forum: let non-members browse a public forum's topic list

ListForumTopics / GetForumTopicsByID / GeneralForumTopic gated on
membership while channel history uses the public-preview path, so a
public forum's topics (General included) were invisible until you joined.
Switch them to getChannelForViewer / channelForViewerLocked; private
forums and write paths keep the membership gate.

internal/rpc/forum_topics_preview_rpc_test.go

@@ -0,0 +1,98 @@
+package rpc
+
+import (
+	"context"
+	"testing"
+
+	"github.com/iamxvbaba/td/clock"
+	"github.com/iamxvbaba/td/tg"
+	"go.uber.org/zap/zaptest"
+
+	appchannels "telesrv/internal/app/channels"
+	appusers "telesrv/internal/app/users"
+	"telesrv/internal/domain"
+	"telesrv/internal/store/memory"
+)
+
+// A public forum's topic list is browsable before joining, like its history.
+// Regression: getForumTopics used the member-only access path and returned
+// CHANNEL_PRIVATE / an empty list to non-members, so the topic list (and even
+// General) was invisible until they joined.
+func TestGetForumTopicsVisibleToPublicNonMember(t *testing.T) {
+	ctx := context.Background()
+	userStore := memory.NewUserStore()
+	owner, _ := userStore.Create(ctx, domain.User{AccessHash: 81, Phone: "15550008101", FirstName: "Owner"})
+	outsider, _ := userStore.Create(ctx, domain.User{AccessHash: 82, Phone: "15550008102", FirstName: "Outsider"})
+	channelStore := memory.NewChannelStore()
+	channelSvc := appchannels.NewService(channelStore)
+	r := New(Config{}, Deps{
+		Users:    appusers.NewService(userStore),
+		Channels: channelSvc,
+	}, zaptest.NewLogger(t), clock.System)
+
+	ownerCtx := WithUserID(ctx, owner.ID)
+	outsiderCtx := WithUserID(ctx, outsider.ID)
+
+	created, err := r.onChannelsCreateChannel(ownerCtx, &tg.ChannelsCreateChannelRequest{Title: "Public Forum", Megagroup: true})
+	if err != nil {
+		t.Fatalf("create channel: %v", err)
+	}
+	channel := created.(*tg.Updates).Chats[0].(*tg.Channel)
+	input := &tg.InputChannel{ChannelID: channel.ID, AccessHash: channel.AccessHash}
+	forumPeer := &tg.InputPeerChannel{ChannelID: channel.ID, AccessHash: channel.AccessHash}
+
+	if _, err := r.onChannelsToggleForum(ownerCtx, &tg.ChannelsToggleForumRequest{Channel: input, Enabled: true, Tabs: true}); err != nil {
+		t.Fatalf("toggle forum: %v", err)
+	}
+	if _, err := channelSvc.UpdateUsername(ctx, owner.ID, domain.UpdateChannelUsernameRequest{
+		ChannelID: channel.ID,
+		Username:  "publicforum",
+	}); err != nil {
+		t.Fatalf("set channel username: %v", err)
+	}
+	if _, err := r.onMessagesCreateForumTopic(ownerCtx, &tg.MessagesCreateForumTopicRequest{
+		Peer:      forumPeer,
+		Title:     "Test",
+		IconColor: domain.DefaultForumTopicIconColor,
+		RandomID:  8101001,
+	}); err != nil {
+		t.Fatalf("create forum topic: %v", err)
+	}
+
+	res, err := r.onMessagesGetForumTopics(outsiderCtx, &tg.MessagesGetForumTopicsRequest{
+		Peer:  forumPeer,
+		Limit: 100,
+	})
+	if err != nil {
+		t.Fatalf("getForumTopics as non-member: %v", err)
+	}
+	titles := map[string]bool{}
+	for _, tc := range res.Topics {
+		switch topic := tc.(type) {
+		case *tg.ForumTopic:
+			titles[topic.Title] = true
+		case *tg.ForumTopicDeleted:
+		}
+	}
+	if !titles["General"] {
+		t.Fatalf("non-member did not see the General topic: %+v", res.Topics)
+	}
+	if !titles["Test"] {
+		t.Fatalf("non-member did not see the Test topic: %+v", res.Topics)
+	}
+
+	// A private forum still refuses a non-member.
+	priv, err := r.onChannelsCreateChannel(ownerCtx, &tg.ChannelsCreateChannelRequest{Title: "Private Forum", Megagroup: true})
+	if err != nil {
+		t.Fatalf("create private channel: %v", err)
+	}
+	privCh := priv.(*tg.Updates).Chats[0].(*tg.Channel)
+	privInput := &tg.InputChannel{ChannelID: privCh.ID, AccessHash: privCh.AccessHash}
+	privPeer := &tg.InputPeerChannel{ChannelID: privCh.ID, AccessHash: privCh.AccessHash}
+	if _, err := r.onChannelsToggleForum(ownerCtx, &tg.ChannelsToggleForumRequest{Channel: privInput, Enabled: true, Tabs: true}); err != nil {
+		t.Fatalf("toggle private forum: %v", err)
+	}
+	if _, err := r.onMessagesGetForumTopics(outsiderCtx, &tg.MessagesGetForumTopicsRequest{Peer: privPeer, Limit: 100}); err == nil {
+		t.Fatal("non-member read a private forum's topic list")
+	}
+}

