Reentrancy danger on controller’s onTransfer() callback

[3esmit]

The check-effects-interactions pattern was ignored during
MiniMeToken.doTransfer(), where after making a callback it uses stale balances during
the updates that come afterward.
Although the controller is a trusted entity of the system (and current implementation isn’t
vulnerable), This can lead to needless risk for future or more complex controller
implementations (as leveraging the reentrancy does not require any special permissions, just
the execution time at that moment).
Another way to look at this is that there’s an assumption of the system that the controller must
not give arbitrary 3rd party execution. Currently, this assumption has to be enforced on every
controller implementation.
Scenario: Let’s assume a non-malicious controller that can give another callback to the
user during onTransfer() call
When an attacker would transfer some X amount of funds in a way that gets the 2nd callback,
they can just do another Transfer for another X amount, and their balance would only decrease
by X.
That’s because in the first call, the MiniMeToken already checked the balance requirements and
calculated the balanceBefore but didn’t send it yet. Making a reentrant call here would double
spend the sent amount.