Clients are overcharged for mutable shares that grow enough to change their price

[exarkun]

This is a specific case split off of #221 .

If at time T0 a client uploads a mutable that costs N ZKAPs then they must provide N ZKAPs for the operation to succeed and they are granted a lease on the stored data that expires at time Te. If they modify the mutable at time T1 (<Te) so that the mutable costs an additional M (>0) ZKAPs then they must provide M ZKAPs for the operation to succeed. At this point they have paid for (N + M) × (Te - T0) units of storage-time for one lease period but they are only going to receive (N × (Te - T0) + M × (Te - T1)) units of storage-time. Since T1 > T0 we know that this will be less than what they paid for.