Alpha/Vote: fix potential overflow in quorum computation

This patch prevents a potential overflow if the number of rolls get
too big in a future update. This bug cannot happen with the current
parent 09794e40
......@@ -55,22 +55,25 @@ let check_approval_and_update_quorum ctxt =
Vote.get_ballots ctxt >>=? fun ballots ->
Vote.listing_size ctxt >>=? fun maximum_vote ->
Vote.get_current_quorum ctxt >>=? fun expected_quorum ->
(* FIXME check overflow ??? *)
let casted_vote = Int32.add ballots.yay ballots.nay in
let actual_vote = Int32.add casted_vote ballots.pass in
let actual_quorum =
Int32.div (Int32.mul actual_vote 100_00l) maximum_vote in
let supermajority = Int32.div (Int32.mul 8l casted_vote) 10l in
(* Note overflows: considering a maximum of 8e8 tokens, with roll size as
small as 1e3, there is a maximum of 8e5 rolls and thus votes.
In 'participation' an Int64 is used because in the worst case 'all_votes is
8e5 and after the multiplication is 8e9, making it potentially overflow a
signed Int32 which is 2e9. *)
let casted_votes = Int32.add ballots.yay ballots.nay in
let all_votes = Int32.add casted_votes ballots.pass in
let supermajority = Int32.div (Int32.mul 8l casted_votes) 10l in
let participation = (* in centile of percentage *)
(Int64.mul (Int64.of_int32 all_votes) 100_00L)
(Int64.of_int32 maximum_vote)) in
let outcome = Compare.Int32.(participation >= expected_quorum &&
ballots.yay >= supermajority) in
let updated_quorum =
(Int32.add (Int32.mul 8l expected_quorum)
(Int32.mul 2l actual_quorum))
10l in
Int32.div (Int32.add (Int32.mul 8l expected_quorum) (Int32.mul 2l participation)) 10l in
Vote.set_current_quorum ctxt updated_quorum >>=? fun ctxt ->
Compare.Int32.(actual_quorum >= expected_quorum
&& ballots.yay >= supermajority))
return (ctxt, outcome)
(** Implements the state machine of the amendment procedure.
Note that [freeze_listings], that computes the vote weight of each delegate,
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