Background
The server only applies stricter index key pattern validation to v:2 indexes (as of SERVER-26659). This means that the server does not apply stricter validation on pre-3.4 legacy indexes. Pre-3.4 index key values could be 0, empty string, numbers other than -1 and 1, non-numeric / string values, etc. See SERVER-11064 for more information. 3.4+ server versions still handle pre-3.4 legacy indexes because these legacy indexes can persist across server upgrades.

Solution
We should confirm how pre-3.4 index key values behave on 3.4+ server versions. This includes (but is not limited to):

• 0
• empty string
• non-numeric / non string values
• really small ints / floats

Any index key value that was accepted in pre-3.4 but not in 3.4+ should be investigated. We should also document this (internally or publicly).

Impact on Mongosync / Mongomirror / Mongorestore
Mongosyc and Mongomirror automatically convert source legacy index specs when building them on the destination. Mongorestore also does this if --convertLegacyIndexes option is enabled. Verifying pre-3.4 index behavior will ensure that mongosync/mongomirror/mongorestore converts them in the right way. For example, mongosync/mongomirror/mongorestore must convert a legacy index key value of 0 to 1 instead -1. Otherwise, the destination cluster will treat the index differently compared to the source cluster, which is a correctness issue for mongosync/mongomirror/mongorestore.