Add on_idle hook (#8209)

* add in idle hook

* remaining weight passed through to on_idle

* added weight return

* remove TODO

* weight adjustment fix

* added adjusted weight into tuple

* Update frame/support/src/dispatch.rs

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

* Update frame/support/src/dispatch.rs

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

* Update frame/support/src/dispatch.rs

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

* Update frame/support/src/dispatch.rs

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

* Update frame/support/src/dispatch.rs

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

* Update frame/support/src/dispatch.rs

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

* Update frame/support/src/dispatch.rs

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

* compile errors for on_idle in dispatch

* Update frame/support/src/dispatch.rs

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

* Update frame/support/src/dispatch.rs

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

* Update frame/support/src/dispatch.rs

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

* on idle tuple clean up

* register reduced weight

* collect and add reduced wait from on idle call

* better demo example

* Update frame/support/procedural/src/pallet/expand/hooks.rs

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

* added tests to dispatch.rs

* idle test on executive

* skip on idle if remaining weight is 0

* Update frame/executive/src/lib.rs

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>

* Update frame/support/src/dispatch.rs

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>

* abstract common logic out to functions

* docs

* remove demo example

* remove debug

* spacing

* docs

* revert template pallet to master

* change reduced weight to used weight

* remove empty line

* lint

* spacing

* Update frame/support/src/traits.rs

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>

* documentation

* Update frame/support/procedural/src/pallet/expand/hooks.rs

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

* docs

* Update frame/support/src/traits.rs

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>

* docs

* Update frame/support/src/traits.rs

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>

* Update frame/support/src/traits.rs

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>

* Update frame/support/src/traits.rs

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>
Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>
Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>

substrate/frame/executive/src/lib.rs

@@ -119,7 +119,7 @@
 use sp_std::{prelude::*, marker::PhantomData};
 use frame_support::{
 	weights::{GetDispatchInfo, DispatchInfo, DispatchClass},
-	traits::{OnInitialize, OnFinalize, OnRuntimeUpgrade, OffchainWorker, ExecuteBlock},
+	traits::{OnInitialize, OnIdle, OnFinalize, OnRuntimeUpgrade, OffchainWorker, ExecuteBlock},
 	dispatch::PostDispatchInfo,
 };
 use sp_runtime::{
@@ -160,6 +160,7 @@ impl<
 	AllModules:
 		OnRuntimeUpgrade +
 		OnInitialize<System::BlockNumber> +
+		OnIdle<System::BlockNumber> +
 		OnFinalize<System::BlockNumber> +
 		OffchainWorker<System::BlockNumber>,
 	COnRuntimeUpgrade: OnRuntimeUpgrade,
@@ -186,6 +187,7 @@ impl<
 		UnsignedValidator,
 		AllModules: OnRuntimeUpgrade
 			+ OnInitialize<System::BlockNumber>
+			+ OnIdle<System::BlockNumber>
 			+ OnFinalize<System::BlockNumber>
 			+ OffchainWorker<System::BlockNumber>,
 		COnRuntimeUpgrade: OnRuntimeUpgrade,
@@ -349,8 +351,8 @@ where
 
 		// post-extrinsics book-keeping
 		<frame_system::Module<System>>::note_finished_extrinsics();
-		<frame_system::Module<System> as OnFinalize<System::BlockNumber>>::on_finalize(block_number);
+
-		<AllModules as OnFinalize<System::BlockNumber>>::on_finalize(block_number);
+		Self::idle_and_finalize_hook(block_number);
 	}
 
 	/// Finalize the block - it is up the caller to ensure that all header fields are valid
@@ -360,12 +362,36 @@ where
 		sp_tracing::enter_span!( sp_tracing::Level::TRACE, "finalize_block" );
 		<frame_system::Module<System>>::note_finished_extrinsics();
 		let block_number = <frame_system::Module<System>>::block_number();
-		<frame_system::Module<System> as OnFinalize<System::BlockNumber>>::on_finalize(block_number);
+
-		<AllModules as OnFinalize<System::BlockNumber>>::on_finalize(block_number);
+		Self::idle_and_finalize_hook(block_number);
 
