[host] linux: change file if in a lxc container.

host/host_linux.go

@@ -95,7 +95,18 @@ func BootTime() (uint64, error) {
 	if t != 0 {
 		return t, nil
 	}
-	filename := common.HostProc("stat")
+
+	system, role, err := Virtualization()
+	if err != nil {
+		return 0, err
+	}
+	statFile := "stat"
+	if system == "lxc" && role == "guest" {
+		// if lxc, /proc/uptime is used.
+		statFile = "uptime"
+	}
+
+	filename := common.HostProc(statFile)
 	lines, err := common.ReadLines(filename)
 	if err != nil {
 		return 0, err

host/host_test.go

@@ -40,11 +40,13 @@ func TestBoot_time(t *testing.T) {
 	if v < 946652400 {
 		t.Errorf("Invalid Boottime, older than 2000-01-01")
 	}
+	t.Logf("first boot time: %d", v)
 
 	v2, err := BootTime()
 	if v != v2 {
 		t.Errorf("cached boot time is different")
 	}
+	t.Logf("second boot time: %d", v2)
 }
 
 func TestUsers(t *testing.T) {