Add version 6.5

[ZenGround0]

This is motivated by the effort to fix lotus pricelist to rely on version numbers filecoin-project/lotus#6652

Turns out versions are already an enum so this was easy.

[arajasek]

noice

[Stebalien]

Discussed in standup:

This should work, but it's exposed over the API so there are some concerns that users are using it. The main use is, as far as I know, the lotus miner.

The correct way to do this update is to make a breaking version change to the API, which is always a bit annoying.

[ZenGround0]

@Stebalien how terrible would it be to change from int to float if we make a breaking api change? The benefit would be that version numbers would look like users would expect: 6.5, 13.0. The cost would be using floats.

[Stebalien]

Awful? Checking equality between floats is generally a bad idea and not every (base-10) decimal can even be represented (accurately) as a float (which are effectively base-2 decimals).

If we're going to make a breaking change, I'd make these versions:

• 100
• 200
• 300
  ...

So we can stick a 250 in there.

We'd have to... rework a few cases where we assume that versions are mostly sequential, but that's not really a problem.

[whyrusleeping]

Hey friends, people were using this and it not just breaks, it causes panics

[whyrusleeping]

The panic we encountered comes from here: https://github.com/filecoin-project/lotus/blob/master/chain/actors/version.go#L37

Any version mismatch between this code and where youre getting the version number from will likely hit this.

[Stebalien]

Ok, I think we can sort of fix this by "aliasing" the old versions in that code. I.e., manually checking:

if version < 100 {
    version = version*100
}