chore: report aggregate race conditions in test

This is effectively the same thing as both wait on `<-sf.term` but it makes for a clearer test that demonstrates the bug because `terminate(false)` is documented as blocking until "all disk loads finish".

core/state/trie_prefetcher_test.go

@@ -18,6 +18,8 @@ package state
 
 import (
 	"math/big"
+	"sync"
+	"sync/atomic"
 	"testing"
 
 	"github.com/ethereum/go-ethereum/common"
@@ -66,6 +68,57 @@ func TestUseAfterTerminate(t *testing.T) {
 	}
 }
 
+func TestSchdeulerTerminationRaceCondition(t *testing.T) {
+	// The lock-based implementation of [subfetcher] had a race condition
+	// whereby schedule() could obtain the lock after the <-sf.stop branch of
+	// loop() had already checked for an empty queue. Although probabilistic,
+	// this test reliably triggered at a rates of (~4 in 10k) and (~100 in 50k)
+	// on an Apple M3 Max chip.
+
+	t.Parallel()
+	db := filledStateDB()
+	skey := common.HexToHash("aaa")
+
+	// Maximise concurrency by synchronising all scheduling and termination.
+	start := make(chan struct{})
+	var (
+		wg          sync.WaitGroup
+		raceInduced atomic.Uint64
+	)
+
+	const numTrials = 50_000
+	for i := 0; i < numTrials; i++ {
+		wg.Add(2)
+		fetcher := newSubfetcher(db.db, db.originalRoot, common.Hash{}, db.originalRoot, common.Address{})
+
+		var gotScheduleErr error
+		doneScheduling := make(chan struct{})
+		go func() {
+			defer wg.Done()
+			<-start
+			gotScheduleErr = fetcher.schedule(nil, []common.Hash{skey}, false)
+			close(doneScheduling)
+		}()
+
+		go func() {
+			defer wg.Done()
+			<-start
+			fetcher.terminate(false /*async*/)
+
+			<-doneScheduling
+			if gotScheduleErr == nil && len(fetcher.tasks) > 0 {
+				raceInduced.Add(1)
+			}
+		}()
+	}
+
+	close(start)
+	wg.Wait()
+	if got := raceInduced.Load(); got > 0 {
+		t.Errorf("In %d/%d concurrent trials %T.schedule() returned nil error but >0 tasks remain in queue after %[3]T.terminate([blocking]) returned", got, numTrials, &subfetcher{})
+	}
+}
+
 func TestVerklePrefetcher(t *testing.T) {
 	disk := rawdb.NewMemoryDatabase()
 	db := triedb.NewDatabase(disk, triedb.VerkleDefaults)