Source file staking_pseudotokens_storage.ml
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(** {0} Introduction
This module is responsible for maintaining the
{!Storage.Contract.Frozen_deposits_pseudotokens} and
{!Storage.Contract.Staking_pseudotokens} tables.
{1} Terminology
Even so a registered delegate is always technically in a delegation
relation with itself, in this module, when we use the word
"delegator", we always mean a delegator different from the
delegate itself. The word "staker" means for a delegator who
participates in staking. The word "staker" means any participant
in staking, either a delegate or a staker. In this module, we
use the word "contract" to mean either a delegate or a delegator.
{1} Full staking balance of a delegate
For each delegate, the {!Stake_storage} module is responsible to
track three tez quantities which can be requested with the
{!Stake_storage.get_full_staking_balance} function: [own_frozen]
is the frozen deposits of the delegate, [staked_frozen] is the
sum of all frozen deposits of its stakers, and [delegate] is the
sum of all tez delegated to the delegate (some of which may belong
to the delegate itself). This module is in charge of tracking the
frozen deposits of each staker. Since we already have access to
their sum ([staked_frozen]) we only need to track the proportion
of this sum owned by each staker.
{1} Pseudo-tokens
The {!Storage.Contract.Frozen_deposits_pseudotokens} and
{!Storage.Contract.Staking_pseudotokens} tables are used to keep
track of this proportion. The amounts stored in these tables don't
have a fixed value in tez, they can be seen as shares of the total
frozen deposits of a delegate's stakers, we call them
pseudotokens. Pseudotokens are minted when a staker increases
its share using the stake pseudo-operation; they are burnt when a
staker decreases its share using the request-unstake
pseudo-operation. Events which modify uniformly the frozen
deposits of all the stakers of a delegate (reward distribution
and slashing) don't lead to minting nor burning any pseudotokens;
that's the main motivation for using these pseudotokens: thanks to
them we never need to iterate over the stakers of a delegate
(whose number is unbounded).
{1} Conversion rate:
The conversion rate between pseudotokens and mutez (the value in
mutez of a pseudotoken) should be given by the ratio between the
delegate's current staked frozen deposits and the total number
of pseudotokens of the delegate; it's actually the case when this
total number of pseudotokens is positive. When the total number of
pseudotokens of a delegate is null, the conversion rate could
theoretically have any value but extreme values are dangerous
because of overflows and loss of precision; for these reasons, we
use one as the conversion rate when the total number of
pseudotokens is null, which can happen in two situations:
- the first time a delegator stakes since the
migration which created the pseudotoken tables, and
- when stakers empty their delegate's staked frozen deposits and later
receive rewards.
{2} Implementation:
The {!Storage.Contract.Staking_pseudotokens} table stores for
each staker its {i staking balance pseudotokens} which is the
number of pseudotokens owned by the staker.
The {!Storage.Contract.Frozen_deposits_pseudotokens} table stores
for each delegate the {i frozen deposits pseudotokens} of the
delegate which is defined as the sum of all the staking balance
pseudotokens of its stakers.
For both tables, pseudotokens are represented using the
[Pseudotoken_repr.t] type which is, like [Tez_repr.t], stored on
non-negative signed int64.
{2} Invariants:
{3} Invariant 1: frozen deposits pseudotokens initialization
For {!Storage.Contract.Frozen_deposits_pseudotokens}, a missing
key is equivalent to a value of [0]. This case means that there are
no pseudotokens, the delegate has no staker, the conversion rate is [1].
{3} Invariant 2: staking balance pseudotokens initialization
For {!Storage.Contract.Staking_pseudotokens}, a missing key is
equivalent to a value of [0].
{3} Invariant 3: relationship between frozen deposits and staking
balance pseudotokens
For a given delegate, their frozen deposits pseudotokens equal
the sum of all staking pseudotokens of their delegators.
{3} Invariant 4: delegates have no staking pseudotokens.