 		<frame_system::Module<System>>::finalize()
 	}
 
+	fn idle_and_finalize_hook(block_number: NumberFor<Block>) {
+		let weight =  <frame_system::Module<System>>::block_weight();
+		let max_weight =  <System::BlockWeights as frame_support::traits::Get<_>>::get().max_block;
+		let mut remaining_weight = max_weight.saturating_sub(weight.total());
+
+		if remaining_weight > 0 {
+			let mut used_weight =
+                <frame_system::Module<System> as OnIdle<System::BlockNumber>>::on_idle(
+                    block_number,
+                    remaining_weight
+                );
+			remaining_weight = remaining_weight.saturating_sub(used_weight);
+			used_weight = <AllModules as OnIdle<System::BlockNumber>>::on_idle(
+                block_number,
+                remaining_weight
+            )
+            .saturating_add(used_weight);
+			<frame_system::Module::<System>>::register_extra_weight_unchecked(used_weight, DispatchClass::Mandatory);
+		}
+
+		<frame_system::Module<System> as OnFinalize<System::BlockNumber>>::on_finalize(block_number);
+		<AllModules as OnFinalize<System::BlockNumber>>::on_finalize(block_number);
+	}
+
 	/// Apply extrinsic outside of the block execution function.
 	///
 	/// This doesn't attempt to validate anything regarding the block, but it builds a list of uxt
@@ -555,6 +581,11 @@ mod tests {
 					175
 				}
 
+				fn on_idle(n: T::BlockNumber, remaining_weight: Weight) -> Weight {
+					println!("on_idle{}, {})", n, remaining_weight);
+					175
+				}
+
 				fn on_finalize() {
 					println!("on_finalize(?)");
 				}
@@ -769,7 +800,7 @@ mod tests {
 				header: Header {
 					parent_hash: [69u8; 32].into(),
 					number: 1,
-					state_root: hex!("2c01e6f33d595793119823478b45b36978a8f65a731b5ae3fdfb6330b4cd4b11").into(),
+					state_root: hex!("6e70de4fa07bac443dc7f8a812c8a0c941aacfa892bb373c5899f7d511d4c25b").into(),
 					extrinsics_root: hex!("03170a2e7597b7b7e3d84c05391d139a62b157e78786d8c082f29dcf4c111314").into(),
 					digest: Digest { logs: vec![], },
 				},
@@ -1006,10 +1037,11 @@ mod tests {
 		new_test_ext(1).execute_with(|| {
 
 			Executive::initialize_block(&Header::new_from_number(1));
+			Executive::finalize_block();
 			// NOTE: might need updates over time if new weights are introduced.
 			// For now it only accounts for the base block execution weight and
 			// the `on_initialize` weight defined in the custom test module.
-			assert_eq!(<frame_system::Module<Runtime>>::block_weight().total(), 175 + 10);
+			assert_eq!(<frame_system::Module<Runtime>>::block_weight().total(), 175 + 175  + 10);
 		})
 	}
 

substrate/frame/support/procedural/src/pallet/expand/hooks.rs

@@ -63,6 +63,22 @@ pub fn expand_hooks(def: &mut Def) -> proc_macro2::TokenStream {
 			}
 		}
 
+		impl<#type_impl_gen>
+			#frame_support::traits::OnIdle<<T as #frame_system::Config>::BlockNumber>
+			for #pallet_ident<#type_use_gen> #where_clause
+		{
+			fn on_idle(
+				n: <T as #frame_system::Config>::BlockNumber,
+				remaining_weight: #frame_support::weights::Weight
+			) -> #frame_support::weights::Weight {
+				<
+					Self as #frame_support::traits::Hooks<
+						<T as #frame_system::Config>::BlockNumber
+					>
+				>::on_idle(n, remaining_weight)
+			}
+		}
+
 		impl<#type_impl_gen>
 			#frame_support::traits::OnInitialize<<T as #frame_system::Config>::BlockNumber>
 			for #pallet_ident<#type_use_gen> #where_clause

