devmapper: Make sure device is a thin pool device

Right now we check for the existence of device but don't make sure it is
a thin pool device. We assume it is a thin pool device and call poolStatus()
on the device which returns an error EOF. And that error does not tell
anything.

So before we reach the stage of calling poolStatus() make sure we are working
with a thin pool device otherwise error out.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>

daemon/graphdriver/devmapper/deviceset.go

@@ -897,6 +897,33 @@ func (devices *DeviceSet) createBaseImage() error {
 	return nil
 }
 
+// Returns if thin pool device exists or not. If device exists, also makes
+// sure it is a thin pool device and not some other type of device.
+func (devices *DeviceSet) thinPoolExists(thinPoolDevice string) (bool, error) {
+	logrus.Debugf("devmapper: Checking for existence of the pool %s", thinPoolDevice)
+
+	info, err := devicemapper.GetInfo(thinPoolDevice)
+	if err != nil {
+		return false, fmt.Errorf("devmapper: GetInfo() on device %s failed: %v", thinPoolDevice, err)
+	}
+
+	// Device does not exist.
+	if info.Exists == 0 {
+		return false, nil
+	}
+
+	_, _, deviceType, _, err := devicemapper.GetStatus(thinPoolDevice)
+	if err != nil {
+		return false, fmt.Errorf("devmapper: GetStatus() on device %s failed: %v", thinPoolDevice, err)
+	}
+
+	if deviceType != "thin-pool" {
+		return false, fmt.Errorf("devmapper: Device %s is not a thin pool", thinPoolDevice)
+	}
+
+	return true, nil
+}
+
 func (devices *DeviceSet) checkThinPool() error {
 	_, transactionID, dataUsed, _, _, _, err := devices.poolStatus()
 	if err != nil {
@@ -1441,10 +1468,8 @@ func (devices *DeviceSet) initDevmapper(doInit bool) error {
 	logrus.Debugf("Generated prefix: %s", devices.devicePrefix)
 
 	// Check for the existence of the thin-pool device
-	logrus.Debugf("Checking for existence of the pool '%s'", devices.getPoolName())
-	info, err := devicemapper.GetInfo(devices.getPoolName())
-	if info == nil {
-		logrus.Debugf("Error device devicemapper.GetInfo: %s", err)
+	poolExists, err := devices.thinPoolExists(devices.getPoolName())
+	if err != nil {
 		return err
 	}
 
@@ -1459,7 +1484,7 @@ func (devices *DeviceSet) initDevmapper(doInit bool) error {
 	createdLoopback := false
 
 	// If the pool doesn't exist, create it
-	if info.Exists == 0 && devices.thinPoolDevice == "" {
+	if !poolExists && devices.thinPoolDevice == "" {
 		logrus.Debugf("Pool doesn't exist. Creating it.")
 
 		var (
@@ -1542,7 +1567,7 @@ func (devices *DeviceSet) initDevmapper(doInit bool) error {
 	// we probably created pool earlier and could not remove it as some
 	// containers were still using it. Detect some of the properties of
 	// pool, like is it using loop devices.
-	if info.Exists != 0 && devices.thinPoolDevice == "" {
+	if poolExists && devices.thinPoolDevice == "" {
 		if err := devices.loadThinPoolLoopBackInfo(); err != nil {
 			logrus.Debugf("Failed to load thin pool loopback device information:%v", err)
 			return err