handle error when smartcl returns an empty response

fixes #608

pkg/capacity/capacity.go

@@ -5,6 +5,7 @@ import (
 	"strings"
 
 	"github.com/pkg/errors"
+	"github.com/rs/zerolog/log"
 	"github.com/shirou/gopsutil/host"
 	"github.com/threefoldtech/zos/pkg/capacity/dmi"
 	"github.com/threefoldtech/zos/pkg/capacity/smartctl"
@@ -78,6 +79,11 @@ type Disks struct {
 // Disks list and parse the hardware information using smartctl
 func (r *ResourceOracle) Disks() (d Disks, err error) {
 	devices, err := smartctl.ListDevices()
+	if errors.Is(err, smartctl.ErrEmpty) {
+		// TODO: for now we allow to not have the smartctl dump of all the disks
+		log.Warn().Err(err).Msg("smartctl did not found any disk on the system")
+		return d, nil
+	}
 	if err != nil {
 		return
 	}

pkg/capacity/smartctl/smartctl.go

@@ -1,6 +1,7 @@
 package smartctl
 
 import (
+	"errors"
 	"fmt"
 	"os/exec"
 	"regexp"
@@ -11,6 +12,9 @@ var reScan = regexp.MustCompile(`(?m)^([^\s]+)\s+-d\s+([^\s]+)\s+#`)
 var reHeader = regexp.MustCompile(`(?m)([^\[]+)\[([^\[]+)\]`)
 var reInfo = regexp.MustCompile(`(?m)([^:]+):\s+(.+)`)
 
+// ErrEmpty is return when smatctl doesn't find any device
+var ErrEmpty = errors.New("smartctl returned an empty response")
+
 // Device represents a device as returned by "smartctl --scan"
 type Device struct {
 	Type string
@@ -25,6 +29,10 @@ func ListDevices() ([]Device, error) {
 		return nil, err
 	}
 
+	if len(output) == 0 {
+		return nil, ErrEmpty
+	}
+
 	return parseScan(output)
 }