substrate/frame/support/src/dispatch.rs

@@ -300,6 +300,12 @@ impl<T> Parameter for T where T: Codec + EncodeLike + Clone + Eq + fmt::Debug {}
 ///   * `fn on_initialize(n: BlockNumber) -> frame_support::weights::Weight` or
 ///   * `fn on_initialize() -> frame_support::weights::Weight`
 ///
+/// * `on_idle`: Executes at the end of a block. Passes a remaining weight to provide a threshold
+/// for when to execute non vital functions. Using this function will implement the
+/// [`OnIdle`](./traits/trait.OnIdle.html) trait.
+/// Function signature is:
+///   * `fn on_idle(n: BlockNumber, remaining_weight: Weight) -> frame_support::weights::Weight`
+///
 /// * `on_finalize`: Executes at the end of a block. Using this function will
 /// implement the [`OnFinalize`](./traits/trait.OnFinalize.html) trait.
 /// Function signature can be either:
@@ -340,6 +346,7 @@ macro_rules! decl_module {
 			{}
 			{}
 			{}
+			{}
 			[]
 			$($t)*
 		);
@@ -375,6 +382,7 @@ macro_rules! decl_module {
 			{}
 			{}
 			{}
+			{}
 			[]
 			$($t)*
 		);
@@ -389,6 +397,7 @@ macro_rules! decl_module {
 		{}
 		{ $( $on_initialize:tt )* }
 		{ $( $on_runtime_upgrade:tt )* }
+		{ $( $on_idle:tt )* }
 		{ $( $on_finalize:tt )* }
 		{ $( $offchain:tt )* }
 		{ $( $constants:tt )* }
@@ -407,6 +416,7 @@ macro_rules! decl_module {
 			{ $vis fn deposit_event() = default; }
 			{ $( $on_initialize )* }
 			{ $( $on_runtime_upgrade )* }
+			{ $( $on_idle )* }
 			{ $( $on_finalize )* }
 			{ $( $offchain )* }
 			{ $( $constants )* }
@@ -424,6 +434,7 @@ macro_rules! decl_module {
 		{}
 		{ $( $on_initialize:tt )* }
 		{ $( $on_runtime_upgrade:tt )* }
+		{ $( $on_idle:tt )* }
 		{ $( $on_finalize:tt )* }
 		{ $( $offchain:tt )* }
 		{ $( $constants:tt )* }
@@ -449,6 +460,7 @@ macro_rules! decl_module {
 		{ $( $deposit_event:tt )+ }
 		{ $( $on_initialize:tt )* }
 		{ $( $on_runtime_upgrade:tt )* }
+		{ $( $on_idle:tt )* }
 		{ $( $on_finalize:tt )* }
 		{ $( $offchain:tt )* }
 		{ $( $constants:tt )* }
@@ -470,6 +482,7 @@ macro_rules! decl_module {
 		{ $( $deposit_event:tt )* }
 		{ $( $on_initialize:tt )* }
 		{ $( $on_runtime_upgrade:tt )* }
+		{ $( $on_idle:tt )* }
 		{}
 		{ $( $offchain:tt )* }
 		{ $( $constants:tt )* }
@@ -488,6 +501,7 @@ macro_rules! decl_module {
 			{ $( $deposit_event )* }
 			{ $( $on_initialize )* }
 			{ $( $on_runtime_upgrade )* }
+			{ $( $on_idle )* }
 			{
 				fn on_finalize( $( $param_name : $param ),* ) { $( $impl )* }
 			}
@@ -508,6 +522,7 @@ macro_rules! decl_module {
 		{ $( $deposit_event:tt )* }
 		{ $( $on_initialize:tt )* }
 		{ $( $on_runtime_upgrade:tt )* }
+		{ $( $on_idle:tt )* }
 		{}
 		{ $( $offchain:tt )* }
 		{ $( $constants:tt )* }
@@ -535,6 +550,7 @@ macro_rules! decl_module {
 		{ $( $deposit_event:tt )* }
 		{ $( $on_initialize:tt )* }
 		{ $( $on_runtime_upgrade:tt )* }
+		{ $( $on_idle:tt )* }
 		{ $( $on_finalize:tt )+ }
 		{ $( $offchain:tt )* }
 		{ $( $constants:tt )* }
@@ -548,6 +564,72 @@ macro_rules! decl_module {
 	) => {
 		compile_error!("`on_finalize` can only be passed once as input.");
 	};
+
+	// Add on_idle
+	(@normalize
+		$(#[$attr:meta])*
+		pub struct $mod_type:ident<$trait_instance:ident: $trait_name:ident$(<I>, I: $instantiable:path $(= $module_default_instance:path)?)?>
+		for enum $call_type:ident where origin: $origin_type:ty, system = $system:ident
+		{ $( $other_where_bounds:tt )* }
+		{ $( $deposit_event:tt )* }
+		{ $( $on_initialize:tt )* }
+		{ $( $on_runtime_upgrade:tt )* }
+		{}
+		{ $( $on_finalize:tt )* }
+		{ $( $offchain:tt )* }
+		{ $( $constants:tt )* }
+		{ $( $error_type:tt )* }
+		{ $( $integrity_test:tt )* }
+		[ $( $dispatchables:tt )* ]
+		$(#[doc = $doc_attr:tt])*
+		fn on_idle($param_name1:ident : $param1:ty, $param_name2:ident: $param2:ty $(,)? ) -> $return:ty { $( $impl:tt )* }
+		$($rest:tt)*
+	) => {
+		$crate::decl_module!(@normalize
+			$(#[$attr])*
+			pub struct $mod_type<$trait_instance: $trait_name$(<I>, I: $instantiable $(= $module_default_instance)?)?>
+			for enum $call_type where origin: $origin_type, system = $system
+			{ $( $other_where_bounds )* }
+			{ $( $deposit_event )* }
+			{ $( $on_initialize )* }
+			{ $( $on_runtime_upgrade )* }
+			{
+				fn on_idle( $param_name1: $param1, $param_name2: $param2 ) -> $return { $( $impl )* }
