property described with multipleOf does not pass the validation

[jpaliwoda13]

In my openapi file, I have one property that is described as follows:

Quantity:
    type: number
    maximum: 999999999.99
    minimum: -999999999.99
    multipleOf: 0.01

The problem is that not every value provided as Quantity passes validation.

This part of lib code seems to be responsible for random outcome: kin-openapi/openapi3/schema.go#L1508 as the division value/0.01 sometimes gives a rounded value (so it’s Int and it passes validation), but sometimes it gives decimal outcome and the validation returns an error.

Here are some examples of Quantity for which division by 0.01 returns Int, so the validation passes:
8.11
68.15
2130.46
5895083

And here are some examples of Quantity for which division by 0.01 returns non-Int, so the validation fails:
8.1
2.07
19628.87
323.39
40428.2

I believe some adjustments to this validation should be provided.

[fenollp]

Hello! Indeed IEEE floats seem to be getting in the way. https://pkg.go.dev/math/big#Rat should be used instead.
If you're down to open a PR with this fix as well as with regression tests, I'll gladly review it! Thanks

[maxxant]

Good discussion and proposal for golang "decimal float types (IEEE 754-2008)" :
golang/go#19787

Good explanation "Why don't you just use big.Rat" for money and trading or financial application:
https://github.com/shopspring/decimal#why-dont-you-just-use-bigrat

another decimal implementation woodsbury/decimal128 that can do a few things faster:
woodsbury/decimal128#4

[fenollp]

Hey @maxxant thanks for doing this!

Indeed big.Rat usage is full of footguns. decimal is too short on precision for JSON (float64). decimal128 looks good enough to go with it!
We should keep that choice of lib hidden (to the API) so as to be able to switch to a future golang/std impl.
This means tweaking openapi3.Schema de-serialization of Min Max and MultipleOf, also json.Number.
If we can't manage this well we'll that'd just be a breaking change and new version.

Feel free to publish your attempts at this! I'll help :)