forum: let non-members preview topic replies in a public channel

ListChannelReplies used getChannelForMemberOrLinkedGuest, so messages.getReplies
was member-only. ListChannelHistory (flat history) uses getChannelForViewer and
already allows a public channel's non-members to preview it. The mismatch meant
that on a public forum you could preview the flat history but not the topics -
and after leaving, tdesktop's topic view got CHANNEL_PRIVATE and sat on
"Loading..." forever instead of rendering a preview.

Switch the primary channel lookup in ListChannelReplies (both stores) to the
viewer-scope path. Private channels still return CHANNEL_PRIVATE to non-members;
the broadcast comment-thread lookup is unchanged.

internal/rpc/forum_topics_preview_rpc_test.go

@@ -67,10 +67,14 @@ func TestGetForumTopicsVisibleToPublicNonMember(t *testing.T) {
 		t.Fatalf("getForumTopics as non-member: %v", err)
 	}
 	titles := map[string]bool{}
+	testTopicID := 0
 	for _, tc := range res.Topics {
 		switch topic := tc.(type) {
 		case *tg.ForumTopic:
 			titles[topic.Title] = true
+			if topic.Title == "Test" {
+				testTopicID = topic.ID
+			}
 		case *tg.ForumTopicDeleted:
 		}
 	}
@@ -81,6 +85,19 @@ func TestGetForumTopicsVisibleToPublicNonMember(t *testing.T) {
 		t.Fatalf("non-member did not see the Test topic: %+v", res.Topics)
 	}
 
+	// A non-member can also read the replies inside a topic (preview), the same
+	// way ListChannelHistory lets them preview a public group's flat history.
+	if testTopicID == 0 {
+		t.Fatal("no Test topic id to open")
+	}
+	if _, err := r.onMessagesGetReplies(outsiderCtx, &tg.MessagesGetRepliesRequest{
+		Peer:  forumPeer,
+		MsgID: testTopicID,
+		Limit: 20,
+	}); err != nil {
+		t.Fatalf("getReplies as non-member of a public forum: %v", err)
+	}
+
 	// The forum's own channel must come back with left=true so the client still
 	// offers a Join button instead of treating the forum as already joined.
 	var forumChat *tg.Channel
@@ -110,4 +127,7 @@ func TestGetForumTopicsVisibleToPublicNonMember(t *testing.T) {
 	if _, err := r.onMessagesGetForumTopics(outsiderCtx, &tg.MessagesGetForumTopicsRequest{Peer: privPeer, Limit: 100}); err == nil {
 		t.Fatal("non-member read a private forum's topic list")
 	}
+	if _, err := r.onMessagesGetReplies(outsiderCtx, &tg.MessagesGetRepliesRequest{Peer: privPeer, MsgID: 1, Limit: 20}); err == nil {
+		t.Fatal("non-member read a private forum topic's replies")
+	}
 }

internal/store/memory/channel_topics.go

@@ -501,7 +501,9 @@ func (s *ChannelStore) GetForumTopicsByID(_ context.Context, viewerUserID, chann
 func (s *ChannelStore) ListChannelReplies(_ context.Context, viewerUserID int64, filter domain.ChannelRepliesFilter) (domain.ChannelHistory, error) {
 	s.mu.RLock()
 	defer s.mu.RUnlock()
-	source, member, err := s.channelAndMemberOrLinkedGuestLocked(viewerUserID, filter.ChannelID)
+	// Viewer scope (not strict membership): non-members can preview topic
+	// replies in a public channel/supergroup, matching ListChannelHistory.
+	source, member, _, err := s.channelForViewerLocked(viewerUserID, filter.ChannelID)
 	if err != nil {
 		return domain.ChannelHistory{}, err
 	}

internal/store/postgres/channel_topics.go

@@ -588,7 +588,9 @@ ORDER BY pinned DESC, pinned_order DESC, date DESC, topic_id DESC`, channelID, m
 }
 
 func (s *ChannelStore) ListChannelReplies(ctx context.Context, viewerUserID int64, filter domain.ChannelRepliesFilter) (domain.ChannelHistory, error) {
-	source, member, err := s.getChannelForMemberOrLinkedGuest(ctx, s.db, viewerUserID, filter.ChannelID)
+	// Viewer口径(非严格 member):公开频道/超级群的非成员可预览话题回复,与
+	// ListChannelHistory 一致。私有频道非成员仍是 ErrChannelPrivate。
+	source, member, _, err := s.getChannelForViewer(ctx, s.db, viewerUserID, filter.ChannelID)
 	if err != nil {
 		return domain.ChannelHistory{}, err
 	}