Fix thread safety issues within BasicStatusManager

[mches]

Fixes thread safety issues within BasicStatusManager, with respect to count and level. As it stands, updates to count and level could be temporarily or permanently invisible to other threads, or simply lost, due to lack of synchronization and non-atomic read-modify-write operations. Adopts the well-suited LongAdder for count and AtomicInteger for level. Without synchronization, level needs to be set using CAS to avoid a race.

[ruanwenjun]

How about

synchronized (statusListLock) {
      count++;
       if (newStatus.getLevel() > level) {
           level = newStatus.getLevel();
       }
       if (statusList.size() < MAX_HEADER_COUNT) {
          statusList.add(newStatus);
      } else {
          tailBuffer.add(newStatus);
      }
  }

The code might be more simple.

[mches]

Yes, we certainly could take advantage of the existing lock that's going to be taken out.

However, for correctness, the reads would also need to use the same lock:

    public int getLevel() {
        synchronized (statusListLock) {
            return level;
        }
    }

    public int getCount() {
        synchronized (statusListLock) {
            return count;
        }
    }

Otherwise the fields would need to be made volatile:

    volatile int count = 0;
    volatile int level = Status.INFO;

It also occurred to me that perhaps the level should be reset when the count is reset:

    public void clear() {
        synchronized (statusListLock) {
            count = 0;
            level = Level.INFO;
            statusList.clear();
            tailBuffer.clear();
        }
    }

[mches]

Here's an alternate approach to CAS

Suggested change

	count.increment();
	int newLevel = newStatus.getLevel();
	int currentLevel;
	do {
	currentLevel = level.get();
	if (newLevel <= currentLevel) {
	break;
	}
	} while (!level.compareAndSet(currentLevel, newLevel));
	synchronized (statusListLock) {
	synchronized (statusListLock) {
	count.increment();
	int newLevel = newStatus.getLevel();
	if (newLevel > level.get()) {
	level.set(newLevel);
	}

[mches]

Should level be reset as well?

Suggested change

	count.reset();
	count.reset();
	level.set(Status.INFO);

[mches]

Should StatusManager#getLevel() be uncommented?

package ch.qos.logback.core.status;
…
public interface StatusManager {
…
    /**
     * Return the highest level of all the statii.
     * 
     * @return
     */
    // int getLevel();
…
}

Should @Override be added to all implemented methods? (Available since Java 6)

See also #845