Refactor attach loop device in pure Go

graphdriver/devmapper/devmapper.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"github.com/dotcloud/docker/utils"
 	"runtime"
+	"unsafe"
 )
 
 type DevmapperLogger interface {
@@ -177,15 +178,6 @@ func (t *Task) GetNextTarget(next uintptr) (nextPtr uintptr, start uint64,
 		start, length, targetType, params
 }
 
-func AttachLoopDevice(filename string) (*osFile, error) {
-	var fd int
-	res := DmAttachLoopDevice(filename, &fd)
-	if res == "" {
-		return nil, ErrAttachLoopbackDevice
-	}
-	return &osFile{File: osNewFile(uintptr(fd), res)}, nil
-}
-
 func getLoopbackBackingFile(file *osFile) (uint64, uint64, error) {
 	dev, inode, err := DmGetLoopbackBackingFile(file.Fd())
 	if err != 0 {
@@ -223,11 +215,10 @@ func FindLoopDeviceFor(file *osFile) *osFile {
 			continue
 		}
 
-		dev, inode, err := getLoopbackBackingFile(&osFile{File: file})
+		dev, inode, err := getLoopbackBackingFile(file)
 		if err == nil && dev == targetDevice && inode == targetInode {
-			return &osFile{File: file}
+			return file
 		}
-
 		file.Close()
 	}
 
@@ -420,7 +411,7 @@ func suspendDevice(name string) error {
 		return err
 	}
 	if err := task.Run(); err != nil {
-		return fmt.Errorf("Error running DeviceSuspend")
+		return fmt.Errorf("Error running DeviceSuspend: %s", err)
 	}
 	return nil
 }
@@ -437,7 +428,7 @@ func resumeDevice(name string) error {
 	}
 
 	if err := task.Run(); err != nil {
-		return fmt.Errorf("Error running DeviceSuspend")
+		return fmt.Errorf("Error running DeviceResume")
 	}
 
 	UdevWait(cookie)
@@ -574,3 +565,109 @@ func (devices *DeviceSet) createSnapDevice(poolName string, deviceId int, baseNa
 
 	return nil
 }
+
+type LoopInfo64 struct {
+	loDevice           uint64 /* ioctl r/o */
+	loInode            uint64 /* ioctl r/o */
+	loRdevice          uint64 /* ioctl r/o */
+	loOffset           uint64
+	loSizelimit        uint64 /* bytes, 0 == max available */
+	loNumber           uint32 /* ioctl r/o */
+	loEncrypt_type     uint32
+	loEncrypt_key_size uint32 /* ioctl w/o */
+	loFlags            uint32 /* ioctl r/o */
+	loFileName         [LoNameSize]uint8
+	loCryptName        [LoNameSize]uint8
+	loEncryptKey       [LoKeySize]uint8 /* ioctl w/o */
+	loInit             [2]uint64
+}
+
+// attachLoopDevice attaches the given sparse file to the next
+// available loopback device. It returns an opened *osFile.
+func attachLoopDevice(filename string) (loop *osFile, err error) {
+	startIndex := 0
+
+	// Try to retrieve the next available loopback device via syscall.
+	// If it fails, we discard error and start loopking for a
+	// loopback from index 0.
+	if f, err := osOpenFile("/dev/loop-control", osORdOnly, 0644); err == nil {
+		if index, _, err := sysSyscall(sysSysIoctl, f.Fd(), LoopCtlGetFree, 0); err != 0 {
+			utils.Debugf("Error retrieving the next available loopback: %s", err)
+		} else if index > 0 {
+			startIndex = int(index)
+		}
+		f.Close()
+	}
+
+	// Open the given sparse file (use OpenFile because Open sets O_CLOEXEC)
+	f, err := osOpenFile(filename, osORdWr, 0644)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+
+	var (
+		target   string
+		loopFile *osFile
+	)
+	// Start looking for a free /dev/loop
+	for i := startIndex; ; {
+		target = fmt.Sprintf("/dev/loop%d", i)
+
+		fi, err := osStat(target)
+		if err != nil {
+			if osIsNotExist(err) {
+				utils.Errorf("There are no more loopback device available.")
+			}
+		}
+
+		// FIXME: Check here if target is a block device (in C: S_ISBLK(mode))
+		if fi.IsDir() {
+		}
+
+		// Open the targeted loopback (use OpenFile because Open sets O_CLOEXEC)
+		loopFile, err = osOpenFile(target, osORdWr, 0644)
+		if err != nil {
+			return nil, err
+		}
+
+		// Try to attach to the loop file
+		if _, _, err := sysSyscall(sysSysIoctl, loopFile.Fd(), LoopSetFd, f.Fd()); err != 0 {
+			loopFile.Close()
+			// If the error is EBUSY, then try the next loopback
+			if err != sysEBusy {
+				utils.Errorf("Cannot set up loopback device %s: %s", target, err)
+				return nil, err
+			}
+		} else {
+			// In case of success, we finished. Break the loop.
+			break
+		}
+		// In case of EBUSY error, the loop keep going.
+	}
+
+	// This can't happen, but let's be sure
+	if loopFile == nil {
+		return nil, fmt.Errorf("Unreachable code reached! Error attaching %s to a loopback device.", filename)
+	}
+
+	// Set the status of the loopback device
+	var loopInfo LoopInfo64
+
+	// Due to type incompatibility (string vs [64]uint8), we copy data
+	copy(loopInfo.loFileName[:], target[:])
+	loopInfo.loOffset = 0
+	loopInfo.loFlags = LoFlagsAutoClear
+
+	if _, _, err := sysSyscall(sysSysIoctl, loopFile.Fd(), LoopSetStatus64, uintptr(unsafe.Pointer(&loopInfo))); err != 0 {
+		// If the call failed, then free the loopback device
+		utils.Errorf("Cannot set up loopback device info: %s", err)
+		if _, _, err := sysSyscall(sysSysIoctl, loopFile.Fd(), LoopClrFd, 0); err != 0 {
+			utils.Errorf("Error while cleaning up the loopback device")
+		}
+		loopFile.Close()
+		return nil, err
+	}
+
+	return loopFile, nil
+}