Problem

max.hirschhorn@mongodb.com had a comment in the legacy index conversion IA:

Additionally, a key pattern with a literal NaN value won't be coerced. New index builds have their key patterns validated as v:2 indexes would and so a literal NaN value would result in the index build failing.

An index with value NaN can be created with pre-3.4 server versions and upgraded to a supported source version. Mongosync is not handling it as part of legacy index conversion, and thus causing failure when creating it on the destination.

Solution & Acceptance Criteria

Since it's treated as an ascending index according to the server test, we should convert NaN index key value to 1.

Impact

One less edge case that mongorestore fails upon.