*)
(** When a delegate gets totally slashed, the value of its
pseudotokens becomes 0 and before minting any new token we would
need to iterate over all stakers to empty their pseudotoken
balances. We want to avoid iterating over stakers so we forbid
{b stake} in this case. *)
type error += Cannot_stake_on_fully_slashed_delegate
(** These two types are not exported, they are views to the portions
of the storage which are relevant in this module when a delegate
or a staker are considered. *)
type delegate_balances = {
delegate : Signature.public_key_hash;
frozen_deposits_staked_tez : Tez_repr.t;
frozen_deposits_pseudotokens : Staking_pseudotoken_repr.t;
}
type delegator_balances = {
delegator : Contract_repr.t;
pseudotoken_balance : Staking_pseudotoken_repr.t;
delegate_balances : delegate_balances;
}
(** {0} Functions reading from the storage *)
(** [get_frozen_deposits_staked_tez ctxt ~delegate] returns the sum of frozen
deposits, in tez, of the delegate's stakers. *)
let get_frozen_deposits_staked_tez ctxt ~delegate =
let open Lwt_result_syntax in
let+ staking_balance = Stake_storage.get_full_staking_balance ctxt delegate in
Full_staking_balance_repr.staked_frozen staking_balance
let get_own_frozen_deposits ctxt ~delegate =
let open Lwt_result_syntax in
let+ staking_balance = Stake_storage.get_full_staking_balance ctxt delegate in
Full_staking_balance_repr.own_frozen staking_balance
(** [get_frozen_deposits_pseudotokens ctxt ~delegate] returns the total
number of pseudotokens in circulation for the given
[delegate]. This should, by invariant 3 be the sum of the
staking balance (in pseudotokens) of all its delegators.
To preserve invariant 1, this should be the only function of this
module reading from the
{!Storage.Contract.Frozen_deposits_pseudotokens} table. *)
let get_frozen_deposits_pseudotokens ctxt ~delegate =
let open Lwt_result_syntax in
let+ frozen_deposits_pseudotokens_opt =
Storage.Contract.Frozen_deposits_pseudotokens.find ctxt (Implicit delegate)
in
Option.value
frozen_deposits_pseudotokens_opt
~default:Staking_pseudotoken_repr.zero
(** [staking_pseudotokens_balance ctxt ~delegator] returns
[delegator]'s current staking balance in pseudotokens.
To preserve invariant 2, this should be the only function of this
module reading from the {!Storage.Contract.Staking_pseudotokens}
table.
*)
let staking_pseudotokens_balance ctxt ~delegator =
let open Lwt_result_syntax in
let+ staking_pseudotokens_opt =
Storage.Contract.Staking_pseudotokens.find ctxt delegator
in
Option.value ~default:Staking_pseudotoken_repr.zero staking_pseudotokens_opt
(** [get_delegate_balances ctxt ~delegate] records the staked frozen deposits
in tez and pseudotokens of a given delegate.
Postcondition:
delegate = result.delegate /\
get_frozen_deposits_staked_tez ctxt ~delegate = return result.frozen_deposits_staked_tez /\
get_frozen_deposits_pseudotokens ctxt ~delegate = return result.frozen_deposits_pseudotokens
*)
let get_delegate_balances ctxt ~delegate =
let open Lwt_result_syntax in
let* frozen_deposits_staked_tez =
get_frozen_deposits_staked_tez ctxt ~delegate
in
let+ frozen_deposits_pseudotokens =
get_frozen_deposits_pseudotokens ctxt ~delegate
in
{delegate; frozen_deposits_staked_tez; frozen_deposits_pseudotokens}
(** [get_delegator_balances ctxt ~delegator ~delegate_balances] enriches
the [delegate_balances] with [delegator]'s pseudotoken balance.
