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Pure OCaml Wayland protocol library

• API documentation

Wayland is a communications protocol intended for use between processes on a single computer. It is mainly used by graphical applications (clients) to talk to display servers, but nothing about the protocol is specific to graphics and it could be used for other things.

A client connects to a server over a Unix-domain socket (e.g. by opening /run/user/1000/wayland-0). Conceptually, the server hosts a number of objects (initially, a single object with ID 1). The client can send "requests" to objects, and the server can send "events" from objects. On the wire, a message is an object ID, an operation ID (request or event), and a list of arguments.

The protocol is described in more detail in The Wayland Protocol (which is however only half written, as of Feb 2021).

Features

• File descriptor passing: You can use any Unix file descriptor as a message argument and it will be sent over the Unix-domain socket using SCM_RIGHTS. For example, to send a frame of video you can get a file descriptor to some shared memory, draw the image to it, and then pass the FD to the server for rendering, with no need to copy the data.

• Asynchronous: Multiple messages can be sent in a row by either side. They will be processed in order.

• Schema files: Protocols are defined using a (fairly simple) XML format. Then a language-specific tool (e.g. wayland-scanner-ocaml from this package) generates typed bindings for it.

• Extensible: Multiple services can be added to a registry object and queried at runtime.

• Versioning: The schema file indicates which operations were added in each version.

Limitations

• Local only: It is only possible to attach file descriptors to Unix-domain sockets, so the system is not very useful over TCP (unless you avoid using FDs completely). Also, there is no support for access-control (beyond Unix file permissions on the socket), encryption, etc.

• Limited type system: Argument types are limited to 32-bit integers (signed and unsigned), 24.8 fixed point, strings, byte-arrays, object IDs, and file descriptors. If you want to e.g. send a list of strings then you would create a list object, then add each item to it, then pass the list object to the target, then destroy the list. However, this can all be done in one go and is fairly efficient.

• Middleware is tricky: If you want to write a proxy process that sits between a client and a server, it will need to understand the schema (including any extensions). This is because the framing does not say which FDs go with which messages.

If you are looking for an RPC protocol for use over the Internet, consider using Cap'n Proto RPC instead.

Getting started

example/test.ml contains a simple client, based on the example in "The Wayland Protocol" book. To run it:

dune exec -- ./example/test.exe

It shows a scrolling grid of squares.

See the API documentation for more information.

Interface versions

Each proxy object has phantom types for the interface and the version(s) of that interface. e.g. a value of type ([`Wl_surface], [`V3], [`Client]) Proxy.t is a client's proxy to a version 3 surface object.

Functions that send messages require a compatible version. For example, Wl_surface.set_buffer_transform was introduced in version 2, and so it can be used with objects supporting version 2, 3 or 4:

module Wl_surface : sig
  ...
  val set_buffer_transform : [< `V2 | `V3 | `V4 ] t ->
    transform:Wayland_proto.Wl_output.Transform.t -> unit
  ...
end

In Wayland, there are two ways of introducing new objects to a conversation:

• In most cases, the schema gives the interface for the created object. In this case, the new object will have the same version as the object from which it was created.

• The registry's bind operation does not specify the interface. Instead, the client passes the interface and version as arguments.

When creating an object, you must give handlers for each message that could be received related to that object. For example, when using the compositor object to create a surface you must supply a compatible set of handlers:

module Wl_compositor : sig
  ...
  val create_surface : [< `V1 | `V2 | `V3 | `V4 ] as 'v t ->
    ([ `Wl_surface ], 'v) Proxy.Handler.t ->
    ([ `Wl_surface ], 'v) Proxy.t
  ...
end

This says that create_surface can be used with any version 'v of the compositor, but you must supply handlers for version 'v of the surface object. The return value will be a surface proxy with the same version as the compositor.

There is one handler class for each distinct version. For example, to create a handler for version 4 (or later) of the compositor interface you would have your handlers inherit from Wl_compositor.v4 (or just use it directly, since in this case there are no events to be handled). e.g.

let compositor = Registry.bind reg @@ new Wl_compositor.v4 in
(* [compositor] has type [[`V4] Wl_compositor.t]. *)

This means that you can send any compositor request that was available in version 4 (but the bind will fail if the server doesn't support this version). When you create new objects using the compositor, it will know that they also will be version 4.