+			}
+			{ $( $on_finalize:tt )* }
+			{ $( $offchain )* }
+			{ $( $constants )* }
+			{ $( $error_type )* }
+			{ $( $integrity_test)* }
+			[ $( $dispatchables )* ]
+			$($rest)*
+		);
+	};
+	// compile_error for invalid on_idle function signature in decl_module
+	(@normalize
+		$(#[$attr:meta])*
+		pub struct $mod_type:ident<
+			$trait_instance:ident: $trait_name:ident$(<I>, I: $instantiable:path $(= $module_default_instance:path)?)?
+		>
+		for enum $call_type:ident where origin: $origin_type:ty, system = $system:ident
+		{ $( $other_where_bounds:tt )* }
+		{ $( $deposit_event:tt )* }
+		{ $( $on_initialize:tt )* }
+		{ $( $on_runtime_upgrade:tt )* }
+		{ $( $on_finalize:tt )* }
+		{ $( $offchain:tt )* }
+		{ $( $constants:tt )* }
+		{ $( $error_type:tt )* }
+		{ $( $integrity_test:tt )* }
+		[ $( $dispatchables:tt )* ]
+		$(#[doc = $doc_attr:tt])*
+		$(#[weight = $weight:expr])?
+		fn on_idle
+		$($rest:tt)*
+	) => {
+		compile_error!("`on_idle` method is reserved and syntax doesn't match expected syntax.");
+	};
+
 	// compile_error on_runtime_upgrade, without a given weight removed syntax.
 	(@normalize
 		$(#[$attr:meta])*
@@ -559,6 +641,7 @@ macro_rules! decl_module {
 		{ $( $deposit_event:tt )* }
 		{ $( $on_initialize:tt )* }
 		{}
+		{ $( $on_idle:tt )* }
 		{ $( $on_finalize:tt )* }
 		{ $( $offchain:tt )* }
 		{ $( $constants:tt )* }
@@ -584,6 +667,7 @@ macro_rules! decl_module {
 		{ $( $deposit_event:tt )* }
 		{ $( $on_initialize:tt )* }
 		{}
+		{ $( $on_idle:tt )* }
 		{ $( $on_finalize:tt )* }
 		{ $( $offchain:tt )* }
 		{ $( $constants:tt )* }
@@ -611,6 +695,7 @@ macro_rules! decl_module {
 		{ $( $deposit_event:tt )* }
 		{ $( $on_initialize:tt )* }
 		{}
+		{ $( $on_idle:tt )* }
 		{ $( $on_finalize:tt )* }
 		{ $( $offchain:tt )* }
 		{ $( $constants:tt )* }
@@ -631,6 +716,7 @@ macro_rules! decl_module {
 			{
 				fn on_runtime_upgrade( $( $param_name : $param ),* ) -> $return { $( $impl )* }
 			}
+			{ $( $on_idle )* }
 			{ $( $on_finalize )* }
 			{ $( $offchain )* }
 			{ $( $constants )* }
@@ -651,6 +737,7 @@ macro_rules! decl_module {
 		{ $( $deposit_event:tt )* }
 		{ $( $on_initialize:tt )* }
 		{ $( $on_runtime_upgrade:tt )+ }
+		{ $( $on_idle:tt )* }
 		{ $( $on_finalize:tt )* }
 		{ $( $offchain:tt )* }
 		{ $( $constants:tt )* }
@@ -674,6 +761,7 @@ macro_rules! decl_module {
 		{ $( $deposit_event:tt )* }
 		{ $( $on_initialize:tt )* }
 		{ $( $on_runtime_upgrade:tt )* }
+		{ $( $on_idle:tt )* }
 		{ $( $on_finalize:tt )* }
 		{ $( $offchain:tt )* }
 		{ $( $constants:tt )* }
@@ -692,6 +780,7 @@ macro_rules! decl_module {
 			{ $( $deposit_event )* }
 			{ $( $on_initialize )* }
 			{ $( $on_runtime_upgrade )* }
+			{ $( $on_idle )* }
 			{ $( $on_finalize )* }
 			{ $( $offchain )* }
 			{ $( $constants )* }
@@ -715,6 +804,7 @@ macro_rules! decl_module {
 		{ $( $deposit_event:tt )* }
 		{ $( $on_initialize:tt )* }
 		{ $( $on_runtime_upgrade:tt )* }
+		{ $( $on_idle:tt )* }
 		{ $( $on_finalize:tt )* }
 		{ $( $offchain:tt )* }
 		{ $( $constants:tt )* }
@@ -738,6 +828,7 @@ macro_rules! decl_module {
 		{ $( $deposit_event:tt )* }
 		{}
 		{ $( $on_runtime_upgrade:tt )* }
+		{ $( $on_idle:tt )* }
 		{ $( $on_finalize:tt )* }
 		{ $( $offchain:tt )* }
 		{ $( $constants:tt )* }
@@ -763,6 +854,7 @@ macro_rules! decl_module {
 		{ $( $deposit_event:tt )* }
 		{}
 		{ $( $on_runtime_upgrade:tt )* }
+		{ $( $on_idle:tt )* }
 		{ $( $on_finalize:tt )* }
 		{ $( $offchain:tt )* }
 		{ $( $constants:tt )* }
@@ -790,6 +882,7 @@ macro_rules! decl_module {
 		{ $( $deposit_event:tt )* }
 		{}
 		{ $( $on_runtime_upgrade:tt )* }
+		{ $( $on_idle:tt )* }
 		{ $( $on_finalize:tt )* }
 		{ $( $offchain:tt )* }
 		{ $( $constants:tt )* }
@@ -810,6 +903,7 @@ macro_rules! decl_module {
 				fn on_initialize( $( $param_name : $param ),* ) -> $return { $( $impl )* }
 			}
 			{ $( $on_runtime_upgrade )* }
+			{ $( $on_idle )* }
 			{ $( $on_finalize )* }
 			{ $( $offchain )* }
 			{ $( $constants )* }
@@ -830,6 +924,7 @@ macro_rules! decl_module {
 		{ $( $deposit_event:tt )* }
 		{ $( $on_initialize:tt )+ }
 		{ $( $on_runtime_upgrade:tt )* }
+		{ $( $on_idle:tt )* }
 		{ $( $on_finalize:tt )* }
 		{ $( $offchain:tt )* }
 		{ $( $constants:tt )* }
