Right now both listIndexes and $indexStats return index specifications as they’re stored on disk. This causes problems when the server has accepted a “weird” index specification like {foo: NaN} because anything that needs to compare two index specs has to normalize the specifications.

This is nontrivial to do. (example: mongo-tools)

Ideally the server could just normalize all index specifications internally; however, that will probably (as noted in SERVER-27464) require an index version bump. In the meantime, C2C (and likely others?) could benefit significantly from having normalization built into listIndexes and $indexStats so that we’d never see {foo: 2} or {bar: -0.0000000002}, but always just 1 or -1.

Suggested interface

Add an optional `indexNormalization` parameter that accepts two strings: `normalized` and `none` (default). (It should fail if any other value is given.) If `normalized` is given, then all index spec values should either be 1, -1, or a string like `2d`. Identical indexes will always have the same normalized spec, and nonidentical indexes will never have the same normalized spec.

Behavior notes

• Pre-3.4 index key values should also be addressed (docs). For example, if a pre-3.4 index key value is 0, it should be normalized to be 1.
• Ignored fields (e.g., background) should be removed.