Implements a variable deposit base calculation for EPM signed submissions (#1547)

**Note**: This is a lift-and-shift PR from the old substrate and
polkadot repos, both PRs have been reviewed and audited
(https://github.com/paritytech/substrate/pull/13983,
https://github.com/paritytech/polkadot/pull/7140)

---

This PR implements a generic `BaseDeposit` calculation for signed
submissions, based on the size of the submission queue.

It adds a new associated type to EPM's config, `type SignedDepositBase`,
that implements `Convert<usize, BalanceOf<T>>`, which is used to
calculate the base deposit for signed submissions based on the size of
the signed submissions queue.

`struct GeometricDepositBase<Balance, Fixed, Inc>` implements the
convert trait so that the deposit value increases as a geometric
progression. The deposit base is calculated by `deposit_base =
fixed_deposit_base * (1 + increase_factor)^n`, where `n` is the term of
the progression (i.e. the number of signed submissions in the queue).
`Fixed` and `Inc` generic params are getters for `Balance` and
`IncreaseFactor` to compute the geometric progression. If
`IncreaseFactor = 0`, then the signed deposit is constant and equal to
`Fixed` regardless of the size of the queue.

### Runtime configs

In Kusama, the progression with 10% increase without changing the
current signed fixed deposit is: (term == size of the queue)

Term 1: `1,333,333,332,000`
Term 2: `1,333,333,332,000 * 1.10 = 1,466,666,665,200`
Term 3: `1,333,333,332,000 * 1.10^2 = 1,613,333,331,200`
Term 4: `1,333,333,332,000 * 1.10^3 = 1,774,666,664,320`
Term 5: `1,333,333,332,000 * 1.10^4 = 1,952,133,330,752`
Term 6: `1,333,333,332,000 * 1.10^5 = 2,147,346,663,827.20`
Term 7: `1,333,333,332,000 * 1.10^6 = 2,362,081,330,210.92`
Term 8: `1,333,333,332,000 * 1.10^7 = 2,598,289,463,231.01`
Term 9: `1,333,333,332,000 * 1.10^8 = 2,858,118,409,554.11`
Term 10: `1,333,333,332,000 * 1.10^9 = 3,143,930,250,509.52`

Westend:

Term 1: `2,000,000,000,000`
Term 2: `2,000,000,000,000 * 1.10 = 2,200,000,000,000`
Term 3: `2,000,000,000,000 * 1.10^2 = 2,420,000,000,000`
Term 4: `2,000,000,000,000 * 1.10^3 = 2,662,000,000,000`
Term 5: `2,000,000,000,000 * 1.10^4 = 2,928,200,000,000`
Term 6: `2,000,000,000,000 * 1.10^5 = 3,221,020,000,000`
Term 7: `2,000,000,000,000 * 1.10^6 = 3,543,122,000,000`
Term 8: `2,000,000,000,000 * 1.10^7 = 3,897,434,200,000`
Term 9: `2,000,000,000,000 * 1.10^8 = 4,287,177,620,000`
Term 10: `2,000,000,000,000 * 1.10^9 = 4,715,895,382,000`

and in Polkadot, the deposit increase is disabled in the current state
of the PR, as the increase factor is 0% -- so nothing changes from the
current behaviour.

Closes https://github.com/paritytech-secops/srlabs_findings/issues/189

substrate/frame/election-provider-multi-phase/src/mock.rs

@@ -16,7 +16,7 @@
 // limitations under the License.
 
 use super::*;
-use crate::{self as multi_phase, unsigned::MinerConfig};
+use crate::{self as multi_phase, signed::GeometricDepositBase, unsigned::MinerConfig};
 use frame_election_provider_support::{
 	bounds::{DataProviderBounds, ElectionBounds},
 	data_provider, onchain, ElectionDataProvider, NposSolution, SequentialPhragmen,
@@ -44,8 +44,8 @@ use sp_npos_elections::{
 use sp_runtime::{
 	bounded_vec,
 	testing::Header,
-	traits::{BlakeTwo256, IdentityLookup},
-	BuildStorage, PerU16,
+	traits::{BlakeTwo256, Convert, IdentityLookup},
+	BuildStorage, PerU16, Percent,
 };
 use std::sync::Arc;
 
@@ -283,7 +283,11 @@ parameter_types! {
 	pub static UnsignedPhase: BlockNumber = 5;
 	pub static SignedMaxSubmissions: u32 = 5;
 	pub static SignedMaxRefunds: u32 = 1;
-	pub static SignedDepositBase: Balance = 5;
+	// for tests only. if `EnableVariableDepositBase` is true, the deposit base will be calculated
+	// by `Multiphase::DepositBase`. Otherwise the deposit base is `SignedFixedDeposit`.
+	pub static EnableVariableDepositBase: bool = false;
+	pub static SignedFixedDeposit: Balance = 5;
+	pub static SignedDepositIncreaseFactor: Percent = Percent::from_percent(10);
 	pub static SignedDepositByte: Balance = 0;
 	pub static SignedDepositWeight: Balance = 0;
 	pub static SignedRewardBase: Balance = 7;
@@ -393,7 +397,7 @@ impl crate::Config for Runtime {
 	type OffchainRepeat = OffchainRepeat;
 	type MinerTxPriority = MinerTxPriority;
 	type SignedRewardBase = SignedRewardBase;
-	type SignedDepositBase = SignedDepositBase;
+	type SignedDepositBase = Self;
 	type SignedDepositByte = ();
 	type SignedDepositWeight = ();
 	type SignedMaxWeight = SignedMaxWeight;
@@ -414,6 +418,18 @@ impl crate::Config for Runtime {
 	type ElectionBounds = ElectionsBounds;
 }
 
+impl Convert<usize, BalanceOf<Runtime>> for Runtime {
+	/// returns the geometric increase deposit fee if `EnableVariableDepositBase` is set, otherwise
+	/// the fee is `SignedFixedDeposit`.
+	fn convert(queue_len: usize) -> Balance {
+		if !EnableVariableDepositBase::get() {
+			SignedFixedDeposit::get()
+		} else {
+			GeometricDepositBase::<Balance, SignedFixedDeposit, SignedDepositIncreaseFactor>::convert(queue_len)
+		}
+	}
+}
+
 impl<LocalCall> frame_system::offchain::SendTransactionTypes<LocalCall> for Runtime
 where
 	RuntimeCall: From<LocalCall>,
@@ -553,8 +569,14 @@ impl ExtBuilder {
 		<SignedMaxSubmissions>::set(count);
 		self
 	}
+	pub fn signed_base_deposit(self, base: u64, variable: bool, increase: Percent) -> Self {
+		<EnableVariableDepositBase>::set(variable);
+		<SignedFixedDeposit>::set(base);
+		<SignedDepositIncreaseFactor>::set(increase);
+		self
+	}
 	pub fn signed_deposit(self, base: u64, byte: u64, weight: u64) -> Self {
-		<SignedDepositBase>::set(base);
+		<SignedFixedDeposit>::set(base);
 		<SignedDepositByte>::set(byte);
 		<SignedDepositWeight>::set(weight);
 		self