@@ -853,6 +948,7 @@ macro_rules! decl_module {
 		{ $( $deposit_event:tt )* }
 		{ $( $on_initialize:tt )* }
 		{ $( $on_runtime_upgrade:tt )* }
+		{ $( $on_idle:tt )* }
 		{ $( $on_finalize:tt )* }
 		{ }
 		{ $( $constants:tt )* }
@@ -873,6 +969,7 @@ macro_rules! decl_module {
 			{ $( $deposit_event )* }
 			{ $( $on_initialize )* }
 			{ $( $on_runtime_upgrade )* }
+			{ $( $on_idle )* }
 			{ $( $on_finalize )* }
 			{ fn offchain_worker( $( $param_name : $param ),* ) { $( $impl )* } }
 			{ $( $constants )* }
@@ -893,6 +990,7 @@ macro_rules! decl_module {
 		{ $( $deposit_event:tt )* }
 		{ $( $on_initialize:tt )* }
 		{ $( $on_runtime_upgrade:tt )* }
+		{ $( $on_idle:tt )* }
 		{ $( $on_finalize:tt )* }
 		{ $( $offchain:tt )+ }
 		{ $( $constants:tt )* }
@@ -917,6 +1015,7 @@ macro_rules! decl_module {
 		{ $( $deposit_event:tt )* }
 		{ $( $on_initialize:tt )* }
 		{ $( $on_runtime_upgrade:tt )* }
+		{ $( $on_idle:tt )* }
 		{ $( $on_finalize:tt )* }
 		{ $( $offchain:tt )* }
 		{ $( $constants:tt )* }
@@ -938,6 +1037,7 @@ macro_rules! decl_module {
 			{ $( $deposit_event )* }
 			{ $( $on_initialize )* }
 			{ $( $on_runtime_upgrade )* }
+			{ $( $on_idle )* }
 			{ $( $on_finalize )* }
 			{ $( $offchain )* }
 			{
@@ -964,6 +1064,7 @@ macro_rules! decl_module {
 		{ $( $deposit_event:tt )* }
 		{ $( $on_initialize:tt )* }
 		{ $( $on_runtime_upgrade:tt )* }
+		{ $( $on_idle:tt )* }
 		{ $( $on_finalize:tt )* }
 		{ $( $offchain:tt )* }
 		{ $( $constants:tt )* }
@@ -984,6 +1085,7 @@ macro_rules! decl_module {
 			{ $( $deposit_event )* }
 			{ $( $on_initialize )* }
 			{ $( $on_runtime_upgrade )* }
+			{ $( $on_idle )* }
 			{ $( $on_finalize )* }
 			{ $( $offchain )* }
 			{ $( $constants )* }
@@ -1005,6 +1107,7 @@ macro_rules! decl_module {
 		{ $( $deposit_event:tt )* }
 		{ $( $on_initialize:tt )* }
 		{ $( $on_runtime_upgrade:tt )* }
+		{ $( $on_idle:tt )* }
 		{ $( $on_finalize:tt )* }
 		{ $( $offchain:tt )* }
 		{ $( $constants:tt )* }
@@ -1023,6 +1126,7 @@ macro_rules! decl_module {
 			{ $( $deposit_event )* }
 			{ $( $on_initialize )* }
 			{ $( $on_runtime_upgrade )* }
+			{ $( $on_idle )* }
 			{ $( $on_finalize )* }
 			{ $( $offchain )* }
 			{ $( $constants )* }
@@ -1045,6 +1149,7 @@ macro_rules! decl_module {
 		{ $( $deposit_event:tt )* }
 		{ $( $on_initialize:tt )* }
 		{ $( $on_runtime_upgrade:tt )* }
+		{ $( $on_idle:tt )* }
 		{ $( $on_finalize:tt )* }
 		{ $( $offchain:tt )* }
 		{ $( $constants:tt )* }
@@ -1069,6 +1174,7 @@ macro_rules! decl_module {
 			{ $( $deposit_event )* }
 			{ $( $on_initialize )* }
 			{ $( $on_runtime_upgrade )* }
+			{ $( $on_idle )* }
 			{ $( $on_finalize )* }
 			{ $( $offchain )* }
 			{ $( $constants )* }
@@ -1099,6 +1205,7 @@ macro_rules! decl_module {
 		{ $( $deposit_event:tt )* }
 		{ $( $on_initialize:tt )* }
 		{ $( $on_runtime_upgrade:tt )* }
+		{ $( $on_idle:tt )* }
 		{ $( $on_finalize:tt )* }
 		{ $( $offchain:tt )* }
 		{ $( $constants:tt )* }
@@ -1127,6 +1234,7 @@ macro_rules! decl_module {
 		{ $( $deposit_event:tt )* }
 		{ $( $on_initialize:tt )* }
 		{ $( $on_runtime_upgrade:tt )* }
+		{ $( $on_idle:tt )* }
 		{ $( $on_finalize:tt )* }
 		{ $( $offchain:tt )* }
 		{ $( $constants:tt )* }
@@ -1155,6 +1263,7 @@ macro_rules! decl_module {
 		{ $( $deposit_event:tt )* }
 		{ $( $on_initialize:tt )* }
 		{ $( $on_runtime_upgrade:tt )* }
+		{ $( $on_idle:tt )* }
 		{ $( $on_finalize:tt )* }
 		{ $( $offchain:tt )* }
 		{ $( $constants:tt )* }
@@ -1183,6 +1292,7 @@ macro_rules! decl_module {
 		{ $( $deposit_event:tt )* }
 		{ $( $on_initialize:tt )* }
 		{ $( $on_runtime_upgrade:tt )* }
+		{ $( $on_idle:tt )* }
 		{ $( $on_finalize:tt )* }
 		{ $( $offchain:tt )* }
 		{ $( $constants:tt )* }
@@ -1212,6 +1322,7 @@ macro_rules! decl_module {
 		{ $( $deposit_event:tt )* }
 		{ $( $on_initialize:tt )* }
 		{ $( $on_runtime_upgrade:tt )* }
+		{ $( $on_idle:tt )* }
 		{ $( $on_finalize:tt )* }
 		{ $( $offchain:tt )* }
 		{ $( $constants:tt )* }
@@ -1229,6 +1340,7 @@ macro_rules! decl_module {
 			{ $( $deposit_event )* }
 			{ $( $on_initialize )* }
 			{ $( $on_runtime_upgrade )* }
+			{ $( $on_idle )* }
 			{ $( $on_finalize )* }
 			{ $( $offchain )* }
 			{ $( $constants )* }
@@ -1485,6 +1597,35 @@ macro_rules! decl_module {
 		}
 	};
 
