portal-impl: Only return found implementation if it launched

[jadahl]

If no portal backend for a given interface is found, a fallback is always tried anyway, despite that fallback not being listed as compatible with the current desktop environment.

Sometimes it's good that a fallback is returned; e.g. the xdg-desktop-portal-gtk file chooser backend is technically usable anywhere, however, some backends might be specifically designed to only work in a specific desktop environment, e.g. xdg-desktop-portal-gnome.

In order to avoid creating portals with non-functional backends, make sure it's possible to create a proxy object for the interface and D-Bus name, and that it launched successfully (i.e. has no name owner after creating the proxy).

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Opening this against xdg-desktop-portals-1.16 because hopefully #955 will make this obsolete.

Not sure how this will work on non-systemd systems; it relies on using g_dbus_proxy_get_name_owner() being set after D-Bus activation did its thing, which is true for the portals I have managed to test (xdg-desktop-portal-{gnome,gtk,kde})

[smcv]

it relies on using g_dbus_proxy_get_name_owner() being set after D-Bus activation did its thing

That should be equally valid for traditional activation in dbus-daemon: in traditional activation, the signal that the service is ready is exactly the name being requested.

It would be good to get a dbus-monitor trace with each of dbus-daemon and dbus-broker, to make sure that the NameOwnerChanged signal happens before the successful method return.

[jadahl]

Gather some logs on a system using dbus-broker (F38 beta):

method call time=1679309435.630877 sender=:1.2 -> destination=org.freedesktop.systemd1 serial=121 path=/org/freedesktop/system
d1; interface=org.freedesktop.systemd1.Manager; member=StartUnit
   string "xdg-desktop-portal-gnome.service"
   string "replace"

method return time=1679309435.633568 sender=:1.1 -> destination=:1.2 serial=1691 reply_serial=121
   object path "/org/freedesktop/systemd1/job/554"

signal time=1679309435.831621 sender=org.freedesktop.DBus -> destination=(null destination) serial=4294967295 path=/org/freedesktop/DBus; interface=org.freedesktop.DBus; member=NameOwnerChanged
   string "org.freedesktop.impl.portal.desktop.gnome"
   string ""
   string ":1.89"

In other words, it looks like NameOwnerChanged happens after the method is returned. For GDBus that's not a problem; it calls GetNameOwner after StartServiceByName returned and gets the name that way.

[jadahl]

@smcv any concerns left?

[GeorgesStavracas]

Let's go with this for 1.16. For 1.18, the plan is to land Emmanuele's rework anyway.