message-worker: stop re-stamping parent thread_room_id on every subsequent reply

The parent message's thread_room_id is immutable once set and is stamped on the first
reply (handleFirstThreadReply). The subsequent-reply path re-issued the same
UpdateParentMessageThreadRoomID Cassandra write on every reply purely as a crash-window
self-heal. Drop it: one fewer Cassandra write per subsequent reply.

Trade-off (consistent with the parent-subscription removal): if the first reply created
the thread room but crashed before its stamp landed, the parent stays unstamped — a
narrow window not worth a write on every reply.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VzvswJ23JQB1nYyskjBpi9

Author: claude

message-worker/handler.go

@@ -415,30 +415,11 @@ func (h *Handler) handleSubsequentThreadReply(ctx context.Context, msg *model.Me
 		return "", false, fmt.Errorf("update thread room last message: %w", err)
 	}
 
-	// Re-stamp handles redelivery: first attempt may have created the thread room
-	// but crashed before the stamp landed. IF EXISTS in the store prevents phantom rows.
-	switch {
-	case parentFound && msg.ThreadParentMessageCreatedAt != nil:
-		if err := h.store.UpdateParentMessageThreadRoomID(ctx, msg.ThreadParentMessageID, msg.RoomID, *msg.ThreadParentMessageCreatedAt, existingRoom.ID); err != nil {
-			return "", false, fmt.Errorf("stamp thread_room_id on parent message: %w", err)
-		}
-	case !parentFound:
-		slog.ErrorContext(ctx, "subsequent thread reply: parent not found in messages_by_id, thread_room_id stamp skipped",
-			"request_id", natsutil.RequestIDFromContext(ctx),
-			"replyID", msg.ID,
-			"parentMessageID", msg.ThreadParentMessageID,
-			"threadRoomID", existingRoom.ID,
-			"room_id", msg.RoomID,
-		)
-	default: // msg.ThreadParentMessageCreatedAt == nil
-		slog.ErrorContext(ctx, "subsequent thread reply: ThreadParentMessageCreatedAt is nil, parent thread_room_id stamp skipped",
-			"request_id", natsutil.RequestIDFromContext(ctx),
-			"replyID", msg.ID,
-			"parentMessageID", msg.ThreadParentMessageID,
-			"threadRoomID", existingRoom.ID,
-			"room_id", msg.RoomID,
-		)
-	}
+	// The parent message's thread_room_id is stamped once, on the first reply
+	// (handleFirstThreadReply). It never changes for a thread, so re-stamping it on every
+	// subsequent reply is a redundant Cassandra write — skipped here. Trade-off: if the
+	// first reply created the room but crashed before its stamp landed, the parent stays
+	// unstamped (no per-reply self-heal) — a narrow window not worth a write on every reply.
 
 	return existingRoom.ID, replierLastSeenAdvanced, nil
 }

message-worker/handler_test.go

@@ -1000,55 +1000,7 @@ func TestHandler_HandleThreadRoomAndSubscriptions(t *testing.T) {
 			wantErr: true,
 		},
 		{
-			name: "subsequent reply — stamps thread_room_id on parent when parentCreatedAt known",
-			msg: &model.Message{
-				ID:                           "msg-reply",
-				RoomID:                       "r1",
-				UserID:                       "u-replier",
-				UserAccount:                  "replier",
-				CreatedAt:                    now,
-				ThreadParentMessageID:        "msg-parent",
-				ThreadParentMessageCreatedAt: ptrTime(now.Add(-5 * time.Minute)),
-			},
-			siteID: "site-a",
-			setupMocks: func(store *MockStore, ts *MockThreadStore) {
-				ts.EXPECT().EnsureThreadRoom(gomock.Any(), gomock.Any()).Return(&model.ThreadRoom{ID: "tr-existing"}, false, nil)
-				store.EXPECT().GetMessageSender(gomock.Any(), "msg-parent").Return(parentSender, nil)
-				// Only the replier's subscription is written (combined upsert+lastSeen); the
-				// parent author's sub is not re-upserted and no parent owner-site lookup runs.
-				ts.EXPECT().UpsertThreadSubscriptionAdvancingLastSeen(gomock.Any(), gomock.Any(), now).Return(nil)
-				ts.EXPECT().UpdateThreadRoomLastMessage(gomock.Any(), "tr-existing", "msg-reply", gomock.Any(), now).Return(nil)
-				store.EXPECT().UpdateParentMessageThreadRoomID(
-					gomock.Any(), "msg-parent", "r1",
-					now.Add(-5*time.Minute),
-					"tr-existing",
-				).Return(nil)
-			},
-		},
-		{
-			name: "subsequent reply — UpdateParentMessageThreadRoomID fails — returns error",
-			msg: &model.Message{
-				ID:                           "msg-reply",
-				RoomID:                       "r1",
-				UserID:                       "u-replier",
-				UserAccount:                  "replier",
-				CreatedAt:                    now,
-				ThreadParentMessageID:        "msg-parent",
-				ThreadParentMessageCreatedAt: ptrTime(now.Add(-5 * time.Minute)),
-			},
-			siteID: "site-a",
-			setupMocks: func(store *MockStore, ts *MockThreadStore) {
-				ts.EXPECT().EnsureThreadRoom(gomock.Any(), gomock.Any()).Return(&model.ThreadRoom{ID: "tr-existing"}, false, nil)
-				store.EXPECT().GetMessageSender(gomock.Any(), "msg-parent").Return(parentSender, nil)
-				ts.EXPECT().UpsertThreadSubscriptionAdvancingLastSeen(gomock.Any(), gomock.Any(), now).Return(nil)
-				ts.EXPECT().UpdateThreadRoomLastMessage(gomock.Any(), "tr-existing", "msg-reply", gomock.Any(), now).Return(nil)
-				store.EXPECT().UpdateParentMessageThreadRoomID(gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any()).
-					Return(errors.New("cassandra: write timeout"))
-			},
-			wantErr: true,
-		},
-		{
-			name: "subsequent reply — parent not found but parentCreatedAt known — skips UpdateParentMessageThreadRoomID",
+			name: "subsequent reply — parent not found but parentCreatedAt known — replier sub still written",
 			msg: &model.Message{
 				ID:                           "msg-reply",
 				RoomID:                       "r1",
@@ -2058,10 +2010,8 @@ func TestHandler_ProcessMessage_ThreadReplyPublish(t *testing.T) {
 		// re-upsert, no parent owner-site lookup, no standalone AdvanceThreadSubscriptionLastSeen.
 		ts.EXPECT().UpsertThreadSubscriptionAdvancingLastSeen(gomock.Any(), gomock.Any(), now).Return(nil)
 		ts.EXPECT().UpdateThreadRoomLastMessage(gomock.Any(), "tr-1", "msg-reply", gomock.Any(), now).Return(nil)
-		// parentFound && ThreadParentMessageCreatedAt != nil → stamps thread_room_id on parent.
-		store.EXPECT().UpdateParentMessageThreadRoomID(
-			gomock.Any(), "msg-parent", "r1", parentCreatedAt, "tr-1",
-		).Return(nil)
+		// Subsequent reply: the parent's thread_room_id was stamped on the first reply, so it is
+		// NOT re-stamped here (no UpdateParentMessageThreadRoomID call).
 	}
 
 	t.Run("publishes MessageEvent to canonical thread reply subject", func(t *testing.T) {