devmapper: construct used device ID map from device Hash map

Currently during startup we walk through all the device files and read
their device ID and mark in a bitmap that device id is used.

We are anyway going through all device files. So we can as well load all
that data into device hash map. This will save us little time when
container is actually launched later.

Also this will help with later patches where cleanup deferred device
wants to go through all the devices and see which have been marked for
deletion and delete these.

So re-organize the code a bit.

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

daemon/graphdriver/devmapper/deviceset.go

@@ -375,6 +375,18 @@ func (devices *DeviceSet) lookupDeviceWithLock(hash string) (*devInfo, error) {
 	return info, err
 }
 
+// This function relies on that device hash map has been loaded in advance.
+// Should be called with devices.Lock() held.
+func (devices *DeviceSet) constructDeviceIDMap() {
+	logrus.Debugf("[deviceset] constructDeviceIDMap()")
+	defer logrus.Debugf("[deviceset] constructDeviceIDMap() END")
+
+	for _, info := range devices.Devices {
+		devices.markDeviceIDUsed(info.DeviceID)
+		logrus.Debugf("Added deviceId=%d to DeviceIdMap", info.DeviceID)
+	}
+}
+
 func (devices *DeviceSet) deviceFileWalkFunction(path string, finfo os.FileInfo) error {
 
 	// Skip some of the meta files which are not device files.
@@ -405,19 +417,16 @@ func (devices *DeviceSet) deviceFileWalkFunction(path string, finfo os.FileInfo)
 		hash = ""
 	}
 
-	dinfo := devices.loadMetadata(hash)
-	if dinfo == nil {
-		return fmt.Errorf("Error loading device metadata file %s", hash)
+	if _, err := devices.lookupDevice(hash); err != nil {
+		return fmt.Errorf("Error looking up device %s:%v", hash, err)
 	}
 
-	devices.markDeviceIDUsed(dinfo.DeviceID)
-	logrus.Debugf("Added deviceID=%d to DeviceIDMap", dinfo.DeviceID)
 	return nil
 }
 
-func (devices *DeviceSet) constructDeviceIDMap() error {
-	logrus.Debugf("[deviceset] constructDeviceIDMap()")
-	defer logrus.Debugf("[deviceset] constructDeviceIDMap() END")
+func (devices *DeviceSet) loadDeviceFilesOnStart() error {
+	logrus.Debugf("[deviceset] loadDeviceFilesOnStart()")
+	defer logrus.Debugf("[deviceset] loadDeviceFilesOnStart() END")
 
 	var scan = func(path string, info os.FileInfo, err error) error {
 		if err != nil {
@@ -568,10 +577,12 @@ func (devices *DeviceSet) initMetaData() error {
 
 	devices.TransactionID = transactionID
 
-	if err := devices.constructDeviceIDMap(); err != nil {
-		return err
+	if err := devices.loadDeviceFilesOnStart(); err != nil {
+		return fmt.Errorf("devmapper: Failed to load device files:%v", err)
 	}
 
+	devices.constructDeviceIDMap()
+
 	if err := devices.processPendingTransaction(); err != nil {
 		return err
 	}