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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.
func devicePrefix() (string, error) {
available := []string{"sd", "xvd"}
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")
}
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