fix: filters.FilterAPI goroutine leak (#254)

## Why this should be merged

This allows closing the `timeoutLoop` goroutine. Closing goroutines is
the best-practice. But, more importantly, this causes the
`filters.Backend` to never be GCed. Which is causing GC tests to fail in
SAE.

## How this works

I tried to make this as minimal of a diff as possible.

## How this was tested

With this change, the SAE tests are passing:
https://github.com/ava-labs/strevm/pull/64

eth/filters/api.go

@@ -64,6 +64,8 @@ type FilterAPI struct {
 	filtersMu sync.Mutex
 	filters   map[rpc.ID]*filter
 	timeout   time.Duration
+
+	quit chan struct{}
 }
 
 // NewFilterAPI returns a new FilterAPI instance.
@@ -73,6 +75,7 @@ func NewFilterAPI(system *FilterSystem, lightMode bool) *FilterAPI {
 		events:  NewEventSystem(system, lightMode),
 		filters: make(map[rpc.ID]*filter),
 		timeout: system.cfg.Timeout,
+		quit:    make(chan struct{}),
 	}
 	go api.timeoutLoop(system.cfg.Timeout)
 
@@ -86,7 +89,11 @@ func (api *FilterAPI) timeoutLoop(timeout time.Duration) {
 	ticker := time.NewTicker(timeout)
 	defer ticker.Stop()
 	for {
-		<-ticker.C
+		select {
+		case <-ticker.C:
+		case <-api.quit:
+			return
+		}
 		api.filtersMu.Lock()
 		for id, f := range api.filters {
 			select {

eth/filters/api.libevm.go

@@ -0,0 +1,22 @@
+// Copyright 2026 the libevm authors.
+//
+// The libevm additions to go-ethereum are free software: you can redistribute
+// them and/or modify them under the terms of the GNU Lesser General Public License
+// as published by the Free Software Foundation, either version 3 of the License,
+// or (at your option) any later version.
+//
+// The libevm additions are distributed in the hope that they will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser
+// General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see
+// <http://www.gnu.org/licenses/>.
+
+package filters
+
+// Close releases resources held by the API.
+func (api *FilterAPI) Close() {
+	close(api.quit)
+}

eth/filters/api.libevm_test.go

@@ -0,0 +1,57 @@
+// Copyright 2026 the libevm authors.
+//
+// The libevm additions to go-ethereum are free software: you can redistribute
+// them and/or modify them under the terms of the GNU Lesser General Public License
+// as published by the Free Software Foundation, either version 3 of the License,
+// or (at your option) any later version.
+//
+// The libevm additions are distributed in the hope that they will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser
+// General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see
+// <http://www.gnu.org/licenses/>.
+
+package filters
+
+import (
+	"testing"
+
+	"go.uber.org/goleak"
+
+	"github.com/ava-labs/libevm/core"
+	"github.com/ava-labs/libevm/core/rawdb"
+	"github.com/ava-labs/libevm/event"
+)
+
+// A closeableTestBackend tracks all subscriptions that it produces, allowing
+// them to be cleaned up to avoid the leak of [EventSystem.eventLoop].
+type closeableTestBackend struct {
+	testBackend
+	subs event.SubscriptionScope
+}
+
+func (b *closeableTestBackend) SubscribeNewTxsEvent(ch chan<- core.NewTxsEvent) event.Subscription {
+	sub := b.testBackend.SubscribeNewTxsEvent(ch)
+	return b.subs.Track(sub)
+}
+
+func (b *closeableTestBackend) Close() {
+	b.subs.Close()
+}
+
+func TestClose(t *testing.T) {
+	defer goleak.VerifyNone(t, goleak.IgnoreCurrent())
+
+	backend := &closeableTestBackend{
+		testBackend: testBackend{
+			db: rawdb.NewMemoryDatabase(),
+		},
+	}
+	defer backend.Close()
+	sys := NewFilterSystem(backend, Config{})
+	api := NewFilterAPI(sys, false)
+	api.Close()
+}

go.mod

@@ -64,6 +64,7 @@ require (
 	github.com/tyler-smith/go-bip39 v1.1.0
 	github.com/urfave/cli/v2 v2.25.7
 	go.uber.org/automaxprocs v1.5.2
+	go.uber.org/goleak v1.3.0
 	golang.org/x/crypto v0.43.0
 	golang.org/x/exp v0.0.0-20231110203233-9a3e6036ecaa
 	golang.org/x/mod v0.29.0

go.sum

@@ -622,6 +622,8 @@ go.opencensus.io v0.22.3/go.mod h1:yxeiOL68Rb0Xd1ddK5vPZ/oVn4vY4Ynel7k9FzqtOIw=
 go.opencensus.io v0.22.4/go.mod h1:yxeiOL68Rb0Xd1ddK5vPZ/oVn4vY4Ynel7k9FzqtOIw=
 go.uber.org/automaxprocs v1.5.2 h1:2LxUOGiR3O6tw8ui5sZa2LAaHnsviZdVOUZw4fvbnME=
 go.uber.org/automaxprocs v1.5.2/go.mod h1:eRbA25aqJrxAbsLO0xy5jVwPt7FQnRgjW+efnwa1WM0=
+go.uber.org/goleak v1.3.0 h1:2K3zAYmnTNqV73imy9J1T3WC+gmCePx2hEGkimedGto=
+go.uber.org/goleak v1.3.0/go.mod h1:CoHD4mav9JJNrW/WLlf7HGZPjdw8EucARQHekz1X6bE=
 golang.org/x/crypto v0.0.0-20180904163835-0709b304e793/go.mod h1:6SG95UA2DQfeDnfUPMdvaQW0Q7yPrPDi9nlGo2tz2b4=
 golang.org/x/crypto v0.0.0-20181203042331-505ab145d0a9/go.mod h1:6SG95UA2DQfeDnfUPMdvaQW0Q7yPrPDi9nlGo2tz2b4=
 golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=