feature: add prepare device for chroot builder

builder/osc/chroot/builder.go

@@ -238,10 +238,20 @@ func (b *Builder) Run(ui packer.Ui, hook packer.Hook, cache packer.Cache) (packe
 		)
 	}
 
+	steps = append(steps,
+		&StepFlock{},
+		&StepPrepareDevice{},
+	)
+
 	// Run!
 	b.runner = common.NewRunner(steps, b.config.PackerConfig, ui)
 	b.runner.Run(state)
 
+	// If there was an error, return that
+	if rawErr, ok := state.GetOk("error"); ok {
+		return nil, rawErr.(error)
+	}
+
 	return nil, nil
 }
 

builder/osc/chroot/device.go

@@ -0,0 +1,70 @@
+package chroot
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// AvailableDevice finds an available device and returns it. Note that
+// you should externally hold a flock or something in order to guarantee
+// that this device is available across processes.
+func AvailableDevice() (string, error) {
+	prefix, err := devicePrefix()
+	if err != nil {
+		return "", err
+	}
+
+	letters := "fghijklmnop"
+	for _, letter := range letters {
+		device := fmt.Sprintf("/dev/%s%c", prefix, letter)
+
+		// If the block device itself, i.e. /dev/sf, exists, then we
+		// can't use any of the numbers either.
+		if _, err := os.Stat(device); err == nil {
+			continue
+		}
+
+		// To be able to build both Paravirtual and HVM images, the unnumbered
+		// device and the first numbered one must be available.
+		// E.g. /dev/xvdf  and  /dev/xvdf1
+		numbered_device := fmt.Sprintf("%s%d", device, 1)
+		if _, err := os.Stat(numbered_device); err != nil {
+			return device, nil
+		}
+	}
+
+	return "", errors.New("available device could not be found")
+}
+
+// devicePrefix returns the prefix ("sd" or "xvd" or so on) of the devices
+// on the system. The "vd" prefix appears on outscale images.
+func devicePrefix() (string, error) {
+	available := []string{"sd", "xvd", "vd"}
+
+	f, err := os.Open("/sys/block")
+	if err != nil {
+		return "", err
+	}
+	defer f.Close()
+
+	dirs, err := f.Readdirnames(-1)
+	if dirs != nil && len(dirs) > 0 {
+		for _, dir := range dirs {
+			dirBase := filepath.Base(dir)
+			for _, prefix := range available {
+				if strings.HasPrefix(dirBase, prefix) {
+					return prefix, nil
+				}
+			}
+		}
+	}
+
+	if err != nil {
+		return "", err
+	}
+
+	return "", errors.New("device prefix could not be detected")
+}

builder/osc/chroot/step_prepare_device.go

@@ -0,0 +1,46 @@
+package chroot
+
+import (
+	"context"
+	"fmt"
+	"log"
+	"os"
+
+	"github.com/hashicorp/packer/helper/multistep"
+	"github.com/hashicorp/packer/packer"
+)
+
+// StepPrepareDevice finds an available device and sets it.
+type StepPrepareDevice struct {
+}
+
+func (s *StepPrepareDevice) Run(_ context.Context, state multistep.StateBag) multistep.StepAction {
+	config := state.Get("config").(*Config)
+	ui := state.Get("ui").(packer.Ui)
+
+	device := config.DevicePath
+	if device == "" {
+		var err error
+		log.Println("Device path not specified, searching for available device...")
+		device, err = AvailableDevice()
+		if err != nil {
+			err := fmt.Errorf("Error finding available device: %s", err)
+			state.Put("error", err)
+			ui.Error(err.Error())
+			return multistep.ActionHalt
+		}
+	}
+
+	if _, err := os.Stat(device); err == nil {
+		err := fmt.Errorf("Device is in use: %s", device)
+		state.Put("error", err)
+		ui.Error(err.Error())
+		return multistep.ActionHalt
+	}
+
+	log.Printf("Device: %s", device)
+	state.Put("device", device)
+	return multistep.ActionContinue
+}
+
+func (s *StepPrepareDevice) Cleanup(state multistep.StateBag) {}