devmapper: Use device id as specified by caller

Currently devicemapper CreateDevice and CreateSnapDevice keep on retrying
device creation till a suitable device id is found. 

With new transaction mechanism we need to store device id in transaction
before it has been created.

So change the logic in such a way that caller decides the devices Id to
use. If that device Id is not available, caller bumps up the device Id
and retries.

That way caller can update transaciton too when it tries a new Id. Transaction
related patches will come later in the series.
  
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>

devicemapper/devmapper.go

@@ -67,6 +67,7 @@ var (
 	ErrGetLoopbackBackingFile = errors.New("Unable to get loopback backing file")
 	ErrLoopbackSetCapacity    = errors.New("Unable set loopback capacity")
 	ErrBusy                   = errors.New("Device is Busy")
+	ErrDeviceIdExists         = errors.New("Device Id Exists")
 
 	dmSawBusy  bool
 	dmSawExist bool
@@ -97,6 +98,16 @@ type (
 	AddNodeType int
 )
 
+// Returns whether error conveys the information about device Id already
+// exist or not. This will be true if device creation or snap creation
+// operation fails if device or snap device already exists in pool.
+// Current implementation is little crude as it scans the error string
+// for exact pattern match. Replacing it with more robust implementation
+// is desirable.
+func DeviceIdExists(err error) bool {
+	return fmt.Sprint(err) == fmt.Sprint(ErrDeviceIdExists)
+}
+
 func (t *Task) destroy() {
 	if t != nil {
 		DmTaskDestroy(t.unmanaged)
@@ -528,10 +539,8 @@ func ResumeDevice(name string) error {
 	return nil
 }
 
-func CreateDevice(poolName string, deviceId *int) error {
+func CreateDevice(poolName string, deviceId int) error {
-	log.Debugf("[devmapper] CreateDevice(poolName=%v, deviceId=%v)", poolName, *deviceId)
+	log.Debugf("[devmapper] CreateDevice(poolName=%v, deviceId=%v)", poolName, deviceId)
-
-	for {
 	task, err := TaskCreateNamed(DeviceTargetMsg, poolName)
 	if task == nil {
 		return err
@@ -541,20 +550,18 @@ func CreateDevice(poolName string, deviceId *int) error {
 		return fmt.Errorf("Can't set sector %s", err)
 	}
 
-		if err := task.SetMessage(fmt.Sprintf("create_thin %d", *deviceId)); err != nil {
+	if err := task.SetMessage(fmt.Sprintf("create_thin %d", deviceId)); err != nil {
 		return fmt.Errorf("Can't set message %s", err)
 	}
 
 	dmSawExist = false // reset before the task is run
 	if err := task.Run(); err != nil {
+		// Caller wants to know about ErrDeviceIdExists so that it can try with a different device id.
 		if dmSawExist {
-				// Already exists, try next id
+			return ErrDeviceIdExists
-				*deviceId++
+		} else {
-				continue
-			}
 			return fmt.Errorf("Error running CreateDevice %s", err)
 		}
-		break
 	}
 	return nil
 }
@@ -607,7 +614,7 @@ func ActivateDevice(poolName string, name string, deviceId int, size uint64) err
 	return nil
 }
 
-func CreateSnapDevice(poolName string, deviceId *int, baseName string, baseDeviceId int) error {
+func CreateSnapDevice(poolName string, deviceId int, baseName string, baseDeviceId int) error {
 	devinfo, _ := GetInfo(baseName)
 	doSuspend := devinfo != nil && devinfo.Exists != 0
 
@@ -617,7 +624,6 @@ func CreateSnapDevice(poolName string, deviceId *int, baseName string, baseDevic
 		}
 	}
 
-	for {
 	task, err := TaskCreateNamed(DeviceTargetMsg, poolName)
 	if task == nil {
 		if doSuspend {
@@ -633,7 +639,7 @@ func CreateSnapDevice(poolName string, deviceId *int, baseName string, baseDevic
 		return fmt.Errorf("Can't set sector %s", err)
 	}
 
-		if err := task.SetMessage(fmt.Sprintf("create_snap %d %d", *deviceId, baseDeviceId)); err != nil {
+	if err := task.SetMessage(fmt.Sprintf("create_snap %d %d", deviceId, baseDeviceId)); err != nil {
 		if doSuspend {
 			ResumeDevice(baseName)
 		}
@@ -642,19 +648,15 @@ func CreateSnapDevice(poolName string, deviceId *int, baseName string, baseDevic
 
 	dmSawExist = false // reset before the task is run
 	if err := task.Run(); err != nil {
-			if dmSawExist {
-				// Already exists, try next id
-				*deviceId++
-				continue
-			}
-
 		if doSuspend {
 			ResumeDevice(baseName)
 		}
+		// Caller wants to know about ErrDeviceIdExists so that it can try with a different device id.
+		if dmSawExist {
+			return ErrDeviceIdExists
+		} else {
 			return fmt.Errorf("Error running DeviceCreate (createSnapDevice) %s", err)
 		}
-
-		break
 	}
 
 	if doSuspend {