libei

libei is a library for Emulated Input, primarily aimed at the Wayland stack. It provides three parts:

• 🥚 EI (Emulated Input) for the client side (libei)
• 🍦 EIS (Emulated Input Server) for the server side (libeis)
• 🍚 REIS (Restrictions for the EIS) for the portal in between (libreis)

The communication between these parts is an implementation detail, neither client nor server need to care about the details. Let's call it the BRidge for EI, or 🥣 brei.

For the purpose of this document, libei refers to the project, libei/libeis/libreis to the libraries provided.

In the Wayland stack, the EIS server component is part of the compositor, the EI client component is part of the Wayland client.

    +--------------------+             +------------------+
    | Wayland compositor |---wayland---| Wayland client B |
    +--------------------+\            +------------------+
    | libinput | libeis  | \_wayland______
    +----------+---------+                \
        |          |           +-------+------------------+
 /dev/input/       +---brei----| libei | Wayland client A |
                               +-------+------------------+

The use-cases libei attempts to solve are:

• on-demand input device emulation, e.g. xdotool or more generically the XTEST extension
• input forwarding, e.g. synergy

libei provides three benefits:

• separation
• distinction
• control

libei provides separation of emulated input from normal input. Emulated input is a distinct channel for the compositor and can thus be handled accordingly. For example, the compositor may show a warning sign in the task bar while emulated input is active.

The second benefit is distinction. Each libei client has its own input device set, the server is always aware of which client is requesting input at any time. It is possible for the server to treat input from different emulated input devices differently.

The server is in control of emulated input - it can filter input or discard at will. For example, if the current focus window is a password prompt, the server can simply discard any emulated input. If the screen is locked, the server can suspend all emulated input devices.

For the use-case of fowarding input (e.g. synergy) libei provides capability monitoring. As with input emulation same benefits apply - input can only be forwarded if the compositor explicitly does so.

Why not $foo?

We start from the baseline of: "there is no emulated input in Wayland (the protocol)".

There is emulated input in X through XTEST but it provides neither separation, distinction nor control in a useful manner. There are however many X clients that require XTEST to work.

There are several suggestions that overlap with libei, with the main proposals being:

• a Wayland protocol for virtual input
• a (compositor-specific) DBus interface for virtual input

Emulated input is not specifically Wayland-y. Clients that emulate input generally don't care about Wayland itself. It's not needed to emulate events on their own surfaces and Wayland does not provide global state. The only connection to Wayland is merely that input events are received through the Wayland protocol. So a Wayland protocol for emulating input is not a great fit, it merely ticks the convenient box of "we already have IPC through the wayland protocol, why not just do it there".

DBus is the most prevalent generic IPC channel on the Linux desktop but it's not available in some compositors. Any other specific side-channel requires an IPC mechanism to be implemented in the sender and receiver.

The current situation looks like that neither proposal will be universally available. Wayland clients (including Xwayland) would need to support any combination of methods.

libei side-steps this issue by making the communication itself a an implementation detail and providing different negotiation backends. A client can attempt to establish a libei context through a Flatpak Portal first and all back onto a public DBus interface and fall back onto e.g. a named UNIX socket. All with a few lines of code only. There is only one spot the client has to care about this, the actual emulation of input is identical regardless of backend.

High-level summary

A pseudo-code implementation for server and client are available in the examples/ directory.

The server starts a libeis context (which can be integrated with flatpak portals) and uses the libeis file descriptor to monitor for client requests.

A client starts a libei context and connects to the server - either directly, via DBus or via a portal. The server (or the portal) approves or denies the client. After successful authentications the client can request the creation of a device with capabilities pointer, keyboard or touch.

The client triggers input events on this device, the server receives those as events through libeis and can forwards them as if they were libinput events. The server has control of the client stream. If the stream is paused, events from the client are discarded. If the stream is resumed, the server will receive the events (but may discard them anyway depending on local state).

The above caters for the xdotool use-case.

The client may request to monitor a capability. When the server deems the client to be in-focus, it forwards events from real devices to the client. The decision of what constitutes logical focus and what events to forward are up to the server.

For a synergy use-case, the setup requires:

• synergy-client on host A monitoring the mouse and keyboard capabilities
• synergy-server on host B requesting a mouse/keyboard capability device
• when synergy-client receives events via libei from compositor A it forwards those to the remote synergy-server which sends them via libei to the compositor B.

The compositor may choose to implement a hotkey to start/stop the events or it may implement the screen edges to be the hot key.

Using REIS

libreis is designed to allow a third-party that does not have a full context to manage restrictions. This is aimed at portals that only have the file descriptor to the EIS implementation but cannot initiate a full EI context.

libreis works so that this third-party can configure the EIS implementation to restrict the abilities of an EI client (later) connected to this implementation. For example, a portal can set the client name using libreis based on the app-id. A client cannot override this name later.

Differences between XTest vs libei

libei functionality is a superset of XTest's input emulation which consists of a single request, XTestFakeInput. This request allows emulation of button, key and motion events, including X Input 1.x events (but not XI2). So libei can be a drop-in replacement since it supports the same functionality and more.

However, XTest is an X protocol extension and users of XTest usually obtain more information out-of-band ("out-of-band" here means "not through XTest but instead other X protocol requests").

One example is xdotool which does window focus and modifier mangling (see below). Window focus notification is not available to a pure libei client and would have to be obtained or handled on a separate channel, e.g. X or Wayland. Having said that, a Wayland client does not usually have acess to query or modifiy the window focus.

