forum: fix reply_to_top_id for replies inside a forum

resolveChannelReply applied discussion-thread logic (reply_to_top_id =
the replied-to message's own id) to forum replies. Replying to a General
message produced reply_to_top_id = <that id>, a topic no client can
resolve: the reply vanished from every topic view and reply-jump on
strict clients said "message doesn't exist".

Forum replies now inherit the target's topic via domain.ForumReplyTopicID
(target's topic, or its own id if it's a topic-create, else General), and
General (topic 1) is accepted as a valid virtual topic everywhere, so
sends carrying top_msg_id: 1 are no longer rejected. Non-forum discussion
threads are unchanged.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/rpc/forum_reply_topic_rpc_test.go

@@ -0,0 +1,120 @@
+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 reply inside a forum must inherit the *target's* topic, never the target's
+// own message id. Regression: replying to a General message produced
+// reply_to_top_id = <that message's id>, a topic that no client can resolve, so
+// the reply vanished from every topic view and reply-jump said "doesn't exist".
+func TestForumReplyInheritsTargetTopic(t *testing.T) {
+	ctx := context.Background()
+	userStore := memory.NewUserStore()
+	owner, _ := userStore.Create(ctx, domain.User{AccessHash: 91, Phone: "15550009101", FirstName: "Owner"})
+	channelStore := memory.NewChannelStore()
+	r := New(Config{}, Deps{
+		Users:    appusers.NewService(userStore),
+		Channels: appchannels.NewService(channelStore),
+	}, zaptest.NewLogger(t), clock.System)
+	ownerCtx := WithUserID(ctx, owner.ID)
+
+	created, err := r.onChannelsCreateChannel(ownerCtx, &tg.ChannelsCreateChannelRequest{Title: "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}
+	peer := &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)
+	}
+	topicUpd, err := r.onMessagesCreateForumTopic(ownerCtx, &tg.MessagesCreateForumTopicRequest{
+		Peer: peer, Title: "Test", IconColor: domain.DefaultForumTopicIconColor, RandomID: 9101001,
+	})
+	if err != nil {
+		t.Fatalf("create topic: %v", err)
+	}
+	testTopicID := forumTopicRootMessageID(t, topicUpd, "Test")
+
+	send := func(text string, randomID int64, reply *tg.InputReplyToMessage) *tg.Message {
+		req := &tg.MessagesSendMessageRequest{Peer: peer, Message: text, RandomID: randomID}
+		if reply != nil {
+			req.SetReplyTo(reply)
+		}
+		upd, err := r.onMessagesSendMessage(ownerCtx, req)
+		if err != nil {
+			t.Fatalf("send %q: %v", text, err)
+		}
+		for _, u := range upd.(*tg.Updates).Updates {
+			if nm, ok := u.(*tg.UpdateNewChannelMessage); ok {
+				if m, ok := nm.Message.(*tg.Message); ok && m.Message == text {
+					return m
+				}
+			}
+		}
+		t.Fatalf("no new message for %q in %+v", text, upd)
+		return nil
+	}
+	topID := func(m *tg.Message) int {
+		h, ok := m.ReplyTo.(*tg.MessageReplyHeader)
+		if !ok {
+			t.Fatalf("message %d has reply header %T, want *MessageReplyHeader", m.ID, m.ReplyTo)
+		}
+		id, _ := h.GetReplyToTopID()
+		if !h.ForumTopic {
+			t.Fatalf("message %d reply header missing forum_topic flag: %+v", m.ID, h)
+		}
+		return id
+	}
+
+	// A plain General message (no reply header).
+	g1 := send("g1", 9101002, nil)
+
+	// Reply to it -> topic must be General (1), not g1.ID.
+	r1 := send("r1", 9101003, &tg.InputReplyToMessage{ReplyToMsgID: g1.ID})
+	if got := topID(r1); got != domain.ForumGeneralTopicID {
+		t.Fatalf("reply to a General message: reply_to_top_id = %d, want %d (General), not the target id %d",
+			got, domain.ForumGeneralTopicID, g1.ID)
+	}
+
+	// Reply again, this time the client also passes top_msg_id: 1 (General).
+	// Previously this was rejected because General has no channel_forum_topics row.
+	replyWithTop := &tg.InputReplyToMessage{ReplyToMsgID: g1.ID}
+	replyWithTop.SetTopMsgID(domain.ForumGeneralTopicID)
+	r2 := send("r2", 9101004, replyWithTop)
+	if got := topID(r2); got != domain.ForumGeneralTopicID {
+		t.Fatalf("reply with top_msg_id=1: reply_to_top_id = %d, want %d", got, domain.ForumGeneralTopicID)
+	}
+
+	// Post directly into the "Test" topic, then reply to a plain message there.
+	tInTopic := &tg.InputReplyToMessage{ReplyToMsgID: 0}
+	tInTopic.SetTopMsgID(testTopicID)
+	m1 := send("t1", 9101005, tInTopic)
+	if got := topID(m1); got != testTopicID {
+		t.Fatalf("message in Test topic: reply_to_top_id = %d, want %d", got, testTopicID)
+	}
+	rt := send("rt", 9101006, &tg.InputReplyToMessage{ReplyToMsgID: m1.ID})
+	if got := topID(rt); got != testTopicID {
+		t.Fatalf("reply inside Test topic: reply_to_top_id = %d, want %d (topic), not %d", got, testTopicID, m1.ID)
+	}
+
+	// Replying to a General message while claiming a mismatched topic is rejected.
+	bad := &tg.InputReplyToMessage{ReplyToMsgID: g1.ID}
+	bad.SetTopMsgID(testTopicID)
+	req := &tg.MessagesSendMessageRequest{Peer: peer, Message: "bad", RandomID: 9101007}
+	req.SetReplyTo(bad)
+	if _, err := r.onMessagesSendMessage(ownerCtx, req); err == nil {
+		t.Fatal("reply with a topic id that doesn't match the target's topic was accepted")
+	}
+}