[RFC] Interface for injection of input events

Christopher James Halse Rogers christopher.halse.rogers at canonical.com
Wed Mar 22 06:35:40 UTC 2017

On Wed, Mar 22, 2017 at 1:24 PM Peter Hutterer <peter.hutterer at who-t.net>

> Hi all,
> This is an RFC for a new interface to generate input events from arbitrary
> clients. Note that this is only a few days old, so **do not** assume this
> is
> anything more a thought experiment right now. This email is supposed to
> start a
> discussion and collection of the various points that need to be addressed.
> First: why? There are some commandline tools like xdotool that allow for
> some
> scripting of desktop interactions. xdotool supports two categories: input
> device emulation and window management.
> This RFC primarily addresses the input device emulation bits but there is
> room for adding window management capabilities. I have a basic repo here:
> http://github.com/whot/woodotool but it doesn't contain much beyond what's
> in this email.
> This will be a discussion of the interface only because the implementations
> are so specific that there is no real code-sharing beyond the interface
> itself. I've talked privately to some of you already, the general mood is
> somewhere around reluctant acceptance.
> So here's a list of talking points:
> == DBus ==
> What we need is basic IPC and not directly Wayland related, DBus provides a
> bunch of extras over the wayland protocol: introspection, ease of
> extensibility, bindings, etc. Also, Mir.
> == Functionality-based interfaces ==
> We need a mix of capabilities and features, not all of which will/should be
> available to all clients. Right now, I have two for devices:
>  org.freedesktop.WoodoTool.Keyboard (Press, Release)
>  org.freedesktop.WoodoTool.Mouse (Press, Release, MoveRelative,
> MoveAbsolute)
> Compositors can implement one, both, either, etc. For future extensions,
> having a Touch interface, Joystick, or whatever is deemed useful is
> technically trivial.
> There's a manager interface too but that's a technical detail, see the repo
> for more details.
> == Control of the event stream ==
> The events are coming in through a custom interface, so it's relatively
> trivial to ignore events based on context, e.g. ignore fake key events
> while
> the screen is locked. Any uinput-based solution would lack this context.
> == Authentication/Identification ==
> The goal is to filter clients based on some white/blacklist, so that e.g.
> xdotool can access this interface but others cannot.
> This is a big ¯\_(ツ)_/¯ for now, I don't now how to do this reliably.
> It's trivial to do per user, but per-process is difficult. DBus filters
> are largely limited to per-users. It's possible to get the process ID of a
> sender but going beyond that is unreliable (kernel doesn't guarantee comm
> being accurate).

For Ubuntu, at least, we use the apparmor dbus integration to filter dbus
on a pretty granular level. If push came to shove it would presumably be
reasonably easy to add an selinux query in the same path to do the same.
*waves hands*

> == Use for testing ==
> Came up several times, I'm not yet convinced this is a good compositor
> testing
> interface beyond basic input. A good testing interface likely requires
> something more compositor-specific where more of the window state is made
> available. But it could work as an interim solution.
> Toolkits seem to have pretty usable introspection capabilities - for
example, there are lots of automated tests of the Ubuntu Touch devices that
use Qt hooks to verify lots of behaviours and need only a way to generate
input events and lookup screen coordinates from client coordinates.

It'll be less useful for testing the compositor itself, though (unless you
happen to have written the compositor in an introspectable toolkit).

> == Input coordinate handling ==
> Keys and mouse buttons are trivial unless we want custom focus (we don't).
> Relative coordinates are ok too, absolute ones are trickier because they
> rely on screen layout not available to the client.
> So the specification needs to include a behaviour we can stick to forever,
> something like "in pixels as measured from the logical top-left of the
> top-left-most screen" etc. Not difficult per se, but this stuff is usually
> prone to corner-case unhappiness.
> == Input coordinate filtering ==
> One use-case that was mentioned to me was effectively "intercept button X
> and send key events 'abc' instead". This is not possible with the current
> proposal and I'm not sure it can be without going overboard with
> specifications. It *may* be possible to provide some global hotkey hooks I
> have not come up with a good solution to that.
> == Window management ==
> This is beyond the current suggestion but also where it gets both
> interesting and difficult.
> I have a sample org.freedesktop.WoodoTool.Desktop interface that would send
> Edge signals to clients. The compositor decides when these are triggered,
> the client can react to these things with custom commmands.
> But this is where we get into the proper scripting territory and that's
> also
> where the opinions will diverge quite quickly.
> For example, xdotool supports things like "search for window with name
> 'foo'
> and activate it". Implementing this is ... tricky. Now we need some
> definition of what a window classifies as and how windows are sorted within
> a process - compare multiple GIMP windows with multiple gnome-terminal
> windows for example. Not specifying exactly what order we return leaves us
> open to behavioural dependencies which may break in the future.
> In other words, an interface to search for windows of a given application
> is technically feasable but extremly hard to get right.
> Anyway, with the separation of interfaces this is not something we need in
> the first iterations. But one legitimate question is whether just an
> implementation for virtual input devices is sufficient or whether it's
> largely pointless without any additional window-management capabilities.
> I'm *only* interested in the virtual input device support, so an
implementation of that is fine for me :).
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