fix: [2.4] Use singleton delete pool and avoid goroutine leakage (#37225)

Cherry-pick from master
pr: #37220
Related to #36887

Previously using newly create pool per request shall cause goroutine
leakage. This PR change this behavior by using singleton delete pool.
This change could also provide better concurrency control over delete
memory usage.

Signed-off-by: Congqi Xia <congqi.xia@zilliz.com>

internal/querynodev2/segments/pool.go

@@ -46,6 +46,9 @@ var (
 	warmupPool atomic.Pointer[conc.Pool[any]]
 	warmupOnce sync.Once
 
+	deletePool     atomic.Pointer[conc.Pool[struct{}]]
+	deletePoolOnce sync.Once
+
 	bfPool      atomic.Pointer[conc.Pool[any]]
 	bfApplyOnce sync.Once
 )
@@ -131,6 +134,13 @@ func initBFApplyPool() {
 	})
 }
 
+func initDeletePool() {
+	deletePoolOnce.Do(func() {
+		pool := conc.NewPool[struct{}](runtime.GOMAXPROCS(0))
+		deletePool.Store(pool)
+	})
+}
+
 // GetSQPool returns the singleton pool instance for search/query operations.
 func GetSQPool() *conc.Pool[any] {
 	initSQPool()
@@ -158,6 +168,11 @@ func GetBFApplyPool() *conc.Pool[any] {
 	return bfPool.Load()
 }
 
+func GetDeletePool() *conc.Pool[struct{}] {
+	initDeletePool()
+	return deletePool.Load()
+}
+
 func ResizeSQPool(evt *config.Event) {
 	if evt.HasUpdated {
 		pt := paramtable.Get()

internal/querynodev2/services.go

@@ -19,7 +19,6 @@ package querynodev2
 import (
 	"context"
 	"fmt"
-	"runtime"
 	"strconv"
 	"sync"
 	"time"
@@ -1475,7 +1474,7 @@ func (node *QueryNode) DeleteBatch(ctx context.Context, req *querypb.DeleteBatch
 
 	// control the execution batch parallel with P number
 	// maybe it shall be lower in case of heavy CPU usage may impacting search/query
-	pool := conc.NewPool[struct{}](runtime.GOMAXPROCS(0))
+	pool := segments.GetDeletePool()
 	futures := make([]*conc.Future[struct{}], 0, len(segs))
 	errSet := typeutil.NewConcurrentSet[int64]()