We reduced the chance of hash collision in SERVER-48528 by using more bits. However, when one of the two hashes collides, we can still report wrong validation errors.

For example, the collection has a record r1 and a matching index entry with hashes {h1, h2}. There is also an extra index entry without a matching record r2 with the hashes {h1, h3}. As a result, we will have both bucket h1 and h3 with non-zero values. The second phase in IndexConsistency::addDocKey() will think that r1 has a missing index entry since bucket h1 is non-zero.

We may solve the problem by storing indexKeyCount as signed integers within IndexKeyBucket.