+	(@impl_on_idle
+		{ $system:ident }
+		$module:ident<$trait_instance:ident: $trait_name:ident$(<I>, $instance:ident: $instantiable:path)?>;
+		{ $( $other_where_bounds:tt )* }
+		fn on_idle($param1:ident : $param1_ty:ty, $param2:ident: $param2_ty:ty) -> $return:ty { $( $impl:tt )* }
+	) => {
+		impl<$trait_instance: $system::Config + $trait_name$(<I>, $instance: $instantiable)?>
+			$crate::traits::OnIdle<<$trait_instance as $system::Config>::BlockNumber>
+			for $module<$trait_instance$(, $instance)?> where $( $other_where_bounds )*
+		{
+			fn on_idle($param1: $param1_ty, $param2: $param2_ty) -> $return {
+				$crate::sp_tracing::enter_span!($crate::sp_tracing::trace_span!("on_idle"));
+				{ $( $impl )* }
+			}
+		}
+	};
+
+	(@impl_on_idle
+		{ $system:ident }
+		$module:ident<$trait_instance:ident: $trait_name:ident$(<I>, $instance:ident: $instantiable:path)?>;
+		{ $( $other_where_bounds:tt )* }
+	) => {
+		impl<$trait_instance: $system::Config + $trait_name$(<I>, $instance: $instantiable)?>
+			$crate::traits::OnIdle<<$trait_instance as $system::Config>::BlockNumber>
+			for $module<$trait_instance$(, $instance)?> where $( $other_where_bounds )*
+		{
+		}
+	};
+
 	(@impl_offchain
 		{ $system:ident }
 		$module:ident<$trait_instance:ident: $trait_name:ident$(<I>, $instance:ident: $instantiable:path)?>;
@@ -1700,6 +1841,7 @@ macro_rules! decl_module {
 		{ $( $deposit_event:tt )* }
 		{ $( $on_initialize:tt )* }
 		{ $( $on_runtime_upgrade:tt )* }
+		{ $( $on_idle:tt )* }
 		{ $( $on_finalize:tt )* }
 		{ $( $offchain:tt )* }
 		{ $( $constants:tt )* }
@@ -1741,6 +1883,14 @@ macro_rules! decl_module {
 			$( $on_finalize )*
 		}
 
