validatorUnbondingCanComplete must take into account (re)bonded validators

[danwt]

By the time a mature VSC is received on the provider side mapping to a validator unbonding operation the validator can already be rebonded!

We must take this into account in the code. Changes will be needed at least here

https://github.com/cosmos/cosmos-sdk/blob/c783aea68fbd856c2b188b2d467a7fa5cb4df1e6/x/staking/keeper/unbonding.go#L326-L346

func (k Keeper) validatorUnbondingCanComplete(ctx sdk.Context, id uint64) (found bool, err error) {
	val, found := k.GetValidatorByUnbondingId(ctx, id)
	if !found {
		return false, nil
	}


	if !val.IsMature(ctx.BlockTime(), ctx.BlockHeight()) {
		val.UnbondingOnHold = false
		k.SetValidator(ctx, val)
	} else {
		// If unbonding is mature complete it
		val = k.UnbondingToUnbonded(ctx, val)
		if val.GetDelegatorShares().IsZero() {
			k.RemoveValidator(ctx, val.GetOperator())
		}


		k.DeleteUnbondingIndex(ctx, id)
	}


	return true, nil
}

In the above, if the validator is already rebonded k.UnbondingToUnbonded can be called triggering a panic.

I am really confirmed about the following case, in addition to what is explicit above. Consider:

1. A validator unbonds which causes VSC 0 to go out
2. The validator rebonds
3. The validator unbonds which causes VSC 1 to go out
4. Maturity for 0 comes in, and the if !val.IsMature(ctx.BlockTime(), ctx.BlockHeight()) branch is entered
5. val.UnbondingOnHold is set false
6. The validator unbonding for 1 completes BEFORE maturity for 1 is received

I will make a PR soon.

Found with diff testing - although, it's interesting because in the diff model I do not model explicity all the data structures. Because of that, the diff model will search the validator unbonding queue for a validator with opID == opID before doing anything, which avoids the problem. Still though, the model is able to generate the example case.

[danwt]

Any fix must take care of state cleanup, which is a bit nuanced. See also this issue

• Fix leak in unbonding hooks #103

for taking care of leaks.

[danwt]

Was fixed by

• cosmos/cosmos-sdk@67c8163

[mpoke]

Was fixed by

cosmos/cosmos-sdk@67c8163

Fixed by cosmos/cosmos-sdk#12796

[danwt]

Closing for now as fixed by cosmos/cosmos-sdk#12796