[a11y] LibreOffice Calc exposes 2^31 children, freezes on `GetChildren`

[Luuk van der Duim]

Hi!

While working on fama11y-tree [1], an AT-SPI2-bus objects tree example, 
I found that LibreOffice Calc has an object that exposes 2^31 children 
on its `Accessible` interface.

(LO version: 24.2.3.2 x86_64, xubuntu 24.04)

The object:

Inspecting object with high child count
Object: name: "Sheet Sheet1", role: "table", description: ""
Interfaces: "InterfaceSet(BitFlags<Interface>(0b10100000000110001, 
Accessible | Collection | Component | Selection | Table))"
child_count: 2147483647, ObjectRef size: 48, collection size: 96.00 GB,
Application: name: soffice, role: application

If `GetChildren` is called on that object, Calc freezes and the AT (eg. 
screen-reader) performing the call would wait for a response.

Each object-reference in `atspi` (Rust crate) [2] is 48 bytes. If all 
children were to be returned, this would be a D-Bus message of 96 GiB. 
This is impractical and cannot be performed legally either, because the 
maximum size of a D-Bus message is 128 MiB. [3]

The main problem however is that calc freezes and that screen-readers 
need to take measures to avoid calc from sending huge numbers of children.

There is an existing bug report by Joanmarie Diggs that is relevant, 
#156657 [4].

 From that discussion I understand that the table already contains more 
children than it exposes, maybe the exposed number of children could be 
reduced further to the maximum number of children mentioned in the 
thread (65535) or to the visible and showing objects only. Either would 
be more practical than trying to send 96 Gigs 😉

The `fama11y-tree` branch linked below, `calc-demo`, checks for any 
object exposing more than 100000 children and then returns information 
on that object and not call `GetChildren`.

The main branch will call `GetChildren` and wait indefinitely for calc 
to finish.

Thanks!

Luuk van der Duim


[1]: <https://github.com/luukvanderduim/fama11y-tree/tree/calc-demo >

[2]: <https://github.com/odilia-app/atspi>

[3]: 
<https://dbus.freedesktop.org/doc/dbus-specification.html#message-protocol-messages> 


[4]: <https://bugs.documentfoundation.org/show_bug.cgi?id=156657>