[CURATOR-459] Support for asynchronous locks in Curator Async

/**

 * A non-reentrant Lock with the ability to react when the lock is acquired async, so that the thread which

 * acquires the lock is not blocked.

 */

public interface CompletableLock {


 /**

 * Acquire the lock non-blocking. The resulting {@link CompletableFuture} can then be used to block

 * until the lock is actually acquired. The {@link CompletableFuture#cancel(boolean)} can be used

 * to abort the acquisition of the lock. As soon as the lock is actually acquired, this method has no

 * effect. The Lock itself is then released with {@link Lease#unlock()}.

 * 


 * Several calls results in several independent locking requests. So it is equivalent to

 * call this method multiple times or get a new lock and then call the method once.

 *

 * @return an instance of {@link CompletableFuture} which represents the actual lock.

 */

 CompletableFuture lock();

 default void withLock(Runnable body) {

 lock().thenAccept(lock -> {

 try {

 body.run();

 } finally {

 lock.unlock();

 }

 });

 }

 /**

 * Same as {@link #lock()} but if the lock can't be acquired within the given amount of time,

 * the {@link CompletableFuture} throws a {@link java.util.concurrent.ExecutionException} with a nested

 * {@link java.util.concurrent.TimeoutException} when it is accessed the next time.

 *

 * @param time the maximum waiting time for acquiring the lock.

 * @param unit the {@link TimeUnit} for the given time.

 * @return an instance of {@link CompletableFuture} which represents the actual lock.

 */

 CompletableFuture lock(int time, TimeUnit unit);

 /**

 * Gets the lock only if possible. If lock is currently acquired, this returns immediately with an undefined option.

 */

 Optional tryLock();

 /**

 * Gets the lock only if possible. If lock is currently acquired, this returns an undefined option after the

 * given timeout.

 */

 Lease tryLock(int time, TimeUnit unit);

 /**

 * For unlocking a successfully acquired lock.

 */

 interface Lease {

 /**

 * Releases the lock.

 */

 void unlock();

 }

}