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![Zbigniew Jędrzejewski-Szmek](/assets/img/avatar_default.png)
This reverts commit 5e8ff010a1
.
This broke all the URLs, we can't have that. (And actually, we probably don't
_want_ to make the change either. It's nicer to have all the pages in one
directory, so one doesn't have to figure out to which collection the page
belongs.)
119 lines
5.5 KiB
Markdown
119 lines
5.5 KiB
Markdown
---
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title: Desktop Environment Integration
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category: Concepts
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layout: default
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SPDX-License-Identifier: LGPL-2.1-or-later
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---
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# Desktop Environments
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NOTE: This document is a work-in-progress.
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## Single Graphical Session
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systemd only supports running one graphical session per user at a time.
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While this might not have always been the case historically, having multiple
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sessions for one user running at the same time is problematic.
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The DBus session bus is shared between all the logins, and services that are
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started must be implicitly assigned to the user's current graphical session.
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In principle it is possible to run a single graphical session across multiple
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logind seats, and this could be a way to use more than one display per user.
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When a user logs in to a second seat, the seat resources could be assigned
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to the existing session, allowing the graphical environment to present it
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is a single seat.
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Currently nothing like this is supported or even planned.
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## Pre-defined systemd units
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[`systemd.special(7)`](https://www.freedesktop.org/software/systemd/man/systemd.special.html)
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defines the `graphical-session.target` and `graphical-session-pre.target` to
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allow cross-desktop integration. Furthermore, systemd defines the three base
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slices `background`, `app` and `session`.
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All units should be placed into one of these slices depending on their purposes:
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* `session.slice`: Contains only processes essential to run the user's graphical session
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* `app.slice`: Contains all normal applications that the user is running
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* `background.slice`: Useful for low-priority background tasks
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The purpose of this grouping is to assign different priorities to the
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applications.
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This could e.g. mean reserving memory to session processes,
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preferentially killing background tasks in out-of-memory situations
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or assigning different memory/CPU/IO priorities to ensure that the session
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runs smoothly under load.
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TODO: Will there be a default to place units into e.g. `app.slice` by default
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rather than the root slice?
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## XDG standardization for applications
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To ensure cross-desktop compatibility and encourage sharing of good practices,
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desktop environments should adhere to the following conventions:
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* Application units should follow the scheme `app[-<launcher>]-<ApplicationID>[@<RANDOM>].service`
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or `app[-<launcher>]-<ApplicationID>-<RANDOM>.scope`
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e.g:
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- `app-gnome-org.gnome.Evince@12345.service`
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- `app-flatpak-org.telegram.desktop@12345.service`
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- `app-KDE-org.kde.okular@12345.service`
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- `app-org.kde.amarok.service`
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- `app-org.gnome.Evince-12345.scope`
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* Using `.service` units instead of `.scope` units, i.e. allowing systemd to
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start the process on behalf of the caller,
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instead of the caller starting the process and letting systemd know about it,
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is encouraged.
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* The RANDOM should be a string of random characters to ensure that multiple instances
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of the application can be launched.
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It can be omitted in the case of a non-transient application services which can ensure
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multiple instances are not spawned, such as a DBus activated application.
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* If no application ID is available, the launcher should generate a reasonable
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name when possible (e.g. using `basename(argv[0])`). This name must not
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contain a `-` character.
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This has the following advantages:
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* Using the `app-<launcher>-` prefix means that the unit defaults can be
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adjusted using desktop environment specific drop-in files.
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* The application ID can be retrieved by stripping the prefix and postfix.
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This in turn should map to the corresponding `.desktop` file when available
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TODO: Define the name of slices that should be used.
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This could be `app-<launcher>-<ApplicationID>-<RANDOM>.slice`.
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TODO: Does it really make sense to insert the `<launcher>`? In GNOME I am
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currently using a drop-in to configure `BindTo=graphical-session.target`,
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`CollectMode=inactive-or-failed` and `TimeoutSec=5s`. I feel that such a
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policy makes sense, but it may make much more sense to just define a
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global default for all (graphical) applications.
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* Should application lifetime be bound to the session?
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* May the user have applications that do not belong to the graphical session (e.g. launched from SSH)?
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* Could we maybe add a default `app-.service.d` drop-in configuration?
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## XDG autostart integration
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To allow XDG autostart integration, systemd ships a cross-desktop generator
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to create appropriate units for the autostart directory
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(`systemd-xdg-autostart-generator`).
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Desktop Environments can opt-in to using this by starting
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`xdg-desktop-autostart.target`. The systemd generator correctly handles
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`OnlyShowIn=` and `NotShowIn=`. It also handles the KDE and GNOME specific
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`X-KDE-autostart-condition=` and `AutostartCondition=` by using desktop-environment-provided
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binaries in an `ExecCondition=` line.
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However, this generator is somewhat limited in what it supports. For example,
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all generated units will have `After=graphical-session.target` set on them,
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and therefore may not be useful to start session services.
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Desktop files can be marked to be explicitly excluded from the generator using the line
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`X-systemd-skip=true`. This should be set if an application provides its own
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systemd service file for startup.
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## Startup and shutdown best practices
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Question here are:
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* Are there strong opinions on how the session-leader process should watch the user's session units?
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* Should systemd/logind/… provide an integrated way to define a session in terms of a running *user* unit?
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* Is having `gnome-session-shutdown.target` that is run with `replace-irreversibly` considered a good practice?
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