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|
//go:build linux
package dmflakey
import (
"errors"
"fmt"
"os"
"time"
"golang.org/x/sys/unix"
)
const (
loopControlDevice = "/dev/loop-control"
loopDevicePattern = "/dev/loop%d"
maxRetryToAttach = 50
)
// attachToLoopDevice associates free loop device with backing file.
//
// There might have race condition. It needs to retry when it runs into EBUSY.
//
// REF: https://man7.org/linux/man-pages/man4/loop.4.html
func attachToLoopDevice(backingFile string) (string, error) {
backingFd, err := os.OpenFile(backingFile, os.O_RDWR, 0)
if err != nil {
return "", fmt.Errorf("failed to open loop device's backing file %s: %w",
backingFile, err)
}
defer backingFd.Close()
for i := 0; i < maxRetryToAttach; i++ {
loop, err := getFreeLoopDevice()
if err != nil {
return "", fmt.Errorf("failed to get free loop device: %w", err)
}
err = func() error {
loopFd, err := os.OpenFile(loop, os.O_RDWR, 0)
if err != nil {
return err
}
defer loopFd.Close()
return unix.IoctlSetInt(int(loopFd.Fd()),
unix.LOOP_SET_FD, int(backingFd.Fd()))
}()
if err != nil {
if errors.Is(err, unix.EBUSY) {
time.Sleep(500 * time.Millisecond)
continue
}
return "", err
}
return loop, nil
}
return "", fmt.Errorf("failed to associate free loop device with backing file %s after retry %v",
backingFile, maxRetryToAttach)
}
// detachLoopDevice disassociates the loop device from any backing file.
//
// REF: https://man7.org/linux/man-pages/man4/loop.4.html
func detachLoopDevice(loopDevice string) error {
loopFd, err := os.Open(loopDevice)
if err != nil {
return fmt.Errorf("failed to open loop %s: %w", loopDevice, err)
}
defer loopFd.Close()
return unix.IoctlSetInt(int(loopFd.Fd()), unix.LOOP_CLR_FD, 0)
}
// getFreeLoopDevice allocates or finds a free loop device for use.
//
// REF: https://man7.org/linux/man-pages/man4/loop.4.html
func getFreeLoopDevice() (string, error) {
control, err := os.OpenFile(loopControlDevice, os.O_RDWR, 0)
if err != nil {
return "", fmt.Errorf("failed to open %s: %w", loopControlDevice, err)
}
idx, err := unix.IoctlRetInt(int(control.Fd()), unix.LOOP_CTL_GET_FREE)
control.Close()
if err != nil {
return "", fmt.Errorf("failed to get free loop device number: %w", err)
}
return fmt.Sprintf(loopDevicePattern, idx), nil
}
|