Precondition:
unchecked: [delegator != delegate_balance.delegate] /\
unchecked: [delegator] delegates to [delegate_balance.delegate]
unchecked: get_delegate_balances ctxt ~delegate = return delegate_balances
Postcondition:
result.delegator = delegator /\
result.delegate_balances = delegate_balances /\
staking_pseudotokens_balance ctxt ~delegator = return result.pseudotoken_balance
*)
let get_delegator_balances ctxt ~delegator ~delegate_balances =
let open Lwt_result_syntax in
let+ pseudotoken_balance = staking_pseudotokens_balance ctxt ~delegator in
{delegator; pseudotoken_balance; delegate_balances}
(** [mint_pseudotokens ctxt delegator_balances_before
pseudotokens_to_mint] mints [pseudotokens_to_mint] pseudotokens
and assign them to [delegator_balances_before.delegator]. Both
tables are updated to maintain invariant 3.
Precondition:
unchecked: get_delegator_balances ctxt delegator_balances_before.delegator = return delegator_balances_before /\
unchecked: invariant3(ctxt)
Postcondition:
get_delegator_balances ctxt delegator_balances_before.delegator =
return {delegator_balances_before with
pseudotoken_balance += pseudotokens_to_mint;
delegate_balances.frozen_deposits_pseudotokens += pseudotokens_to_mint} /\
invariant3(ctxt)
*)
let mint_pseudotokens ctxt (delegator_balances_before : delegator_balances)
pseudotokens_to_mint =
let open Lwt_result_syntax in
let delegator = delegator_balances_before.delegator in
let delegate = delegator_balances_before.delegate_balances.delegate in
let*? new_pseudotoken_balance =
Staking_pseudotoken_repr.(
delegator_balances_before.pseudotoken_balance +? pseudotokens_to_mint)
in
let*? new_delegate_total_frozen_deposits_pseudotokens =
Staking_pseudotoken_repr.(
delegator_balances_before.delegate_balances.frozen_deposits_pseudotokens
+? pseudotokens_to_mint)
in
let*! ctxt =
Storage.Contract.Staking_pseudotokens.add
ctxt
delegator
new_pseudotoken_balance
in
let*! ctxt =
Storage.Contract.Frozen_deposits_pseudotokens.add
ctxt
(Implicit delegate)
new_delegate_total_frozen_deposits_pseudotokens
in
let balance_updates =
Receipt_repr.
[
item
(Staking_delegator_numerator {delegator})
(Credited pseudotokens_to_mint)
Block_application;
item
(Staking_delegate_denominator {delegate})
(Credited pseudotokens_to_mint)
Block_application;
]
in
return (ctxt, balance_updates)
(** [burn_pseudotokens ctxt delegator_balances_before
pseudotokens_to_burn] burns [pseudotokens_to_burn] pseudotokens
from the balance of [delegator_balances_before.delegator]. Both
tables are updated to maintain invariant 3.
Precondition:
unchecked: get_delegator_balances ctxt delegator_balances_before.delegator = return delegator_balances_before /\
unchecked: invariant3(ctxt)
Postcondition:
get_delegator_balances ctxt delegator_balances_before.delegator =
return {delegator_balances_before with
pseudotoken_balance -= pseudotokens_to_mint;
delegate_balances.frozen_deposits_pseudotokens -= pseudotokens_to_mint} /\
invariant3(ctxt)
*)
let burn_pseudotokens ctxt (delegator_balances_before : delegator_balances)
pseudotokens_to_burn =
let open Lwt_result_syntax in
let delegator = delegator_balances_before.delegator in
let delegate = delegator_balances_before.delegate_balances.delegate in
let*? new_pseudotoken_balance =
Staking_pseudotoken_repr.(
delegator_balances_before.pseudotoken_balance -? pseudotokens_to_burn)
in
let*? new_delegate_total_frozen_deposits_pseudotokens =
Staking_pseudotoken_repr.(
delegator_balances_before.delegate_balances.frozen_deposits_pseudotokens
-? pseudotokens_to_burn)
in
let*! ctxt =
Storage.Contract.Staking_pseudotokens.add
ctxt
delegator
new_pseudotoken_balance
in
let*! ctxt =
Storage.Contract.Frozen_deposits_pseudotokens.add
ctxt
(Implicit delegate)
new_delegate_total_frozen_deposits_pseudotokens
in
let balance_updates =
Receipt_repr.