To avoid an explosion of version combinations, the generated handler types require you to handle incoming messages of all later versions in your copy of the schema. For example, you can't ask for exactly version 3 of the compositor and then not bother implementing handlers for v4 events.

This library requires servers to handle all versions of the protocol in the schema (they can't specify a minimum or maximum version).

The version types should prevent you from sending messages the other side won't understand, or failing to handle a message the other side sends. However, they may be inflexible. You can use Proxy.cast_version to escape from the rules and manage things yourself if necessary.

Attaching extra data to objects

Sometimes you may pass an object to the other process and then receive it back again later via another message. This is mostly needed when implementing a server. For example, consider a client that does the following:

1. Create a Wl_region.
2. Add some rectangles to the region.
3. Pass the region to Wl_surface.set_input_region.

When the server gets the region as an argument to set_input_region it will want to recognise it as a region it created earlier, with private state holding the list of rectangles.

You can attach data to a handler by specifying method user_data when creating the handler, and get it back using Proxy.user_data.

Ideally, the application would define the user_data type, but this would require functorising the whole API and all the bindings over the type, which is inconvenient. Instead, Wayland.S.user_data is defined as an open variant type and you should extend that with a single entry for your (GADT) type. e.g.

type 'a my_user_data = 
  | Region : region_data -> [`Wl_region] my_user_data
  | Output : output_data -> [`Wl_output] my_user_data

type ('a, 'role) Wayland.S.user_data +=
    My_user_data : 'a my_user_data -> ('a, [`Client]) Wayland.S.user_data

let user_data (proxy : ('a, _, 'role) Proxy.t) : 'a my_user_data =
  match Wayland.Proxy.user_data proxy with
  | My_user_data x -> x
  | S.No_data -> Fmt.failwith "No data attached to %a!" Proxy.pp proxy
  | _ -> Fmt.failwith "Unexpected data attached to %a!" Proxy.pp proxy

Then when you receive a region proxy, do:

method on_set_input_region _ ~region =
  let Region r = user_data region in
  ...

Because the compiler knows that region is of type Wl_region, it knows that the user data will be of type [`Wl_region] my_user_data, and so there is only case you have to handle. To avoid the exceptions, you just need to ensure that:

1. You don't extend Wayland.S.user_data with any other variants.
2. You don't forget to attach the data when creating the handler.

Resource lifetimes

The Wayland spec is a bit vague about object lifecycles. This is my guess about how it's supposed to work.

Some message types are marked as "destructors". Usually the client sends a destructor request and the server acknowledges it. In some cases (e.g. callbacks, which have no requests), the server calls the destructor.

After sending a destructor message, you cannot reference that object in any further messages. Except that the server can call a destructor and then also send a delete event mentioning the same object.

The client cannot reuse an ID until the server has confirmed the deletion, since otherwise it wouldn't know whether a new message was for the old or new object.

Only servers can confirm object deletion though, so if the server calls a destructor then there has to be some interface-specific method for the client to call to confirm the deletion. Also, servers only send delete confirmations for IDs the client allocated.

Examples:

1. The client creates a surface. Then it calls destroy. The client must no longer send messages to the surface, but continues to receive events about it. When the service sends delete, the client can reuse the ID.

2. The client creates a callback. The service calls done on it (a destructor), and then immediately sends a delete for it. There's no race here because the callback doesn't accept any messages from the client.

3. The server creates data offer. Then it creates another one. This second one implicitly destroys the original. The server must no longer send messages from the original offer, but may receive requests to it from the client. The client responds by calling the release method on the old selection. The server does not send any confirmation for that; the release is itself the confirmation of the implicit deletion from the selection event.

Unlike other handlers, client destructor handlers do not take a proxy argument since the proxy is unusable by this point.

On the server side, the handler will normally respond to a destructor call by calling Proxy.delete immediately. However, when relaying to an upstream service it may be useful to delay this until the upstream service has confirmed the deletion too.

Adding or updating protocols

Each protocol is generated from an xml file vendored under the protocols directory. There is an update.sh shell script which pulls down the latest version of all of the protocols when run.

When adding a new protocol, add an entry in the update.sh and the protocols/dune file. The --open flag is a comma-separated list of the protocol modules which the new protocol is dependent on.