Use SysctlTimeval for boottime collector on BSD

Use SysctlTimeval from the golang.org/x/sys/unix package to
simplify the implementation of the boottime collector for the BSDs and
allows to build it without cgo.

Tested on macOS 11.6, FreeBSD 13 and OpenBSD 7.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>

collector/boot_time_bsd.go

@@ -11,20 +11,20 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-//go:build (freebsd || dragonfly || (openbsd && !amd64) || netbsd || darwin) && !noboottime
-// +build freebsd dragonfly openbsd,!amd64 netbsd darwin
+//go:build (freebsd || dragonfly || openbsd || netbsd || darwin) && !noboottime
+// +build freebsd dragonfly openbsd netbsd darwin
 // +build !noboottime
 
 package collector
 
 import (
 	"github.com/go-kit/log"
 	"github.com/prometheus/client_golang/prometheus"
+	"golang.org/x/sys/unix"
 )
 
 type bootTimeCollector struct {
-	boottime bsdSysctl
-	logger   log.Logger
+	logger log.Logger
 }
 
 func init() {
@@ -34,27 +34,25 @@ func init() {
 // newBootTimeCollector returns a new Collector exposing system boot time on BSD systems.
 func newBootTimeCollector(logger log.Logger) (Collector, error) {
 	return &bootTimeCollector{
-		boottime: bsdSysctl{
-			name:        "boot_time_seconds",
-			description: "Unix time of last boot, including microseconds.",
-			mib:         "kern.boottime",
-			dataType:    bsdSysctlTypeStructTimeval,
-		},
 		logger: logger,
 	}, nil
 }
 
 // Update pushes boot time onto ch
 func (c *bootTimeCollector) Update(ch chan<- prometheus.Metric) error {
-	v, err := c.boottime.Value()
+	tv, err := unix.SysctlTimeval("kern.boottime")
 	if err != nil {
 		return err
 	}
 
+	// This conversion maintains the usec precision.  Using the time
+	// package did not.
+	v := float64(tv.Sec) + (float64(tv.Usec) / float64(1000*1000))
+
 	ch <- prometheus.MustNewConstMetric(
 		prometheus.NewDesc(
-			prometheus.BuildFQName(namespace, "", c.boottime.name),
-			c.boottime.description,
+			prometheus.BuildFQName(namespace, "", "boot_time_seconds"),
+			"Unix time of last boot, including microseconds.",
 			nil, nil,
 		), prometheus.GaugeValue, v)
 

collector/sysctl_bsd.go

@@ -36,7 +36,6 @@ const (
 	// Default to uint32.
 	bsdSysctlTypeUint32 bsdSysctlType = iota
 	bsdSysctlTypeUint64
-	bsdSysctlTypeStructTimeval
 	bsdSysctlTypeCLong
 )
 
@@ -75,8 +74,6 @@ func (b bsdSysctl) Value() (float64, error) {
 	case bsdSysctlTypeUint64:
 		tmp64, err = unix.SysctlUint64(b.mib)
 		tmpf64 = float64(tmp64)
-	case bsdSysctlTypeStructTimeval:
-		tmpf64, err = b.getStructTimeval()
 	case bsdSysctlTypeCLong:
 		tmpf64, err = b.getCLong()
 	}
@@ -92,28 +89,6 @@ func (b bsdSysctl) Value() (float64, error) {
 	return tmpf64, nil
 }
 
-func (b bsdSysctl) getStructTimeval() (float64, error) {
-	raw, err := unix.SysctlRaw(b.mib)
-	if err != nil {
-		return 0, err
-	}
-
-	if len(raw) != int(unsafe.Sizeof(unix.Timeval{})) {
-		// Shouldn't get here.
-		return 0, fmt.Errorf(
-			"length of bytes received from sysctl (%d) does not match expected bytes (%d)",
-			len(raw),
-			unsafe.Sizeof(unix.Timeval{}),
-		)
-	}
-
-	tv := *(*unix.Timeval)(unsafe.Pointer(&raw[0]))
-
-	// This conversion maintains the usec precision.  Using the time
-	// package did not.
-	return (float64(tv.Sec) + (float64(tv.Usec) / float64(1000*1000))), nil
-}
-
 func (b bsdSysctl) getCLong() (float64, error) {
 	raw, err := unix.SysctlRaw(b.mib)
 	if err != nil {