feat(hosts): give a host a pool of SNIs, random per config or shared (#6923)

Rotating SNIs meant one host entry per domain. A host now takes a pool of
SNIs: with more than one, every generated config gets a random entry,
or, with the shared switch on, the whole host group uses one pick per
subscription fetch. Raw, JSON and Clash output, the panel's link preview
and node adoption all carry the pool.

sni stays the pool's first entry, so a rolled-back binary and API callers
that only know the single field keep working; an sni outside the pool
becomes its primary. A host with one SNI renders exactly as before.

internal/sub/host_sub.go

@@ -1,9 +1,11 @@
 package sub
 
 import (
+	"crypto/rand"
 	"encoding/json"
 	"fmt"
 	"maps"
+	"math/big"
 	"slices"
 
 	"github.com/mhsanaei/3x-ui/v3/internal/database"
@@ -86,7 +88,7 @@ func (s *SubService) clientHostEndpoints(inbound *model.Inbound, client model.Cl
 		if h.IsDisabled || slices.Contains(h.ExcludeFromSubTypes, format) {
 			continue
 		}
-		eps = append(eps, hostEndpointMaps(h, defaultDest, inbound.Port)...)
+		eps = append(eps, s.hostEndpointMaps(h, defaultDest, inbound.Port)...)
 	}
 	return eps, true
 }
@@ -136,14 +138,15 @@ func (s *SubService) hostEndpoints(inbound *model.Inbound, format string) []map[
 		if h.IsDisabled || slices.Contains(h.ExcludeFromSubTypes, format) {
 			continue
 		}
-		eps = append(eps, hostEndpointMaps(h, defaultDest, inbound.Port)...)
+		eps = append(eps, s.hostEndpointMaps(h, defaultDest, inbound.Port)...)
 	}
 	return eps
 }
 
 // hostEndpointMaps is one host's externalProxy entries: a CIDR address expands into RandomCount
 // random addresses from the range, any other address is a single entry.
-func hostEndpointMaps(h *model.Host, defaultDest string, defaultPort int) []map[string]any {
+func (s *SubService) hostEndpointMaps(h *model.Host, defaultDest string, defaultPort int) []map[string]any {
+	shared := s.sharedHostSni(h)
 	if IsCIDR(h.Address) {
 		count := max(h.RandomCount, 1)
 		if ips, err := RandomIPsFromCIDR(h.Address, count); err == nil && len(ips) > 0 {
@@ -158,12 +161,57 @@ func hostEndpointMaps(h *model.Host, defaultDest string, defaultPort int) []map[
 						hCopy.Remark = fmt.Sprintf("%d", i+1)
 					}
 				}
+				hCopy.Sni = endpointSni(h, shared)
 				eps = append(eps, hostToExternalProxyMap(&hCopy, defaultDest, defaultPort))
 			}
 			return eps
 		}
 	}
-	return []map[string]any{hostToExternalProxyMap(h, defaultDest, defaultPort)}
+
+	hCopy := *h
+	hCopy.Sni = endpointSni(h, shared)
+	return []map[string]any{hostToExternalProxyMap(&hCopy, defaultDest, defaultPort)}
+}
+
+// endpointSni is the SNI one endpoint of h advertises: the request's shared pick, a fresh random
+// entry of a multi-SNI pool, or the single SNI; the address and blank overrides apply later.
+func endpointSni(h *model.Host, shared string) string {
+	switch {
+	case shared != "":
+		return shared
+	case len(h.Snis) > 1:
+		return randomSni(h.Snis)
+	}
+	return h.Sni
+}
+
+// sharedHostSni picks one SNI of a shared pool per host group and request, so every endpoint of
+// the group carries the same one; "" when h does not share.
+func (s *SubService) sharedHostSni(h *model.Host) string {
+	if !h.SniSharedRandom || len(h.Snis) < 2 {
+		return ""
+	}
+	key := h.GroupId
+	if key == "" {
+		key = fmt.Sprintf("host-%d", h.Id)
+	}
+	if sni, ok := s.sharedGroupSni[key]; ok {
+		return sni
+	}
+	if s.sharedGroupSni == nil {
+		s.sharedGroupSni = make(map[string]string)
+	}
+	sni := randomSni(h.Snis)
+	s.sharedGroupSni[key] = sni
+	return sni
+}
+
+func randomSni(snis []string) string {
+	idx, err := rand.Int(rand.Reader, big.NewInt(int64(len(snis))))
+	if err != nil {
+		return snis[0]
+	}
+	return snis[idx.Int64()]
 }
 
 // hostToExternalProxyMap projects a Host onto the externalProxy entry shape the