/** \page page_access Access Control

This document explains how access control is designed and implemented.

PipeWire implements per client permissions on the objects in the graph.
Permissions include `R` (read), `W` (write), `X` (execute) and `M` (metadata).

- `R`: An object with permission `R` is visible to the client. The client will receive
  registry events for the object and can interact with it.
- `W`: An object with permission `W` can be modified. This is usually done
  through a method that modifies the state of the object. The `W` permission
  usually implies the `X` permission.
- `X`: An object with permission `X` allows invoking methods on the object.
  Some of those methods will only query state, others will modify the object.
  As said above, modifying the object through one of these methods requires
  the `W` permission.
- `M`: An object with `M` permission can be used as the subject in metadata.

Clients with all permissions set are referred to as "ALL" in the
documentation.


# Use Cases

## New Clients Need Their Permissions Configured

A new client is not allowed to communicate with the PipeWire daemon until
it has been configured with permissions.

## Flatpaks Can't Modify Other Stream/Device Volumes

An application running as Flatpak should not be able to modify the state of
certain objects. Permissions of the relevant PipeWire objects should not have
the `W` permission to avoid this.

## Flatpaks Can't Move Other Streams To Different Devices

Streams are moved to another device by setting the `target.node` metadata
on the node ID. By not setting the `M` bit on the other objects, this can be
avoided.

## Application Should Be Restricted In What They Can See

In general, applications should only be able to see the objects that they are
allowed to see. For example, a web browser that was given access to a camera
should not be able to see (and thus receive input data from) audio devices.

## "Manager" Applications Require Full Access

Some applications require full access to the PipeWire graph, including
moving streams between nodes (by setting metadata) and modifying properties
(eg. volume). These applications must work even when running as Flatpak.


# Design

## The PipeWire Daemon

Immediately after a new client connects to the PipeWire daemon and updates
its properties, the client will be registered and made visible to other
clients.

The PipeWire core will emit a `check_access` event in the \ref pw_context_events
context for the the new client. The implementer of this event is responsible
for assigning permissions to the client.

Clients with permission `R` on the core object can continue communicating
with the daemon. Clients without permission `R` on the core are suspended
and are not able to send more messages.

A suspended client can only resume processing after some other client
sets the core permissions to `R`. This other client is usually a session
manager, see e.g. \ref page_session_manager.

## The PipeWire Access Module

The \ref page_module_access hooks into the `check_access` event that is
emitted when a new client is registered. The module checks the permissions of
the client and stores those in the \ref PW_KEY_ACCESS
property on the client object. If this property is already set, the access
module does nothing.

If the property is not set it will go through a set of checks to determine
the permissions for a client. See the \ref page_module_access documentation
for details, particularly on the values documented below. Depending on the
value of the \ref PW_KEY_ACCESS property one the following happens:

- `"allowed"`, `"unrestricted"`:  ALL permissions are set on the core
    object and the client will be able to resume.
- `"restricted"`, `"flatpak"`, `"$access.force"`:  No permissions are set on
    the core object and the client will be suspended.
- `"rejected"`: An error is sent to the client and the client is
    suspended.

As detailed above, the client may be suspended. In that case the session
manager or another client is required to configure permissions on the object
for it to resume.

## The Session Manager

The session manager listens for new clients to appear. It will use the
\ref PW_KEY_ACCESS property to determine what to do.

For clients that are suspended with `"restricted"`, `"flatpak"` or
`"$access.force"` access, the session manager needs to set permissions on the
client for the various PipeWire objects in the graph that it is allowed to
interact with.  To resume a client, the session manager needs to set
permission `R` on the core object for the client.

Permissions of objects for a client can be changed at any time by the
session manager. Removing the client core permission `R` will suspend the
client.

The session manager needs to do additional checks to determine if the
manager permissions can be given to the particular client and then
configure ALL permissions on the client. Possible checks include
permission store checks or ask the user if the application is allowed
full access.

Manager applications (ie. applications that need to modify the graph) will
set the \ref PW_KEY_MEDIA_CATEGORY property in the client object to "Manager".

For details on the pipewire-media-session implementation of access control,
see \ref page_media_session.

*/