+		$crate::decl_module! {
+			@impl_on_idle
+			{ $system }
+			$mod_type<$trait_instance: $trait_name $(<I>, $instance: $instantiable)?>;
+			{ $( $other_where_bounds )* }
+			$( $on_idle )*
+		}
+
 		$crate::decl_module! {
 			@impl_offchain
 			{ $system }
@@ -2415,6 +2565,9 @@ macro_rules! __check_reserved_fn_name {
 	(on_runtime_upgrade $( $rest:ident )*) => {
 		$crate::__check_reserved_fn_name!(@compile_error on_runtime_upgrade);
 	};
+	(on_idle $( $rest:ident )*) => {
+		$crate::__check_reserved_fn_name!(@compile_error on_idle);
+	};
 	(on_finalize $( $rest:ident )*) => {
 		$crate::__check_reserved_fn_name!(@compile_error on_finalize);
 	};
@@ -2459,7 +2612,7 @@ mod tests {
 	use super::*;
 	use crate::weights::{DispatchInfo, DispatchClass, Pays, RuntimeDbWeight};
 	use crate::traits::{
-		CallMetadata, GetCallMetadata, GetCallName, OnInitialize, OnFinalize, OnRuntimeUpgrade,
+		CallMetadata, GetCallMetadata, GetCallName, OnInitialize, OnFinalize, OnIdle, OnRuntimeUpgrade,
 		IntegrityTest, Get, PalletInfo,
 	};
 
