (
 		stop   chan struct{} // Non-nil if background routine is active
 		done   chan struct{} // Non-nil if background routine is active
-		headCh = make(chan ChainHeadEvent)
-		sub    = chain.SubscribeChainHeadEvent(headCh)
+		headCh = indexer.headCh
 	)
-	defer sub.Unsubscribe()
 
 	// Validate the transaction indexes and repair if necessary
 	head := indexer.head.Load()

core/txpool/txpool.go

@@ -76,6 +76,9 @@ type TxPool struct {
 	quit chan chan error         // Quit channel to tear down the head updater
 	term chan struct{}           // Termination channel to detect a closed pool
 
+	newHeadCh  chan core.ChainHeadEvent
+	newHeadSub event.Subscription
+
 	sync chan chan error // Testing / simulator channel to block until internal reset is done
 }
 
@@ -98,13 +101,15 @@ func New(gasTip uint64, chain BlockChain, subpools []SubPool) (*TxPool, error) {
 		return nil, err
 	}
 	pool := &TxPool{
-		subpools: subpools,
-		chain:    chain,
-		state:    statedb,
-		quit:     make(chan chan error),
-		term:     make(chan struct{}),
-		sync:     make(chan chan error),
+		subpools:  subpools,
+		chain:     chain,
+		state:     statedb,
+		quit:      make(chan chan error),
+		term:      make(chan struct{}),
+		sync:      make(chan chan error),
+		newHeadCh: make(chan core.ChainHeadEvent),
 	}
+	pool.newHeadSub = chain.SubscribeChainHeadEvent(pool.newHeadCh)
 	reserver := NewReservationTracker()
 	for i, subpool := range subpools {
 		if err := subpool.Init(gasTip, head, reserver.NewHandle(i)); err != nil {
@@ -150,12 +155,8 @@ func (p *TxPool) loop(head *types.Header) {
 	// Close the termination marker when the pool stops
 	defer close(p.term)
 
-	// Subscribe to chain head events to trigger subpool resets
-	var (
-		newHeadCh  = make(chan core.ChainHeadEvent)
-		newHeadSub = p.chain.SubscribeChainHeadEvent(newHeadCh)
-	)
-	defer newHeadSub.Unsubscribe()
+	newHeadCh := p.newHeadCh
+	defer p.newHeadSub.Unsubscribe()
 
 	// Track the previous and current head to feed to an idle reset
 	var (

eth/backend.go

@@ -49,6 +49,7 @@ import (
 	"github.com/ethereum/go-ethereum/eth/protocols/snap"
 	"github.com/ethereum/go-ethereum/eth/tracers"
 	"github.com/ethereum/go-ethereum/ethdb"
+	"github.com/ethereum/go-ethereum/event"
 	"github.com/ethereum/go-ethereum/internal/ethapi"
 	"github.com/ethereum/go-ethereum/internal/shutdowncheck"
 	"github.com/ethereum/go-ethereum/internal/version"
@@ -110,6 +111,13 @@ type Ethereum struct {
 	filterMaps      *filtermaps.FilterMaps
 	closeFilterMaps chan chan struct{}
 
+	// Chain event subscriptions driving updateFilterMapsHeads. The
+	// subscriptions are registered and consumed in Start.
+	fmHeadEventCh  chan core.ChainEvent
+	fmHeadSub      event.Subscription
+	fmBlockProcCh  chan bool
+	fmBlockProcSub event.Subscription
+
 	APIBackend *EthAPIBackend
 
 	miner    *miner.Miner
@@ -199,6 +207,8 @@ func New(stack *node.Node, config *ethconfig.Config) (*Ethereum, error) {
 		p2pServer:       stack.Server(),
 		discmix:         enode.NewFairMix(discmixTimeout),
 		shutdownTracker: shutdowncheck.NewShutdownTracker(chainDb),
+		fmHeadEventCh:   make(chan core.ChainEvent, 10),
+		fmBlockProcCh:   make(chan bool, 10),
 	}
 	bcVersion := rawdb.ReadDatabaseVersion(chainDb)
 	var dbVer = "<nil>"
@@ -459,6 +469,10 @@ func (s *Ethereum) Start() error {
 	// Start the connection manager
 	s.dropper.Start(s.p2pServer, func() bool { return !s.Synced() })
 
+	// Subscribe to chain events for the filterMaps head updater.
+	s.fmHeadSub = s.blockchain.SubscribeChainEvent(s.fmHeadEventCh)
+	s.fmBlockProcSub = s.blockchain.SubscribeBlockProcessingEvent(s.fmBlockProcCh)
+
 	// start log indexer
 	s.filterMaps.Start()
 	go s.updateFilterMapsHeads()
@@ -473,13 +487,11 @@ func (s *Ethereum) newChainView(head *types.Header) *filtermaps.ChainView {
 }
 
 func (s *Ethereum) updateFilterMapsHeads() {
-	headEventCh := make(chan core.ChainEvent, 10)
-	blockProcCh := make(chan bool, 10)
-	sub := s.blockchain.SubscribeChainEvent(headEventCh)
-	sub2 := s.blockchain.SubscribeBlockProcessingEvent(blockProcCh)
+	headEventCh := s.fmHeadEventCh
+	blockProcCh := s.fmBlockProcCh
 	defer func() {
-		sub.Unsubscribe()
-		sub2.Unsubscribe()
+		s.fmHeadSub.Unsubscribe()
+		s.fmBlockProcSub.Unsubscribe()
 		for {
 			select {
 			case <-headEventCh: