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

[Carlos Garnacho]

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.

>
>> +    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.

>
> 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.

>
> 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.

Cheers,
  Carlos