oomd: implement a prekill varlink event

When a cgroup is selected for termination, send varlink messages
to hooks registered in `/run/systemd/oomd.prekill-hooks/`.
oomd waits up to `PreKillHookTimeoutSec=` seconds for response
before proceeding with the kill.

src/shared/meson.build

@@ -224,6 +224,7 @@ shared_sources = files(
         'varlink-io.systemd.Unit.c',
         'varlink-io.systemd.UserDatabase.c',
         'varlink-io.systemd.oom.c',
+        'varlink-io.systemd.oom.Prekill.c',
         'varlink-io.systemd.service.c',
         'varlink-io.systemd.sysext.c',
         'varlink-serialize.c',

src/shared/varlink-io.systemd.oom.Prekill.c

@@ -0,0 +1,18 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include "varlink-io.systemd.oom.Prekill.h"
+
+/* This is a new Varlink interface for pre-kill notifications from oomd.
+ * It will be available through /run/systemd/oomd.prekill.hook/ */
+
+static SD_VARLINK_DEFINE_METHOD(
+                Notify,
+                SD_VARLINK_FIELD_COMMENT("The cgroup which is going to be killed"),
+                SD_VARLINK_DEFINE_INPUT(cgroup, SD_VARLINK_STRING, 0));
+
+SD_VARLINK_DEFINE_INTERFACE(
+                io_systemd_oom_Prekill,
+                "io.systemd.oom.Prekill",
+                SD_VARLINK_INTERFACE_COMMENT("Prekill notifications from oomd"),
+                SD_VARLINK_SYMBOL_COMMENT("Notify about an imminent OOM kill"),
+                &vl_method_Notify);

src/shared/varlink-io.systemd.oom.Prekill.h

@@ -0,0 +1,6 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+#include "sd-varlink-idl.h"
+
+extern const sd_varlink_interface vl_interface_io_systemd_oom_Prekill;