@@ -2522,6 +2675,10 @@ mod tests {
 			fn operational(_origin) { unreachable!() }
 
 			fn on_initialize(n: T::BlockNumber,) -> Weight { if n.into() == 42 { panic!("on_initialize") } 7 }
+			fn on_idle(n: T::BlockNumber, remaining_weight: Weight,) -> Weight {
+				if n.into() == 42 || remaining_weight == 42  { panic!("on_idle") }
+				7
+			}
 			fn on_finalize(n: T::BlockNumber,) { if n.into() == 42 { panic!("on_finalize") } }
 			fn on_runtime_upgrade() -> Weight { 10 }
 			fn offchain_worker() {}
@@ -2687,6 +2844,23 @@ mod tests {
 		assert_eq!(<Module<TraitImpl> as OnInitialize<u32>>::on_initialize(10), 7);
 	}
 
+	#[test]
+	#[should_panic(expected = "on_idle")]
+	fn on_idle_should_work_1() {
+		<Module<TraitImpl> as OnIdle<u32>>::on_idle(42, 9);
+	}
+
+	#[test]
+	#[should_panic(expected = "on_idle")]
+	fn on_idle_should_work_2() {
+		<Module<TraitImpl> as OnIdle<u32>>::on_idle(9, 42);
+	}
+
+	#[test]
+	fn on_idle_should_work_3() {
+		assert_eq!(<Module<TraitImpl> as OnIdle<u32>>::on_idle(10, 11), 7);
+	}
+
 	#[test]
 	#[should_panic(expected = "on_finalize")]
 	fn on_finalize_should_work() {

substrate/frame/support/src/lib.rs

@@ -1223,7 +1223,7 @@ pub mod pallet_prelude {
 ///
 /// ### Macro expansion:
 ///
-/// The macro implements the traits `OnInitialize`, `OnFinalize`, `OnRuntimeUpgrade`,
+/// The macro implements the traits `OnInitialize`, `OnIdle`, `OnFinalize`, `OnRuntimeUpgrade`,
 /// `OffchainWorker`, `IntegrityTest` using `Hooks` implementation.
 ///
 /// NOTE: OnRuntimeUpgrade is implemented with `Hooks::on_runtime_upgrade` and some additional

substrate/frame/support/src/traits.rs

@@ -1522,6 +1522,38 @@ pub trait OnFinalize<BlockNumber> {
 	fn on_finalize(_n: BlockNumber) {}
 }
 
+/// The block's on idle trait.
+///
+/// Implementing this lets you express what should happen for your pallet before
+/// block finalization (see `on_finalize` hook) in case any remaining weight is left.
+pub trait OnIdle<BlockNumber> {
+	/// The block is being finalized.
+	/// Implement to have something happen in case there is leftover weight.
+	/// Check the passed `remaining_weight` to make sure it is high enough to allow for
+	/// your pallet's extra computation.
+	///
+	/// NOTE: This function is called AFTER ALL extrinsics - including inherent extrinsics -
+	/// in a block are applied but before `on_finalize` is executed.
+	fn on_idle(
+		_n: BlockNumber,
+		_remaining_weight: crate::weights::Weight
+	) -> crate::weights::Weight {
+		0
+	}
+}
+
+#[impl_for_tuples(30)]
+impl<BlockNumber: Clone> OnIdle<BlockNumber> for Tuple {
+	fn on_idle(n: BlockNumber,  remaining_weight: crate::weights::Weight) -> crate::weights::Weight {
+		let mut weight = 0;
+		for_tuples!( #(
+			let adjusted_remaining_weight = remaining_weight.saturating_sub(weight);
+			weight = weight.saturating_add(Tuple::on_idle(n.clone(),  adjusted_remaining_weight));
+		)* );
+		weight
+	}
+}
+
 /// The block initialization trait.
 ///
 /// Implementing this lets you express what should happen for your pallet when the block is
@@ -1539,9 +1571,9 @@ pub trait OnInitialize<BlockNumber> {
 
 #[impl_for_tuples(30)]
 impl<BlockNumber: Clone> OnInitialize<BlockNumber> for Tuple {
-	fn on_initialize(_n: BlockNumber) -> crate::weights::Weight {
+	fn on_initialize(n: BlockNumber) -> crate::weights::Weight {
 		let mut weight = 0;
-		for_tuples!( #( weight = weight.saturating_add(Tuple::on_initialize(_n.clone())); )* );
+		for_tuples!( #( weight = weight.saturating_add(Tuple::on_initialize(n.clone())); )* );
 		weight
 	}
 }
@@ -2039,6 +2071,18 @@ pub trait Hooks<BlockNumber> {
 	/// The block is being finalized. Implement to have something happen.
 	fn on_finalize(_n: BlockNumber) {}
 
+	/// This will be run when the block is being finalized (before `on_finalize`).
+	/// Implement to have something happen using the remaining weight.
+	/// Will not fire if the remaining weight is 0.
+	/// Return the weight used, the hook will subtract it from current weight used
+	/// and pass the result to the next `on_idle` hook if it exists.
+	fn on_idle(
+		_n: BlockNumber,
+		_remaining_weight: crate::weights::Weight
+	) -> crate::weights::Weight {
+		0
+	}
+
 	/// The block is being initialized. Implement to have something happen.
 	///
 	/// Return the non-negotiable weight consumed in the block.