Add synchronized to loaderQueue inside completable future

[prokop7]

Describe the bug
A bug that may cause jobs not to be added to loaderQueue due to race conditions.
DataLoader.load(K key, Object loadContext) has two ways of getting value, first - get immediately or add to queue, second - retrieve from the cache. Retrieving from cache works through consecutive CompletableFuture calls, if the cache misses then we add to queue or load immediately. The first way covered by synchronized on dataLoader but the second isn't.

This PR fixes this issue by adding synchronized on callback if the cache misses

To Reproduce
It's hard to reproduce because of multithreading

[bbakerman]

I am surprised this is working

because that bit of code is only called from one place

    CompletableFuture<V> load(K key, Object loadContext) {
        synchronized (dataLoader) {
            boolean batchingEnabled = loaderOptions.batchingEnabled();
            boolean cachingEnabled = loaderOptions.cachingEnabled();

            stats.incrementLoadCount();

            if (cachingEnabled) {
                return loadFromCache(key, loadContext, batchingEnabled);
            } else {
                return queueOrInvokeLoader(key, loadContext, batchingEnabled);
            }
        }
    }

As you can see its already inside a synchronized call on the data loader above.

Have you tried this on latest master (which was refactored recently)?

[bbakerman]

Ahhh its during the CF callback. Great catch!