[RFC wayland-protocols v4] Add Primary Selection Protocol Version 1

Michal Suchanek hramrach at gmail.com
Mon Feb 22 15:53:43 UTC 2016

On 22 February 2016 at 15:57, Carlos Garnacho <carlosg at gnome.org> wrote:
> Hi Michal,
> On Mon, Feb 22, 2016 at 2:25 PM, Michal Suchanek <hramrach at gmail.com> wrote:
>> Hello,
>> On 20 February 2016 at 01:31, Carlos Garnacho <carlosg at gnome.org> wrote:
>>> +
>>> +  <description summary="Primary selection protocol">
>>> +    This protocol provides the ability to have a primary selection device to
>>> +    match that of the X server. This primary selection is a shortcut to the
>>> +    common clipboard selection, where text just needs to be selected in order
>>> +    to allow copying it elsewhere. The de facto way to perform this action
>>> +    is the middle mouse button, although it is not limited to this one.
>>> +
>>> +    Clients wishing to honor primary selection should create a primary
>>> +    selection source and set it as the selection through
>>> +    wp_primary_selection_device.set_selection whenever the text selection
>>> +    changes. In order to minimize calls in pointer-driven text selection,
>>> +    it should happen only once after the operation finished. Similarly,
>>> +    a NULL source should be set when text is unselected.
>>> +
>>> +    wp_primary_selection_offer objects are first announced through the
>>> +    wp_primary_selection_device.data_offer event. Immediately after this event,
>>> +    the primary data offer will emit wp_primary_selection_offer.offer events
>>> +    to let know of the mime types being offered.
>>> +
>>> +    When the primary selection changes, the client with the keyboard focus
>>> +    will receive wp_primary_selection_device.selection events. Only the client
>> Why keyboard focus?
>> Since paste is done mainly using mouse this has nothing to do with
>> keyboard focus.
> Doing this so allows us to behave just the same than we do with the
> core protocol selection, slightly divergent protocols make sharing
> code harder.
> Conceptually, it also makes some sense to me. I argue that a logical
> "key" focus is needed in compositors, even on lack of wl_keyboard
> capabilities. Things that IMO make sense to tie together in this
> focus, per-seat are:
> - wl_keyboard focus
> - wp_text_input focus
> - focus por (possibly several) pads/buttonsets
> - clipboard selection
> - primary selection
> Of course these are only guidelines, and compositors may attempt to
> implement split foci for these. But still, selection should be tied to
> some definite focus, the other option is broadcasting, and I'd very
> much prefer not to do that.
> I may try to change the wording just to suggest it's loosely attached
> to keyboard focus though.

If you put an Insert sticker on your pad button and bind pasting to
that pad button and the pad focus is not tied to keyboard focus you
have potentially a problem there.

>>> +    with the keyboard focus will receive such events with a non-NULL
>>> +    wp_primary_selection_offer. Across keyboard focus changes, previously
>>> +    focused clients will receive wp_primary_selection_device.events with a
>>> +    NULL wp_primary_selection_offer.
>>> +
>>> +    In order to request the primary selection data, the client must pass
>>> +    a recent serial pertaining to the press event that is triggering the
>>> +    operation, if the compositor deems the serial valid and recent, the
>> Why press event when it has an offer event to base the request on?
>> There is no need to involve other unrelated events.
> IIRC The first protocol drafts attempted to limit the circumstances in
> which a client could read the primary selection. This is a change of
> approach.
>> IMHO the fact that the application receives ANY input event suffices.
>> eg. a pointer entry event.
> Do you mean wl_pointer.enter should be enough to have the application
> read the primary selection? seems open to data leaks to me.
> This serial event is meant to check for user interaction rather than
> "any input event", so just focusing a client is not enough to have it
> retrieve the primary selection.

And why is clicking enough and focusing not?

Accidentally clicking an application can happen as much as
accidentally pointing at it. With touch interface it's pretty much the
same thing. With click-to-focus also. If you want to prevent data
leaks you can unmap windows that should not receive the paste or use a
compositor with per-application access policy for clipboards.

So instead of saying that a button down event should trigger paste in
the protocol specification it is wiser to say that sending the paste
offer to a client can happen at the discretion of the compositor and
suggest some reasonable policy.

Compositor should be free to implement any policy the author finds
reasonable including broadcast on selection change, point to paste,
button press to paste, and per-application different policies.

>> Otherwise you are going to have very fragile protocol that often fails
>> because the application did not happen to receive whatever even is
>> requested by the protocol.
> Note that it just mentions a press event, but does no mention of the
> interface/device it comes from. It can be the serial received on
> wl_pointer.button, wl_keyboard.key, wl_touch.down,
> wp_tablet_tool.down, ...  compositors should ideally check all input
> interfaces.

They need not check anything convoluted if they check that the client
is allowed to paste on sending the offer and only check that the
client acts on valid offer.

So instead of doing some check that it's ok to send offer and then
another check to see if some random event is pasty enough to act on a
paste request you just check that the client is allowed to receive the

>> It's even worse with the keyboard focus. If the event that triggers
>> the paste also triggers getting keyboard focus you are going to have
>> protocol open to all kind of ugly race conditions. If it does not
>> trigger getting the keyboard focus the paste just fails.
> Where do you see race conditions? Because focus is handled by the
> compositor before event emission, AFAICT the client would receive the
> following event sequence:
> -->
> wl_keyboard.enter
> wl_data_device.data_offer
> wl_data_offer.offer
> [...]
> wl_data_device.selection
> zwp_primary_selection_device_v1.data_offer
> zwp_primary_selection_offer_v1.offer
> [...]
> zwp_primary_selection_device_v1.selection
> wl_pointer.button (..., serial_you_want_to_pass)
> <--
> zwp_primary_selection_offer_v1.receive (..., serial_you_want_to_pass)
> Which results in the expected action to take place.
>> There are point-to-type and click-to-type keyboard focus models which
>> should be both supported by the primary selection protocol.
> I think both models agree in that the window will be focused after you
> clicked on it.

ok, so this should work so long as the compositor likes the random
event serial you pick.



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