pkg/rctl: Add a simple FreeBSD RCTL library

Initially just supporting just rctl_get_racct for
(*Container).GetContainerStats.

[NO NEW TESTS NEEDED] we are not running any FreeBSD tests in CI

Signed-off-by: Doug Rabson <dfr@rabson.org>

pkg/rctl/rctl.go

@@ -0,0 +1,47 @@
+//go:build freebsd
+// +build freebsd
+
+package rctl
+
+// #include <sys/rctl.h>
+import "C"
+
+import (
+	"bytes"
+	"fmt"
+	"strconv"
+	"strings"
+	"syscall"
+	"unsafe"
+
+	"github.com/sirupsen/logrus"
+)
+
+func GetRacct(filter string) (map[string]uint64, error) {
+	bp, err := syscall.ByteSliceFromString(filter)
+	if err != nil {
+		return nil, err
+	}
+	var buf [1024]byte
+	_, _, errno := syscall.Syscall6(syscall.SYS_RCTL_GET_RACCT,
+		uintptr(unsafe.Pointer(&bp[0])),
+		uintptr(len(bp)),
+		uintptr(unsafe.Pointer(&buf[0])),
+		uintptr(len(buf)), 0, 0)
+	if errno != 0 {
+		return nil, fmt.Errorf("error calling rctl_get_racct with filter %s: %v", errno)
+	}
+	len := bytes.IndexByte(buf[:], byte(0))
+	entries := strings.Split(string(buf[:len]), ",")
+	res := make(map[string]uint64)
+	for _, entry := range entries {
+		kv := strings.SplitN(entry, "=", 2)
+		key := kv[0]
+		val, err := strconv.ParseUint(kv[1], 10, 0)
+		if err != nil {
+			logrus.Warnf("unexpected rctl entry, ignoring: %s", entry)
+		}
+		res[key] = val
+	}
+	return res, nil
+}