logind: implement generic multi-session

This enables the multi-session capability for seats that don't have VTs.
For legacy seats with VTs, everything stays the same. However, all other
seats now also get the multi-session capability.

The only feature that was missing was session-switching. As logind can
force a session-switch and signal that via the "Active" property, we only
need a way to allow synchronized/delayed session switches. Compositors
need to cleanup some devices before acknowledging the session switch.
Therefore, we use the session-devices to give compositors a chance to
block a session-switch until they cleaned everything up.

If you activate a session on a seat without VTs, we send a PauseDevice
signal to the active session for every active device. Only once the
session acknowledged all these with a PauseDeviceComplete() call, we
perform the final session switch.

One important note is that delayed session-switching is meant for
backwards compatibility. New compositors or other sessions should really
try to deal correctly with forced session switches! They only need to
handle EACCES/EPERM from syscalls and treat them as "PauseDevice" signal.

Following logind patches will add a timeout to session-switches which
forces the switch if the active session does not react in a timely
fashion. Moreover, explicit ForceActivate() calls might also be supported.
Hence, sessions must not crash if their devices get paused.

1 files changed, 1 insertions, 0 deletions

diff --git a/src/login/logind-session-device.h b/src/login/logind-session-device.h
index 511fce0e61..eac7ca7a63 100644
--- a/src/login/logind-session-device.h
+++ b/src/login/logind-session-device.h
@@ -57,3 +57,4 @@ void session_device_complete_pause(SessionDevice *sd);
 
 void session_device_resume_all(Session *s);
 void session_device_pause_all(Session *s);
+unsigned int session_device_try_pause_all(Session *s);