[
item
(Staking_delegate_denominator {delegate})
(Debited pseudotokens_to_burn)
Block_application;
item
(Staking_delegator_numerator {delegator})
(Debited pseudotokens_to_burn)
Block_application;
]
in
return (ctxt, balance_updates)
(** {0} Conversion between tez and pseudotokens *)
(** Tez -> pseudotokens conversion.
Precondition:
tez_amount <> 0 /\
delegate_balances.frozen_deposits_pseudotokens <> 0 /\
delegate_balances.frozen_deposits_staked_tez <> 0.
Postcondition:
result <> 0.
*)
let pseudotokens_of ~rounding (delegate_balances : delegate_balances) tez_amount
=
assert (
Staking_pseudotoken_repr.(
delegate_balances.frozen_deposits_pseudotokens <> zero)) ;
assert (Tez_repr.(delegate_balances.frozen_deposits_staked_tez <> zero)) ;
assert (Tez_repr.(tez_amount <> zero)) ;
Staking_pseudotoken_repr.mul_ratio
~rounding
delegate_balances.frozen_deposits_pseudotokens
~num:(Tez_repr.to_mutez tez_amount)
~den:(Tez_repr.to_mutez delegate_balances.frozen_deposits_staked_tez)
(** Pseudotokens -> tez conversion.
Precondition:
delegate_balances.frozen_deposits_pseudotokens <> 0.
*)
let tez_of ~rounding (delegate_balances : delegate_balances) pseudotoken_amount
=
assert (
Staking_pseudotoken_repr.(
delegate_balances.frozen_deposits_pseudotokens <> zero)) ;
Tez_repr.mul_ratio
~rounding
delegate_balances.frozen_deposits_staked_tez
~num:(Staking_pseudotoken_repr.to_int64 pseudotoken_amount)
~den:
(Staking_pseudotoken_repr.to_int64
delegate_balances.frozen_deposits_pseudotokens)
(** [compute_pseudotoken_credit_for_tez_amount delegate_balances
tez_amount] is a safe wrapper around [pseudotokens_of
delegate_balances tez_amount].
Rounding disadvantages newcomer (gets less pseudotokens for the same tez
value).
*)
let compute_pseudotoken_credit_for_tez_amount delegate_balances tez_amount =
let open Result_syntax in
if Tez_repr.(tez_amount = zero) then
return Staking_pseudotoken_repr.zero
else if
Staking_pseudotoken_repr.(
delegate_balances.frozen_deposits_pseudotokens = zero)
then
return @@ Staking_pseudotoken_repr.init_of_tez tez_amount
else if Tez_repr.(delegate_balances.frozen_deposits_staked_tez = zero) then
tzfail Cannot_stake_on_fully_slashed_delegate
else pseudotokens_of ~rounding:`Down delegate_balances tez_amount
let stake ctxt ~delegator ~delegate tez_amount =
let open Lwt_result_syntax in
let* delegate_balances = get_delegate_balances ctxt ~delegate in
let*? pseudotokens_to_credit =
compute_pseudotoken_credit_for_tez_amount delegate_balances tez_amount
in
let* delegator_balances =
get_delegator_balances ctxt ~delegator ~delegate_balances
in
mint_pseudotokens ctxt delegator_balances pseudotokens_to_credit
(** {0} Exported functions, see the mli file. *)
let stake ctxt ~contract ~delegate tez_amount =
if Contract_repr.(contract = Implicit delegate) then
Lwt_result_syntax.return (ctxt, [])
else stake ctxt ~delegator:contract ~delegate tez_amount
let request_unstake ctxt ~delegator ~delegate requested_amount =
let open Lwt_result_syntax in
