I already posted it on the mailing list, it didn't receive any replies, but I still think it's a bug. Please correct me if I'm wrong.

A tailable cursor on a capped collection can live potentially for a long time and return multiple batches of data.
However, it seems to me that the size of each batch that is read from Mongo is added to the list: DBApiLayer.Result._sizes, in the DBApiLayer.Result.init method, called from the _advance method.

The list is available via DBCursor.getSizes().

If the list grows and grows, especially if there's a lot of data added to the capped collection, eventually it will OOM.

Is that right?

UPDATE:

Added DBCursor.disableBatchSizeTracking() to work around this problem.