fix memory leak of grpc resolver (#4490)

Co-authored-by: nk <kui.niu@akuvox.com>

core/discov/internal/registry.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"slices"
 	"sort"
 	"strings"
 	"sync"
@@ -59,6 +60,17 @@ func (r *Registry) Monitor(endpoints []string, key string, l UpdateListener, exa
 	return c.monitor(key, l, exactMatch)
 }
 
+// Unmonitor cancel monitoring of given endpoints and keys, and remove the listener.
+func (r *Registry) Unmonitor(endpoints []string, key string, l UpdateListener) {
+	c, exists := r.getCluster(endpoints)
+	// if not exists, return.
+	if !exists {
+		return
+	}
+
+	c.unmonitor(key, l)
+}
+
 func (r *Registry) getCluster(endpoints []string) (c *cluster, exists bool) {
 	clusterKey := getClusterKey(endpoints)
 	r.lock.RLock()
@@ -273,6 +285,14 @@ func (c *cluster) monitor(key string, l UpdateListener, exactMatch bool) error {
 	return nil
 }
 
+func (c *cluster) unmonitor(key string, l UpdateListener) {
+	c.lock.Lock()
+	defer c.lock.Unlock()
+	c.listeners[key] = slices.DeleteFunc(c.listeners[key], func(listener UpdateListener) bool {
+		return l == listener
+	})
+}
+
 func (c *cluster) newClient() (EtcdClient, error) {
 	cli, err := NewClient(c.endpoints)
 	if err != nil {

core/discov/internal/registry_test.go

@@ -292,6 +292,31 @@ func TestRegistry_Monitor(t *testing.T) {
 	assert.Error(t, GetRegistry().Monitor(endpoints, "foo", new(mockListener), false))
 }
 
+func TestRegistry_Unmonitor(t *testing.T) {
+	svr, err := mockserver.StartMockServers(1)
+	assert.NoError(t, err)
+	svr.StartAt(0)
+
+	endpoints := []string{svr.Servers[0].Address}
+	GetRegistry().lock.Lock()
+	GetRegistry().clusters = map[string]*cluster{
+		getClusterKey(endpoints): {
+			listeners: map[string][]UpdateListener{},
+			values: map[string]map[string]string{
+				"foo": {
+					"bar": "baz",
+				},
+			},
+		},
+	}
+	GetRegistry().lock.Unlock()
+	l := new(mockListener)
+	assert.Error(t, GetRegistry().Monitor(endpoints, "foo", l, false))
+	assert.Equal(t, 1, len(GetRegistry().clusters[getClusterKey(endpoints)].listeners["foo"]))
+	GetRegistry().Unmonitor(endpoints, "foo", l)
+	assert.Equal(t, 0, len(GetRegistry().clusters[getClusterKey(endpoints)].listeners["foo"]))
+}
+
 type mockListener struct {
 }