plasma-workspace/ksmserver

KDE session manager  (ksmserver)
--------------------------------

Matthias Ettrich <ettrich@kde.org>
Lubos Lunak <l.lunak@kde.org>

ksmserver is KDE's session management server. It talks the
standard X11R6 session management protocol (XSMP).  Thus, in theory,
it should be compatible with all session management compliant X11R6
applications. Unfortunately, there aren't that many of them. To be
precise, I have never seen a single commercial application that
supports it and even within the official X11R6 distribution, 'xclock'
is the only exception. Nevertheless we've chosen to support XSMP
despite the complexity of the protocol in order to provide KDE users
more interoperability with applications that were not explicitly
written with KDE in mind. XSMP, as an official X standard, promised to
be more likely to be supported by third parties than even a superior
KDE-specific protocol. Let's see whether we were right and more
applications will actually talk XSMP. At least all KDE applications do
now.

Here's a short overview on how session management works.

Contents
--------

Starting the server
KDE startup sequence
Establishing the connection
Authorization
Requesting a shutdown


Starting the server
-------------------

The server is usually started from the 'startkde' script. It supports the following options:

  -r, --restore             Restores the previous session if available
  -w, --windowmanager <wm>  Starts 'wm' in case no other window manager is
                            participating in the session. Default is 'kwin'

The default 'startkde' launches 'ksmserver --restore'.  The
'windowmanager' option is useful for users that prefer a window
manager other than kwin. Since the window manager has to participate
in the session (it has to remember window positions and states), it is
usually restarted when the session is restored. To be *really* sure
that this happens, even if the wm might have crashed during the
previous session, ksmserver ensures that. The option specifies, which
windowmanager shall be launched for sure. But again: if the stored
session contains a window manager, the restored one will be used, not
the specified one. As a special feature, ksmserver always starts the
specified window manager first, which results in a much nicer startup
sequence (less flashy).

KDE startup sequence
--------------------

Ksmserver controls the second part of the KDE startup sequence,
after it gets control from the startkde script, which controls
the first part of the startup sequence. All code related to startup
should be in startup.cpp and going down in that source file should
follow the startup order (but note that this is just a documentation
which may get outdated, so in case of doubts the source wins ;) ).

The startkde scripts already launches kdeinit, which in turns launches
KDE daemons like dbus daemon, klauncher and kded. Kded loads autoloaded
kded modules, i.e. those that have X-KDE-Kded-autoload=true in .desktop
files. The exact way autoloading works is controlled by X-KDE-Kded-phase=,
which may be 0, 1 or 2 (the default). Kded phase 0 means the module is
always loaded by kded, even outside of KDE session. It should used only
by kded modules which must be always running. Kded phase 1 modules are
loaded right after kded startup, but only during KDE startup, i.e. it is
for modules that are always needed by the KDE session. Phase 2 modules
will be loaded later. [More information about kded modules could be
found in kdelibs/kded/HOWTO]

Startkde also launches kcminit, which performs initialization done by kcontrol
modules. There are two kcminit phases, 0, 1, controlled
by X-KDE-Init-Phase= in the .desktop file, which defaults to 1. Phase 0 kcminit
modules should be only those that really need to be run early in the startup
process (and those should probably actually use kstartupconfig in startkde
to be done even before kdeinit and daemons). After executing phase 0
modules kcminit returns and waits.

When ksmserver is launched, the first thing it does is launching
the window manager, as the WM is necessary before any windows are possibly
shown. When the WM is ready, ksmserver tells klauncher to perform autostart
phase 0 ($KDEHOME/share/autostart). There are 3 autostart phases, 0, 1 and 2,
defined by X-KDE-autostart-phase, defaulting to 2. Phase 0 is reserved only
for the actual visible base components of KDE, i.e. Plasma, in order to make
the startup appear visually faster. Plasma uses D-Bus calls suspendStartup()
and resumeStartup() to make ksmserver stay waiting for autostart phase 0
until Plasma is ready (note: suspendStartup() needs to be called before
the application registers with ksmserver, i.e. before KApplication ctor is called).

Next step is telling the waiting kcminit to perform phase 1 - all kcminit
modules that should be executed before KDE startup is considered done.
After that ksmserver tells klauncher to perform autostart phase 1,
i.e. launching normal components of KDE that should be available right
after KDE startup, and after this session restore is performed,
i.e. launching all applications that were running during last session
saving (usually logout).

By this time KDE session is considered to be more or less ready and
ksmserver does the knotify startkde event (i.e. plays the login sound).
It also tells klauncher to perform autostart phase 2, kded to load all
remaining autoload (i.e. kded phase 2) modules, and it itself executes
the user Autostart folder.

Technical note: There's a reason why kded modules and items in autostart
default to the latest phase. Before you explicitly use a different phase,
read and understand what's above. You should also consider whether something
really needs to be launched during KDE startup and can't be loaded on-demand
when really needed. Abusing the phases will result in public spanking
for making KDE startup slower.


Establishing the connection
---------------------------

As required by the XSMP specification, the session management server
propagates its network address in the SESSION_MANAGER environment
variable. Probably not the best way to do it, but it's the way it
is. All KDE (and plain Qt) applications simply read this variable and
try to establish a connection to an XSMP server at this address. If
the variable is undefined, nothing happens.

This means, if you want to start a program that should not participate
in the session, simply undefine SESSION_MANAGER in your terminal
window and launch the application. If you want to see an application
desparately trying to connect to something, try setting it to some
bogus value.

In addition, ksmserver propagates both its network address and its
process id in ~/.kde/socket-$HOSTNAME/KSMserver-$DISPLAY. A
utility function KWorkSpace::propagateSessionManager() reads this
setting and sets SESSION_MANAGER accordingly, so that child processes
can pick it up.  The function is called by clients that are started
outside the session (i.e. before ksmserver is started), but want to
launch other processes that should participate in the session.

Authorization
-------------

XSMP is built on top of the Inter Client Exchange (ICE) protocol,
which comes standard as a part of X11R6 and later.
Authorization is done using 'iceauth', with MIT-MAGIC-COOKIE as used
by X. In order to be able to access the session management server, you
need to be able to read ~/.ICEauthority. For security reasons, we do
not provide any host-based authorization.


Requesting a shutdown
---------------------

If an application wants to request a shutdown (i.e. a logout), it does
this via an SmcRequestSaveYourself message to the server. In KDE, a
utility function KWorkSpace::requestShutDown() does exactly
this. It's for example called by KDE's panel or by the context menu of
the desktop.