devicemapper: split out devicemapper bindings

This is a first pass at splitting out devicemapper into separate, usable
bindings.

Signed-off-by: Vincent Batts <vbatts@redhat.com>

devicemapper/attach_loopback.go

@@ -0,0 +1,129 @@
+// +build linux
+
+package devicemapper
+
+import (
+	"fmt"
+	"os"
+	"syscall"
+
+	log "github.com/Sirupsen/logrus"
+)
+
+func stringToLoopName(src string) [LoNameSize]uint8 {
+	var dst [LoNameSize]uint8
+	copy(dst[:], src[:])
+	return dst
+}
+
+func getNextFreeLoopbackIndex() (int, error) {
+	f, err := os.OpenFile("/dev/loop-control", os.O_RDONLY, 0644)
+	if err != nil {
+		return 0, err
+	}
+	defer f.Close()
+
+	index, err := ioctlLoopCtlGetFree(f.Fd())
+	if index < 0 {
+		index = 0
+	}
+	return index, err
+}
+
+func openNextAvailableLoopback(index int, sparseFile *os.File) (loopFile *os.File, err error) {
+	// Start looking for a free /dev/loop
+	for {
+		target := fmt.Sprintf("/dev/loop%d", index)
+		index++
+
+		fi, err := os.Stat(target)
+		if err != nil {
+			if os.IsNotExist(err) {
+				log.Errorf("There are no more loopback devices available.")
+			}
+			return nil, ErrAttachLoopbackDevice
+		}
+
+		if fi.Mode()&os.ModeDevice != os.ModeDevice {
+			log.Errorf("Loopback device %s is not a block device.", target)
+			continue
+		}
+
+		// OpenFile adds O_CLOEXEC
+		loopFile, err = os.OpenFile(target, os.O_RDWR, 0644)
+		if err != nil {
+			log.Errorf("Error opening loopback device: %s", err)
+			return nil, ErrAttachLoopbackDevice
+		}
+
+		// Try to attach to the loop file
+		if err := ioctlLoopSetFd(loopFile.Fd(), sparseFile.Fd()); err != nil {
+			loopFile.Close()
+
+			// If the error is EBUSY, then try the next loopback
+			if err != syscall.EBUSY {
+				log.Errorf("Cannot set up loopback device %s: %s", target, err)
+				return nil, ErrAttachLoopbackDevice
+			}
+
+			// Otherwise, we keep going with the loop
+			continue
+		}
+		// In case of success, we finished. Break the loop.
+		break
+	}
+
+	// This can't happen, but let's be sure
+	if loopFile == nil {
+		log.Errorf("Unreachable code reached! Error attaching %s to a loopback device.", sparseFile.Name())
+		return nil, ErrAttachLoopbackDevice
+	}
+
+	return loopFile, nil
+}
+
+// attachLoopDevice attaches the given sparse file to the next
+// available loopback device. It returns an opened *os.File.
+func AttachLoopDevice(sparseName string) (loop *os.File, err error) {
+
+	// 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.
+	startIndex, err := getNextFreeLoopbackIndex()
+	if err != nil {
+		log.Debugf("Error retrieving the next available loopback: %s", err)
+	}
+
+	// OpenFile adds O_CLOEXEC
+	sparseFile, err := os.OpenFile(sparseName, os.O_RDWR, 0644)
+	if err != nil {
+		log.Errorf("Error opening sparse file %s: %s", sparseName, err)
+		return nil, ErrAttachLoopbackDevice
+	}
+	defer sparseFile.Close()
+
+	loopFile, err := openNextAvailableLoopback(startIndex, sparseFile)
+	if err != nil {
+		return nil, err
+	}
+
+	// Set the status of the loopback device
+	loopInfo := &LoopInfo64{
+		loFileName: stringToLoopName(loopFile.Name()),
+		loOffset:   0,
+		loFlags:    LoFlagsAutoClear,
+	}
+
+	if err := ioctlLoopSetStatus64(loopFile.Fd(), loopInfo); err != nil {
+		log.Errorf("Cannot set up loopback device info: %s", err)
+
+		// If the call failed, then free the loopback device
+		if err := ioctlLoopClrFd(loopFile.Fd()); err != nil {
+			log.Errorf("Error while cleaning up the loopback device")
+		}
+		loopFile.Close()
+		return nil, ErrAttachLoopbackDevice
+	}
+
+	return loopFile, nil
+}