Modifiers in xdotool are handled by obtaining the modifier mask from the X server, identifying any difference to the intended mask and emulating key events to change the modifier state to the intended one. For example, if capslock is on, xdotool would send a capslock key event first (thus disabling capslock) and then the actual key sequence. This is followed by another capslock key event to restore the modifier mask.

This is not possible for a pure libei client as the modifier state is maintained by the windowing system (if any). A client can obtain the modifier state on Wayland on wl_keyboard.enter but when the client is in-focus, there is rarely a need to emulate events.

Overall, it is best to think of libei devices as virtual equivalents to a hardware device.

Open questions

Flatpak integration

Where flatpak portals are in use, libei can communicate with the portal through a custom backend. The above diagram modified for Flatpak would be:

    +--------------------+
    | Wayland compositor |_
    +--------------------+  \
    | libinput | libeis  |   \_wayland______
    +----------+---------+                  \
        |     [eis-0.socket]                 \
 /dev/input/     /   \\       +-------+------------------+
                |      ======>| libei | Wayland client A |
                |      after    +-------+------------------+
         initial|     handover   /
      connection|               / initial request
                |              /  dbus[org.freedesktop.portal.EmulatedInput]
        +--------------------+
        | xdg-desktop-portal |
        +--------------------+

The current approach works so that

• the compositor starts an libeis socket backend at $XDG_RUNTIME_DIR/eis-0
• xdg-desktop-portal provides org.freedesktop.portal.EmulatedInput
• a client connects to the xdg-desktop-portal to request emulated input
• xdg-desktop-portal authenticates a client and opens the initial connection to the libeis socket. It restricts the capabilities available on that socket (e.g. sets the client name based on app-id using libreis).
• xdg-desktop-portal hands over the file descriptor to the client which can initialize a libei context
• from then on, libei and libeis talk directly to each other, the portal has no further influence.

This describes the current implementation. Changes to this approach are likely, e.g. the portal may control suspending/resuming devices (in addition to the server). The UI for this is not yet sorted.

Authentication

Sandboxing is addressed via flatpak portals but a further level is likely desirable, esp. outside flatpak. The simplest solution is the client announcing the name so the UI can be adjusted accordingly. API wise-maybe an opaque key/value system so the exact auth can be left to the implementation.

Triggers

For synergy we need capability monitoring started by triggers, e.g. the client requests a pointer capability monitoring when the real pointer hits the screen edge. Or in response to a keyboard shortcut.

Keyboard layouts

The emulated input may require a specific keyboard layout, for example for softtokens (usually: constant layout "us") or for the synergy case where the remote keyboard should have the same keymap as the local one, even where the remote host is configured otherwise.

libei provides keymap negotation: the client can pick a keymap, the server can accept it, refuse it, or override it with its own. In the latter two cases it is up to the client to handle the result.

Modifier state handling, group handling, etc. is still a private implementation so even where the server supports individual keymaps. So it remains to be seen if this approach is sufficient.

Xwayland and XTEST

There are PoC implementations of using libei within Xwayland and connecting it to a libeis context in the compositor (PoC with Weston). This allows Xwayland to intercept XTEST events and route those through the compositor instead.

    +--------------------+             +------------------+
    | Wayland compositor |---wayland---| Wayland client B |
    +--------------------+\            +------------------+
    | libinput | libeis  | \_wayland______
    +----------+---------+                \
        |          |           +-------+------------------+
 /dev/input/       +---brei----| libei |     XWayland     |
                               +-------+------------------+
                                                |
                                                | XTEST
                                                |
                                         +-----------+
                                         |  X client |
                                         +-----------+

Of course, XWayland is just another Wayland compositor, so the connection between libei and libeis could be handled through a portal.

Short-lived applications

libei is not designed for short-lived fire-and-forget-type applications like xdotool. It provides context and device negotiation between the server and the client - the latter must be able to adjust to limitations the server imposes.

The current implemtation of the protocol does not allow for a libei client to send all requests in bulk and exit. The decision on whether to accept a device is ultimately made by the caller implementation and non-deterministic. For libei to support a batch request, someone would have to wait. It cannot be the server as the exact requirements are unknown: do we pause processing on the client altogether? We may miss a disconnect event? Do we pause processing for one device only? But then we may be re-ordering input events and cause havoc.

It could be libei itself to implement these event queues but this too can mess with the input order. And implementing an event queue is not hard, so this issue is punted to the caller instead. XWayland in its current implementation already does this.

uinput vs libei

uinput is a Linux kernel module that allows creating /dev/input/event-compatible devices. Unlike XTest it is independent of a windowing system but requires write access to /dev/uinput, usually limited to root. uinput devices are effectively identical to physical devices and will thus work on the tty and in any windowing system.

From the POV of a libei client, uinput is a server implementation detail. The client does not need to know that the manager employs uinput to create the devices, it merely connects to an available EIS instance.

    +---------+
    | server  |___auth channel__________
    +---------+                         \
    | libeis  |-             +---------------------+
    +---------+  \____brei___| libei | application |
        |                    +---------------------+
 /dev/uinput/

Note that the server would likely need some authentication channel to verify which client is allowed to emulate input devices at the kernel level (polkit? config files?). This however is out of scope for libei. An example uinput server is implemented in the eis-server-demo in the with libei repository.