Cache the boot time after first query.

When fetching stats on all processes at once there's a non-trivial amount of
time spent in the `BootTime` call. But since this value should never change
during a live process, we can use a cached version for all subsequent calls.

host/host.go

@@ -6,7 +6,10 @@ import (
 	"github.com/shirou/gopsutil/internal/common"
 )
 
-var invoke common.Invoker
+var (
+	invoke         common.Invoker
+	cachedBootTime = uint64(0)
+)
 
 func init() {
 	invoke = common.Invoke{}

host/host_darwin.go

@@ -66,6 +66,9 @@ func Info() (*InfoStat, error) {
 }
 
 func BootTime() (uint64, error) {
+	if cachedBootTime != 0 {
+		return cachedBootTime, nil
+	}
 	values, err := common.DoSysctrl("kern.boottime")
 	if err != nil {
 		return 0, err
@@ -76,8 +79,9 @@ func BootTime() (uint64, error) {
 	if err != nil {
 		return 0, err
 	}
+	cachedBootTime = uint64(boottime)
 
-	return uint64(boottime), nil
+	return cachedBootTime, nil
 }
 
 func uptime(boot uint64) uint64 {

host/host_freebsd.go

@@ -69,6 +69,9 @@ func Info() (*InfoStat, error) {
 }
 
 func BootTime() (uint64, error) {
+	if cachedBootTime != 0 {
+		return cachedBootTime, nil
+	}
 	values, err := common.DoSysctrl("kern.boottime")
 	if err != nil {
 		return 0, err
@@ -80,6 +83,7 @@ func BootTime() (uint64, error) {
 	if err != nil {
 		return 0, err
 	}
+	cachedBootTime = boottime
 
 	return boottime, nil
 }

host/host_linux.go

@@ -86,6 +86,9 @@ func Info() (*InfoStat, error) {
 
 // BootTime returns the system boot time expressed in seconds since the epoch.
 func BootTime() (uint64, error) {
+	if cachedBootTime != 0 {
+		return cachedBootTime, nil
+	}
 	filename := common.HostProc("stat")
 	lines, err := common.ReadLines(filename)
 	if err != nil {
@@ -101,7 +104,8 @@ func BootTime() (uint64, error) {
 			if err != nil {
 				return 0, err
 			}
-			return uint64(b), nil
+			cachedBootTime = uint64(b)
+			return cachedBootTime, nil
 		}
 	}
 

host/host_test.go

@@ -40,6 +40,11 @@ func TestBoot_time(t *testing.T) {
 	if v < 946652400 {
 		t.Errorf("Invalid Boottime, older than 2000-01-01")
 	}
+
+	v2, err := BootTime()
+	if v != v2 {
+		t.Errorf("cached boot time is different")
+	}
 }
 
 func TestUsers(t *testing.T) {

host/host_windows.go

@@ -91,11 +91,15 @@ func bootTime(up uint64) uint64 {
 }
 
 func BootTime() (uint64, error) {
+	if cachedBootTime != 0 {
+		return cachedBootTime, nil
+	}
 	up, err := Uptime()
 	if err != nil {
 		return 0, err
 	}
-	return bootTime(up), nil
+	cachedBootTime = bootTime(up)
+	return cachedBootTime, nil
 }
 
 func PlatformInformation() (platform string, family string, version string, err error) {