internal/store/memory/channel_topic_read.go

@@ -178,7 +178,7 @@ func (s *ChannelStore) GeneralForumTopic(_ context.Context, viewerUserID, channe
 	}
 	s.mu.RLock()
 	defer s.mu.RUnlock()
-	channel, member, err := s.channelAndMemberLocked(viewerUserID, channelID)
+	channel, member, _, err := s.channelForViewerLocked(viewerUserID, channelID)
 	if err != nil {
 		return domain.ChannelForumTopic{}, err
 	}

internal/store/memory/channel_topics.go

@@ -420,7 +420,9 @@ func (s *ChannelStore) DeleteForumTopicHistory(_ context.Context, req domain.Del
 func (s *ChannelStore) ListForumTopics(_ context.Context, viewerUserID int64, filter domain.ChannelForumTopicFilter) (domain.ChannelForumTopicList, error) {
 	s.mu.RLock()
 	defer s.mu.RUnlock()
-	channel, member, err := s.channelAndMemberLocked(viewerUserID, filter.ChannelID)
+	// channelForViewerLocked, not channelAndMemberLocked: a public forum's topic
+	// list is browsable before joining, exactly like its message history.
+	channel, member, _, err := s.channelForViewerLocked(viewerUserID, filter.ChannelID)
 	if err != nil {
 		return domain.ChannelForumTopicList{}, err
 	}
@@ -463,7 +465,7 @@ func (s *ChannelStore) ListForumTopics(_ context.Context, viewerUserID int64, fi
 func (s *ChannelStore) GetForumTopicsByID(_ context.Context, viewerUserID, channelID int64, ids []int) (domain.ChannelForumTopicList, error) {
 	s.mu.RLock()
 	defer s.mu.RUnlock()
-	channel, member, err := s.channelAndMemberLocked(viewerUserID, channelID)
+	channel, member, _, err := s.channelForViewerLocked(viewerUserID, channelID)
 	if err != nil {
 		return domain.ChannelForumTopicList{}, err
 	}

internal/store/postgres/channel_topic_read.go

@@ -198,7 +198,7 @@ func (s *ChannelStore) GeneralForumTopic(ctx context.Context, viewerUserID, chan
 	if viewerUserID == 0 || channelID == 0 {
 		return domain.ChannelForumTopic{}, domain.ErrChannelInvalid
 	}
-	channel, member, err := s.getChannelForMember(ctx, s.db, viewerUserID, channelID)
+	channel, member, _, err := s.getChannelForViewer(ctx, s.db, viewerUserID, channelID)
 	if err != nil {
 		return domain.ChannelForumTopic{}, err
 	}

internal/store/postgres/channel_topics.go

@@ -470,7 +470,9 @@ WHERE channel_id = $1 AND topic_id = $2`, req.ChannelID, req.TopicID); err != ni
 }
 
 func (s *ChannelStore) ListForumTopics(ctx context.Context, viewerUserID int64, filter domain.ChannelForumTopicFilter) (domain.ChannelForumTopicList, error) {
-	channel, member, err := s.getChannelForMember(ctx, s.db, viewerUserID, filter.ChannelID)
+	// getChannelForViewer, not getChannelForMember: a public forum's topic list is
+	// browsable before joining, exactly like its message history.
+	channel, member, _, err := s.getChannelForViewer(ctx, s.db, viewerUserID, filter.ChannelID)
 	if err != nil {
 		return domain.ChannelForumTopicList{}, err
 	}
@@ -539,7 +541,7 @@ LIMIT $`+fmt.Sprint(len(args)), args...)
 }
 
 func (s *ChannelStore) GetForumTopicsByID(ctx context.Context, viewerUserID, channelID int64, ids []int) (domain.ChannelForumTopicList, error) {
-	channel, member, err := s.getChannelForMember(ctx, s.db, viewerUserID, channelID)
+	channel, member, _, err := s.getChannelForViewer(ctx, s.db, viewerUserID, channelID)
 	if err != nil {
 		return domain.ChannelForumTopicList{}, err
 	}