[PATH] core: implement a safe wl_signal_emit

[Daniel Stone]

Hi,

On 22 February 2018 at 14:14, Markus Ongyerth <wl at ongy.net> wrote:
>> It seems that this patch makes that assumption invalid, and we would
>> need patches to weston, enlightenment, and mutter to prevent a
>> use-after-free during the signal emit?  Now I'm seeing valgrind errors
>> on E and weston during buffer destroy.
>>
>> Personally, I don't think we should change this assumption and declare
>> the existing code that's worked for years suddenly buggy. :/
>
> The code was buggy the whole time. Just because it was never triggered, does
> not imply it's not a bug.
> free()ing these struct wl_list without removing them from the signal list
> leaves other struct wl_list that are outside the control of the current code
> in an invalid, prone to use-after-free, state.

There's a difference between something being 'buggy' and a design with
non-obvious details you might not like. If destroy handlers not
removing their list elements were buggy, we would be seeing bugs from
that. But instead it's part of the API contract: when a destroy signal
is invoked, you are guaranteed that this will be the first and only
access to your list member. This implies that anyone trying to remove
their link from the list (accessing other listeners in the list) is
buggy.

> Suddenly allowing this is a breaking API change (*some* struct wl_list inside
> a wl_listener) can suddenly become invalid for reasons outside the users
> control.

I don't know if I've quite parsed this right, but as above, not
removing elements of a destroy listener list, when the listener is
invoked, is our current API.

> Related to this entire thing:
> In [1] you added tests for this and promote something, that is in essence, a
> breaking change.

It's not a breaking change though: it's the API we've pushed on everyone so far.

> It also makes good wrapper implementations into managed languages annoying.
> For example (admittedly my own) [2] ensures a wl_listener can never be lost
> and leak memory. It is freed when the Handle is GC'd.
> To prevent any use-after-free into this wl_listener, it removes the listener
> from the list beforehand.
> I would very much like to keep this code (since it is perfectly valid on the
> current ABI) and is good design in managed languages.

Sure, that is annoying. In hindsight, it probably wasn't a good API
for particularly the new generation of managed languages. In the
meantime, probably the easiest way to do this, and come into line with
all the other users, would be to define a separate destroy-listener
type which intentionally leaks its wl_listener link after being
signaled, rather than removing it.

Cheers,
Daniel