pallet-aura: add feature-flagged explicit slot duration type (#14649)

* aura: add feature-flagged explicit slot duration type

* fmt

* add some comments

* have node-template use new explicit feature

* fix mock

* fmt

* use the experimental feature flag instead

* checkout master Cargo.lock

substrate/bin/node-template/runtime/Cargo.toml

@@ -114,3 +114,4 @@ try-runtime = [
 	"pallet-transaction-payment/try-runtime",
 	"sp-runtime/try-runtime"
 ]
+experimental = ["pallet-aura/experimental"]

substrate/bin/node-template/runtime/src/lib.rs

@@ -207,6 +207,9 @@ impl pallet_aura::Config for Runtime {
 	type DisabledValidators = ();
 	type MaxAuthorities = ConstU32<32>;
 	type AllowMultipleBlocksPerSlot = ConstBool<false>;
+
+	#[cfg(feature = "experimental")]
+	type SlotDuration = pallet_aura::MinimumPeriodTimesTwo<Runtime>;
 }
 
 impl pallet_grandpa::Config for Runtime {

substrate/frame/aura/Cargo.toml

@@ -48,3 +48,4 @@ try-runtime = [
 	"pallet-timestamp/try-runtime",
 	"sp-runtime/try-runtime"
 ]
+experimental = []

substrate/frame/aura/src/lib.rs

@@ -60,6 +60,23 @@ pub use pallet::*;
 
 const LOG_TARGET: &str = "runtime::aura";
 
+/// A slot duration provider which infers the slot duration from the
+/// [`pallet_timestamp::Config::MinimumPeriod`] by multiplying it by two, to ensure
+/// that authors have the majority of their slot to author within.
+///
+/// This was the default behavior of the Aura pallet and may be used for
+/// backwards compatibility.
+///
+/// Note that this type is likely not useful without the `experimental`
+/// feature.
+pub struct MinimumPeriodTimesTwo<T>(sp_std::marker::PhantomData<T>);
+
+impl<T: pallet_timestamp::Config> Get<T::Moment> for MinimumPeriodTimesTwo<T> {
+	fn get() -> T::Moment {
+		<T as pallet_timestamp::Config>::MinimumPeriod::get().saturating_mul(2u32.into())
+	}
+}
+
 #[frame_support::pallet]
 pub mod pallet {
 	use super::*;
@@ -95,6 +112,16 @@ pub mod pallet {
 		/// another pallet which enforces some limitation on the number of blocks authors can create
 		/// using the same slot.
 		type AllowMultipleBlocksPerSlot: Get<bool>;
+
+		/// The slot duration Aura should run with, expressed in milliseconds.
+		/// The effective value of this type should not change while the chain is running.
+		///
+		/// For backwards compatibility either use [`MinimumPeriodTimesTwo`] or a const.
+		///
+		/// This associated type is only present when compiled with the `experimental`
+		/// feature.
+		#[cfg(feature = "experimental")]
+		type SlotDuration: Get<<Self as pallet_timestamp::Config>::Moment>;
 	}
 
 	#[pallet::pallet]
@@ -218,9 +245,17 @@ impl<T: Config> Pallet<T> {
 
 	/// Determine the Aura slot-duration based on the Timestamp module configuration.
 	pub fn slot_duration() -> T::Moment {
-		// we double the minimum block-period so each author can always propose within
+		#[cfg(feature = "experimental")]
-		// the majority of its slot.
+		{
-		<T as pallet_timestamp::Config>::MinimumPeriod::get().saturating_mul(2u32.into())
+			T::SlotDuration::get()
+		}
+
+		#[cfg(not(feature = "experimental"))]
+		{
+			// we double the minimum block-period so each author can always propose within
+			// the majority of its slot.
+			<T as pallet_timestamp::Config>::MinimumPeriod::get().saturating_mul(2u32.into())
+		}
 	}
 
 	/// Ensure the correctness of the state of this pallet.

substrate/frame/aura/src/mock.rs

@@ -30,6 +30,8 @@ use sp_runtime::{testing::UintAuthorityId, traits::IdentityLookup, BuildStorage}
 
 type Block = frame_system::mocking::MockBlock<Test>;
 
+const SLOT_DURATION: u64 = 2;
+
 frame_support::construct_runtime!(
 	pub enum Test
 	{
@@ -68,7 +70,7 @@ impl frame_system::Config for Test {
 impl pallet_timestamp::Config for Test {
 	type Moment = u64;
 	type OnTimestampSet = Aura;
-	type MinimumPeriod = ConstU64<1>;
+	type MinimumPeriod = ConstU64<{ SLOT_DURATION / 2 }>;
 	type WeightInfo = ();
 }
 
@@ -100,6 +102,9 @@ impl pallet_aura::Config for Test {
 	type DisabledValidators = MockDisabledValidators;
 	type MaxAuthorities = ConstU32<10>;
 	type AllowMultipleBlocksPerSlot = AllowMultipleBlocksPerSlot;
+
+	#[cfg(feature = "experimental")]
+	type SlotDuration = ConstU64<SLOT_DURATION>;
 }
 
 fn build_ext(authorities: Vec<u64>) -> sp_io::TestExternalities {