Set oom score for containerd and shims

This adds a config option to set the oom score for the containerd daemon
as well as automatically setting the oom score for the shim's lauched so
that they are not killed until the very end of an out of memory
condition.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

sys/oom_unix.go

@@ -0,0 +1,31 @@
+// +build !windows
+
+package sys
+
+import (
+	"fmt"
+	"os"
+	"strconv"
+
+	"github.com/opencontainers/runc/libcontainer/system"
+)
+
+// OOMScoreMaxKillable is the maximum score keeping the process killable by the oom killer
+const OOMScoreMaxKillable = -999
+
+// SetOOMScore sets the oom score for the provided pid
+func SetOOMScore(pid, score int) error {
+	path := fmt.Sprintf("/proc/%d/oom_score_adj", pid)
+	f, err := os.OpenFile(path, os.O_WRONLY, 0)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+	if _, err = f.WriteString(strconv.Itoa(score)); err != nil {
+		if os.IsPermission(err) && system.RunningInUserNS() {
+			return nil
+		}
+		return err
+	}
+	return nil
+}