core: implement OOMPolicy= and watch cgroups for OOM killings

This adds a new per-service OOMPolicy= (along with a global
DefaultOOMPolicy=) that controls what to do if a process of the service
is killed by the kernel's OOM killer. It has three different values:
"continue" (old behaviour), "stop" (terminate the service), "kill" (let
the kernel kill all the service's processes).

On top of that, track OOM killer events per unit: generate a per-unit
structured, recognizable log message when we see an OOM killer event,
and put the service in a failure state if an OOM killer event was seen
and the selected policy was not "continue". A new "result" is defined
for this case: "oom-kill".

All of this relies on new cgroupv2 kernel functionality: the
"memory.events" notification interface and the "memory.oom.group"
attribute (which makes the kernel kill all cgroup processes
automatically).

1 files changed, 2 insertions, 0 deletions

diff --git a/src/core/dbus-manager.h b/src/core/dbus-manager.h
index d0306adc70..10aa2eccee 100644
--- a/src/core/dbus-manager.h
+++ b/src/core/dbus-manager.h
@@ -12,3 +12,5 @@ void bus_manager_send_reloading(Manager *m, bool active);
 void bus_manager_send_change_signal(Manager *m);
 
 int verify_run_space_and_log(const char *message);
+
+int bus_property_get_oom_policy(sd_bus *bus, const char *path, const char *interface, const char *property, sd_bus_message *reply, void *userdata, sd_bus_error *ret_error);