caffeine: Unintuitive behavior (bug?) with `put()`

Upon migrating from Guava cache to Caffeine in bazel we are seeing some flakiness in tests that heavily exercise weak/soft-valued caches. It appears that the behavior of put() for an already present key in Caffeine diverges from Guava cache (and ConcurrentHashMap) in that a concurrent thread calling getIfPresent can get null, as opposed to either the old value or the new value.

Note that this strange behavior occurs even if the put call does not change the existing value. I believe it’s the result of non-atomically clearing the existing value reference before setting it.

The following reproduces the issue within a few iterations:

Cache<String, String> cache = Caffeine.newBuilder().weakValues().build();
String key = "key";
String val = "val";
cache.put("key", "val");

AtomicReference<Exception> e = new AtomicReference<>();
List<Thread> threads = new ArrayList<>();
for (int i = 0; i < 10; i++) {
  int name = i;
  Thread t =
          new Thread(
              () -> {
                for (int j = 0; j < 1000; j++) {
                  if (Math.random() < .5) {
                    cache.put(key, val);
                  } else if (cache.getIfPresent(key) == null) {
                    e.compareAndSet(
                        null,
                        new IllegalStateException(
                            "Thread " + name + " observed null on iteration " + j));
                    break;
                  }
                }
              });
  threads.add(t);
  t.start();
}

for (Thread t : threads) {
  t.join();
}

if (e.get() != null) {
  throw e.get();
}