mosdns: black_hole: apply Fisher-Yates shuffle algorithm to randomize IP order

* The client seems to always use the first IP address in the response, using a random algorithm to scramble multiple IP addresses so that the client can use all IP addresses as much as possible.

mosdns/patches/201-black_hole-apply-Fisher-Yates-shuffle-algorithm-to-r.patch

@@ -0,0 +1,56 @@
+From 6c00597904ba487615e3a49626ccbb7d5623283f Mon Sep 17 00:00:00 2001
+From: sbwml <admin@cooluc.com>
+Date: Wed, 20 Sep 2023 14:51:19 +0800
+Subject: [PATCH] black_hole: apply Fisher-Yates shuffle algorithm to randomize
+ IP order
+
+---
+ plugin/executable/black_hole/black_hole.go | 15 +++++++++++++++
+ 1 file changed, 15 insertions(+)
+
+diff --git a/plugin/executable/black_hole/black_hole.go b/plugin/executable/black_hole/black_hole.go
+index 775253d..f955019 100644
+--- a/plugin/executable/black_hole/black_hole.go
++++ b/plugin/executable/black_hole/black_hole.go
+@@ -27,6 +27,8 @@ import (
+ 	"github.com/miekg/dns"
+ 	"net/netip"
+ 	"strings"
++	"math/rand"
++	"sync"
+ )
+ 
+ const PluginType = "black_hole"
+@@ -40,6 +42,7 @@ var _ sequence.Executable = (*BlackHole)(nil)
+ type BlackHole struct {
+ 	ipv4 []netip.Addr
+ 	ipv6 []netip.Addr
++	shuffleMutex sync.Mutex
+ }
+ 
+ // QuickSetup format: [ipv4|ipv6] ...
+@@ -65,9 +68,21 @@ func NewBlackHole(ips []string) (*BlackHole, error) {
+ 	return b, nil
+ }
+ 
++func (b *BlackHole) shuffleIPs() {
++	b.shuffleMutex.Lock()
++	defer b.shuffleMutex.Unlock()
++	rand.Shuffle(len(b.ipv4), func(i, j int) {
++		b.ipv4[i], b.ipv4[j] = b.ipv4[j], b.ipv4[i]
++	})
++	rand.Shuffle(len(b.ipv6), func(i, j int) {
++		b.ipv6[i], b.ipv6[j] = b.ipv6[j], b.ipv6[i]
++	})
++}
++
+ // Exec implements sequence.Executable. It set a response with given ips if
+ // query has corresponding qtypes.
+ func (b *BlackHole) Exec(_ context.Context, qCtx *query_context.Context) error {
++	b.shuffleIPs()
+ 	if r := b.Response(qCtx.Q()); r != nil {
+ 		qCtx.SetResponse(r)
+ 	}
+-- 
+2.34.8
+