Fix a race and some comments in ClipStorage.

 * Provides support for storing lists of documents identified by Uri.

 *

 * This class uses a ring buffer to recycle clip file slots, to mitigate the issue of clip file

 * deletions.

 * deletions. Below is the directory layout:

 * [cache dir]

 *      - [dir] 1

 *      - [dir] 2

 *      - ... to {@link #NUM_OF_SLOTS}

 * When a clip data is actively being used:

 * [cache dir]

 *      - [dir] 1

 *          - [file] primary

 *          - [symlink] 1 > primary # copying to location X

 *          - [symlink] 2 > primary # copying to location Y

 */

public final class ClipStorage {

    synchronized int claimStorageSlot() {

        int curPos = mNextPos;

        for (int i = 0; i < NUM_OF_SLOTS; ++i, curPos = (curPos + 1) % NUM_OF_SLOTS) {

            createSlotFile(curPos);

            createSlotFileObject(curPos);

            if (!mSlots[curPos].exists()) {

                break;

     * counting method. When someone is done using this symlink, it's responsible to delete it.

     * Therefore we can have a neat way to track how many things are still using this slot.

     */

    public File getFile(int tag) throws IOException {

        createSlotFile(tag);

    public synchronized File getFile(int tag) throws IOException {

        createSlotFileObject(tag);

        File primary = toSlotDataFile(tag);

        return new File(mSlots[pos], PRIMARY_DATA_FILE_NAME);

    }

    private void createSlotFile(int pos) {

    private void createSlotFileObject(int pos) {

        if (mSlots[pos] == null) {

            mSlots[pos] = new File(mOutDir, Integer.toString(pos));

        }