Lock issue when iterating map.

[MrSleazy]

I spend a bit of time debugging this simple case.

@Test
    public void iteratingMapError() {
        db = DBMaker.memoryDB().transactionEnable().make();
        Map<String, String> myStringMap = db.hashMap("fileReadyMap", Serializer.STRING, Serializer.ELSA).createOrOpen();
        myStringMap.put("myKey1", "value1");

        myStringMap.forEach((key, value) -> myStringMap.remove(key)); // stuck forever
        myStringMap.entrySet().removeIf(stringStringEntry -> true); // not stuck
    }

It's a very simple use case:

• Insert an entry to a map.
• Execute for each loop within the map
• Remove entry from map.

I spent some time debugging the locking mechanism in the ReentrantReadWriteLock and HTreeMap classes to see what was going on. Particularly this piece of code here:

private inline fun <E> segmentWrite(segment:Int, body:()->E):E{
        val lock = locks[segment]?.writeLock()
        lock?.lock()
        try {
            return body()
        }finally{
            lock?.unlock()
        }
    }

I filed this as a bug because I encountered this very strange case upon calling remove(key) while doing a for-each in the map. The READ lock is blocking the remove operation and stays stuck forever. You can see here what's going on:

After a quick google search, I realized this kind of operation is illegal and that's why you must use an iterator to do this: https://stackoverflow.com/questions/6092642/how-to-remove-a-key-from-hashmap-while-iterating-over-it

However, I think your implementation should check for the READ lock and throw a similar java.util.ConcurrentModificationException in this case, to avoid forever-locking issues like these.