Add support for flatpak portal

[gicmo]

Add support to transparently use the portal when running inside a flatpak. This is necessary because flatpak uses pid namespace isolation (see #73) and therefore process ids need to be translated for the gamemode daemon .

This is a draft PR because it depends on flatpak/xdg-desktop-portal#314

[mdiluz]

lib/client_impl.c:106 should use the correct bus name on the error message. Other than that, really nice simple change on the part of gamemode - glad a solution was found for Flatpak

[gicmo]

lib/client_impl.c:106 should use the correct bus name on the error message.

Good point! Fixed.

[gicmo]

The portal landed, so I guess this can go in.

[aejsmith]

Merged. Thanks a lot for working on a solution for this!

[FLAGEL]

Disclaimer: I am just a tinkerer, and not familiar with the inner workings of GameMode, Flatpak, or DBus.

Executing gamemoderun %command% within the Steam Flatpak [1] enables and disables GameMode on the host [2]. This can be verified via dbus-monitor --session (below) or gamemoded -s [3]. Within the Steam Flatpak it can also be verified via mangohud gamemoderun %command%, given that MangoHud is available.

However, the following errors are logged to Steam Flatpak stderr as gamemoderun exits:

** (process:562044): WARNING **: 17:55:10.780: Invalid client serial
dbus[3720]: arguments to dbus_pending_call_block() were incorrect, assertion "pending != NULL" failed in file ../../dbus/dbus-pending-call.c line 766.
This is normally a bug in some application using the D-Bus library.

dbus[3720]: arguments to dbus_pending_call_steal_reply() were incorrect, assertion "pending != NULL" failed in file ../../dbus/dbus-pending-call.c line 733.
This is normally a bug in some application using the D-Bus library.

GameMode ERROR: D-Bus error: Did not receive a reply
gamemodeauto: D-Bus error: Did not receive a reply

DBus registration:

method call time=1704210321.074698 sender=:1.3277 -> destination=org.freedesktop.portal.Desktop serial=26 path=/org/freedesktop/portal/desktop; interface=org.freedesktop.portal.GameMode; member=RegisterGameByPIDFd
   file descriptor
         inode: 2099
         type:    file descriptor
         inode: 2099
         type: method call time=1704210321.074737 sender=:1.44 -> destination=org.freedesktop.DBus serial=7943 path=/org/freedesktop/DBus; interface=org.freedesktop.DBus; member=GetConnectionUnixProcessID
   string ":1.3277"
method return time=1704210321.074746 sender=org.freedesktop.DBus -> destination=:1.44 serial=1739 reply_serial=7943
   uint32 562044
signal time=1704210321.109850 sender=:1.3018 -> destination=(null destination) serial=384 path=/com/feralinteractive/GameMode; interface=com.feralinteractive.GameMode; member=GameRegistered
   int32 565669
   object path "/com/feralinteractive/GameMode/Games/565669"
signal time=1704210321.109889 sender=:1.3018 -> destination=(null destination) serial=385 path=/com/feralinteractive/GameMode; interface=org.freedesktop.DBus.Properties; member=PropertiesChanged
   string "com.feralinteractive.GameMode"
   array [
      dict entry(
         string "ClientCount"
         variant             int32 1
      )
   ]
   array [
   ]
method return time=1704210321.109935 sender=:1.3018 -> destination=:1.44 serial=386 reply_serial=7945
   int32 0
signal time=1704210321.110067 sender=:1.44 -> destination=(null destination) serial=7946 path=/org/freedesktop/portal/desktop; interface=org.freedesktop.DBus.Properties; member=PropertiesChanged
   string "org.freedesktop.portal.GameMode"
   array [
      dict entry(
         string "Active"
         variant             boolean true
      )
   ]
   array [
   ]

DBus deregistration:

signal time=1704210743.041465 sender=:1.3018 -> destination=(null destination) serial=408 path=/com/feralinteractive/GameMode; interface=com.feralinteractive.GameMode; member=GameUnregistered
   int32 565685
   object path "/com/feralinteractive/GameMode/Games/565685"
signal time=1704210743.041517 sender=:1.3018 -> destination=(null destination) serial=409 path=/com/feralinteractive/GameMode; interface=org.freedesktop.DBus.Properties; member=PropertiesChanged
   string "com.feralinteractive.GameMode"
   array [
      dict entry(
         string "ClientCount"
         variant             int32 0
      )
   ]
   array [
   ]
signal time=1704210743.041752 sender=:1.44 -> destination=(null destination) serial=8002 path=/org/freedesktop/portal/desktop; interface=org.freedesktop.DBus.Properties; member=PropertiesChanged
   string "org.freedesktop.portal.GameMode"
   array [
      dict entry(
         string "Active"
         variant             boolean false
      )
   ]
   array [
   ]

Setup
[1] com.valvesoftware.Steam 1.0.0.78
[2] Linux host 6.6.8-arch1-1 #1 SMP PREEMPT_DYNAMIC Thu, 21 Dec 2023 19:01:01 +0000 x86_64 GNU/Linux
[3] dbus 1.14.10-1, gamemode 1.8.1-1