let* delegate_balances = get_delegate_balances ctxt ~delegate in
if Tez_repr.(delegate_balances.frozen_deposits_staked_tez = zero) then
return (ctxt, Tez_repr.zero, [])
else
let* delegator_balances =
get_delegator_balances ctxt ~delegator ~delegate_balances
in
if Staking_pseudotoken_repr.(delegator_balances.pseudotoken_balance = zero)
then return (ctxt, Tez_repr.zero, [])
else (
assert (
Staking_pseudotoken_repr.(
delegate_balances.frozen_deposits_pseudotokens <> zero)) ;
let*? max_to_request =
tez_of
~rounding:`Down
delegate_balances
delegator_balances.pseudotoken_balance
in
let requested_amount = Tez_repr.min max_to_request requested_amount in
let*? requested_pseudotokens =
pseudotokens_of ~rounding:`Up delegate_balances requested_amount
in
let pseudotokens_to_unstake =
Staking_pseudotoken_repr.min
requested_pseudotokens
delegator_balances.pseudotoken_balance
in
let*? tez_to_unstake =
tez_of ~rounding:`Down delegate_balances pseudotokens_to_unstake
in
let*? pseudotokens_to_unstake, tez_to_unstake =
let open Result_syntax in
if Tez_repr.(tez_to_unstake > requested_amount) then (
let pseudotokens_to_unstake =
match Staking_pseudotoken_repr.pred pseudotokens_to_unstake with
| None -> assert false
| Some pt -> pt
in
let* tez_to_unstake =
tez_of ~rounding:`Down delegate_balances pseudotokens_to_unstake
in
assert (Tez_repr.(tez_to_unstake <= requested_amount)) ;
return (pseudotokens_to_unstake, tez_to_unstake))
else return (pseudotokens_to_unstake, tez_to_unstake)
in
let+ ctxt, balance_updates =
burn_pseudotokens ctxt delegator_balances pseudotokens_to_unstake
in
(ctxt, tez_to_unstake, balance_updates))
let request_unstake ctxt ~contract ~delegate requested_amount =
let open Lwt_result_syntax in
if Tez_repr.(requested_amount = zero) then return (ctxt, Tez_repr.zero, [])
else if Contract_repr.(contract = Implicit delegate) then
let+ delegate_own_frozen_deposits =
get_own_frozen_deposits ctxt ~delegate
in
(ctxt, Tez_repr.min delegate_own_frozen_deposits requested_amount, [])
else request_unstake ctxt ~delegator:contract ~delegate requested_amount
module For_RPC = struct
let staked_balance ctxt ~delegator ~delegate =
let open Lwt_result_syntax in
let* delegate_balances = get_delegate_balances ctxt ~delegate in
let* delegator_balances =
get_delegator_balances ctxt ~delegator ~delegate_balances
in
if
Staking_pseudotoken_repr.(
delegate_balances.frozen_deposits_pseudotokens <> zero)
then
Lwt.return
@@ tez_of
~rounding:`Down
delegate_balances
delegator_balances.pseudotoken_balance
else (
assert (
Staking_pseudotoken_repr.(delegator_balances.pseudotoken_balance = zero)) ;
return Tez_repr.zero)
let staked_balance ctxt ~contract ~delegate =
if Contract_repr.(contract = Implicit delegate) then
get_own_frozen_deposits ctxt ~delegate
else staked_balance ctxt ~delegator:contract ~delegate
let staking_pseudotokens_balance = staking_pseudotokens_balance
let get_frozen_deposits_pseudotokens = get_frozen_deposits_pseudotokens
let get_frozen_deposits_staked_tez = get_frozen_deposits_staked_tez
end