Add device mapper functionality

Signed-off-by: Amory Hoste <[email protected]>

Author: amohoste

devmapper/deviceSnapshot.go

@@ -0,0 +1,176 @@
+package devmapper
+
+import (
+	"fmt"
+	"github.com/pkg/errors"
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"strings"
+	"sync"
+	"syscall"
+)
+
+// DeviceSnapshot represents a device mapper snapshot
+type DeviceSnapshot struct {
+	sync.Mutex
+	poolName           string
+	deviceName         string
+	deviceId           string
+	mountDir           string
+	mountedReadonly    bool
+	numMounted         int
+	numActivated       int
+}
+
+// GetDevicePath returns the path to the snapshot device.
+func (dsnp *DeviceSnapshot) GetDevicePath() string {
+	return fmt.Sprintf("/dev/mapper/%s", dsnp.deviceName)
+}
+
+// getPoolpath returns the path of the thin pool used by the snapshot.
+func (dsnp *DeviceSnapshot) getPoolPath() string {
+	return fmt.Sprintf("/dev/mapper/%s", dsnp.poolName)
+}
+
+// NewDeviceSnapshot initializes a new device mapper snapshot.
+func NewDeviceSnapshot(poolName, deviceName, deviceId string) *DeviceSnapshot {
+	dsnp := new(DeviceSnapshot)
+	dsnp.poolName = poolName
+	dsnp.deviceName = deviceName
+	dsnp.deviceId = deviceId
+	dsnp.mountDir = ""
+	dsnp.mountedReadonly = false
+	dsnp.numMounted = 0
+	dsnp.numActivated = 0
+	return dsnp
+}
+
+// Activate creates a snapshot.
+func (dsnp *DeviceSnapshot) Activate() error {
+	dsnp.Lock()
+	defer dsnp.Unlock()
+
+	if dsnp.numActivated == 0 {
+		tableEntry := fmt.Sprintf("0 20971520 thin %s %s", dsnp.getPoolPath(), dsnp.deviceId)
+
+		cmd := exec.Command("sudo", "dmsetup", "create", dsnp.deviceName, "--table", fmt.Sprintf("%s", tableEntry))
+		err := cmd.Run()
+		if err != nil {
+			return errors.Wrapf(err, "activating snapshot %s", dsnp.deviceName)
+		}
+
+	}
+
+	dsnp.numActivated += 1
+
+	return nil
+}
+
+// Deactivate removes a snapshot.
+func (dsnp *DeviceSnapshot) Deactivate() error {
+	dsnp.Lock()
+	defer dsnp.Unlock()
+
+	if dsnp.numActivated == 1 {
+		cmd := exec.Command("sudo", "dmsetup", "remove", dsnp.deviceName)
+		err := cmd.Run()
+		if err != nil {
+			return errors.Wrapf(err, "deactivating snapshot %s", dsnp.deviceName)
+		}
+	}
+
+	dsnp.numActivated -= 1
+	return nil
+}
+
+// Mount mounts a snapshot device and returns the path where it is mounted. For better performance and efficiency,
+// a snapshot is only mounted once and shared if it is already mounted.
+func (dsnp *DeviceSnapshot) Mount(readOnly bool) (string, error) {
+	dsnp.Lock()
+	defer dsnp.Unlock()
+
+	if dsnp.numActivated == 0 {
+		return "", errors.New("failed to mount: snapshot not activated")
+	}
+
+	if dsnp.numMounted != 0 && (!dsnp.mountedReadonly || dsnp.mountedReadonly && !readOnly) {
+		return "", errors.New("failed to mount: can't mount snapshot for both reading and writing")
+	}
+
+	if dsnp.numMounted == 0 {
+		mountDir, err := ioutil.TempDir("", dsnp.deviceName)
+		if err != nil {
+			return "", err
+		}
+		mountDir = removeTrailingSlash(mountDir)
+
+		err = mountExt4(dsnp.GetDevicePath(), mountDir, readOnly)
+		if err != nil {
+			return "", errors.Wrapf(err, "mounting %s at %s", dsnp.GetDevicePath(), mountDir)
+		}
+		dsnp.mountDir = mountDir
+		dsnp.mountedReadonly = readOnly
+	}
+
+	dsnp.numMounted += 1
+
+	return dsnp.mountDir, nil
+}
+
+// UnMounts a device snapshot. Due to mounted snapshot being shared, a snapshot is only actually unmounted if it is not
+// in use by anyone else.
+func (dsnp *DeviceSnapshot) UnMount() error {
+	dsnp.Lock()
+	defer dsnp.Unlock()
+
+	if dsnp.numMounted == 1 {
+		err := unMountExt4(dsnp.mountDir)
+		if err != nil {
+			return errors.Wrapf(err, "unmounting %s", dsnp.mountDir)
+		}
+
+		err = os.RemoveAll(dsnp.mountDir)
+		if err != nil {
+			return errors.Wrapf(err, "removing %s", dsnp.mountDir)
+		}
+		dsnp.mountDir = ""
+	}
+
+	dsnp.numMounted -= 1
+	return nil
+}
+
+// mountExt4 mounts a snapshot device available at devicePath at the specified mountPath.
+func mountExt4(devicePath, mountPath string, readOnly bool) error {
+	// Specify flags for faster mounting and performance:
+	// * Do not update access times for (all types of) files on this filesystem.
+	// * Do not allow access to devices (special files) on this filesystem.
+	// * Do not allow programs to be executed from this filesystem.
+	// * Do not honor set-user-ID and set-group-ID bits or file  capabilities when executing programs from this filesystem.
+	// * Suppress the display of certain (printk()) warning messages in the kernel log.
+	var flags uintptr = syscall.MS_NOATIME | syscall.MS_NODEV | syscall.MS_NOEXEC | syscall.MS_NOSUID | syscall.MS_SILENT
+	options := make([]string, 0)
+
+	if readOnly {
+		// Mount filesystem read-only.
+		flags |= syscall.MS_RDONLY
+		options = append(options, "noload")
+	}
+
+	return syscall.Mount(devicePath, mountPath, "ext4", flags, strings.Join(options, ","))
+}
+
+// unMountExt4 unmounts a snapshot device mounted at mountPath.
+func unMountExt4(mountPath string) error {
+	return syscall.Unmount(mountPath, syscall.MNT_DETACH)
+}
+
+// removeTrailingSlash returns a path with the trailing slash removed.
+func removeTrailingSlash(path string) string {
+	if strings.HasSuffix(path, "/") {
+		return path[:len(path)-1]
+	} else {
+		return path
+	}
+}