[doc] Example MBM pallet (#2119)

## Basic example showcasing a migration using the MBM framework

This PR has been built on top of
https://github.com/paritytech/polkadot-sdk/pull/1781 and adds two new
example crates to the `examples` pallet

### Changes Made:

Added the `pallet-example-mbm` crate: This crate provides a minimal
example of a pallet that uses MBM. It showcases a storage migration
where values are migrated from a `u32` to a `u64`.

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>

substrate/frame/migrations/src/lib.rs

@@ -276,9 +276,10 @@ pub mod pallet {
 	#[pallet::pallet]
 	pub struct Pallet<T>(_);
 
-	#[pallet::config]
+	#[pallet::config(with_default)]
 	pub trait Config: frame_system::Config {
 		/// The overarching event type of the runtime.
+		#[pallet::no_default_bounds]
 		type RuntimeEvent: From<Event<Self>> + IsType<<Self as frame_system::Config>::RuntimeEvent>;
 
 		/// All the multi-block migrations to run.
@@ -286,12 +287,14 @@ pub mod pallet {
 		/// Should only be updated in a runtime-upgrade once all the old migrations have completed.
 		/// (Check that [`Cursor`] is `None`).
 		#[cfg(not(feature = "runtime-benchmarks"))]
+		#[pallet::no_default]
 		type Migrations: SteppedMigrations;
 
 		/// Mocked migrations for benchmarking only.
 		///
 		/// Should be configured to [`crate::mock_helpers::MockedMigrations`] in benchmarks.
 		#[cfg(feature = "runtime-benchmarks")]
+		#[pallet::no_default]
 		type Migrations: MockedMigrations;
 
 		/// The maximal length of an encoded cursor.
@@ -323,6 +326,45 @@ pub mod pallet {
 		type WeightInfo: WeightInfo;
 	}
 
+	/// Default implementations of [`DefaultConfig`], which can be used to implement [`Config`].
+	pub mod config_preludes {
+		use super::{inject_runtime_type, DefaultConfig};
+		use frame_support::{
+			derive_impl,
+			migrations::FreezeChainOnFailedMigration,
+			pallet_prelude::{ConstU32, *},
+		};
+		use frame_system::limits::BlockWeights;
+
+		/// Provides a viable default config that can be used with
+		/// [`derive_impl`](`frame_support::derive_impl`) to derive a testing pallet config
+		/// based on this one.
+		///
+		/// See `Test` in the `default-config` example pallet's `test.rs` for an example of
+		/// a downstream user of this particular `TestDefaultConfig`
+		pub struct TestDefaultConfig;
+
+		frame_support::parameter_types! {
+			/// Maximal weight per block that can be spent on migrations in tests.
+			pub TestMaxServiceWeight: Weight = <<TestDefaultConfig as frame_system::DefaultConfig>::BlockWeights as Get<BlockWeights>>::get().max_block.div(2);
+		}
+
+		#[derive_impl(frame_system::config_preludes::TestDefaultConfig, no_aggregated_types)]
+		impl frame_system::DefaultConfig for TestDefaultConfig {}
+
+		#[frame_support::register_default_impl(TestDefaultConfig)]
+		impl DefaultConfig for TestDefaultConfig {
+			#[inject_runtime_type]
+			type RuntimeEvent = ();
+			type CursorMaxLen = ConstU32<{ 1 << 16 }>;
+			type IdentifierMaxLen = ConstU32<{ 256 }>;
+			type MigrationStatusHandler = ();
+			type FailedMigrationHandler = FreezeChainOnFailedMigration;
+			type MaxServiceWeight = TestMaxServiceWeight;
+			type WeightInfo = ();
+		}
+	}
+
 	/// The currently active migration to run and its cursor.
 	///
 	/// `None` indicates that no migration is running.
@@ -421,12 +463,11 @@ pub mod pallet {
 			}
 
 			// The per-block service weight is sane.
-			#[cfg(not(test))]
 			{
 				let want = T::MaxServiceWeight::get();
 				let max = <T as frame_system::Config>::BlockWeights::get().max_block;
 
-				assert!(want.all_lte(max), "Service weight is larger than a block: {want} > {max}",);
+				assert!(want.all_lte(max), "Service weight is larger than a block: {want} > {max}");
 			}
 
 			// Cursor MEL
@@ -726,7 +767,8 @@ impl<T: Config> Pallet<T> {
 		}
 	}
 
-	fn exec_migration_max_weight() -> Weight {
+	/// The maximal weight of calling the private `Self::exec_migration` function.
+	pub fn exec_migration_max_weight() -> Weight {
 		T::WeightInfo::exec_migration_complete()
 			.max(T::WeightInfo::exec_migration_completed())
 			.max(T::WeightInfo::exec_migration_skipped_historic())

substrate/frame/migrations/src/mock.rs

@@ -28,7 +28,7 @@ use frame_support::{
 	weights::Weight,
 };
 use frame_system::EventRecord;
-use sp_core::{ConstU32, H256};
+use sp_core::H256;
 
 type Block = frame_system::mocking::MockBlock<Test>;
 
@@ -51,15 +51,14 @@ frame_support::parameter_types! {
 	pub const MaxServiceWeight: Weight = Weight::MAX.div(10);
 }
 
+#[derive_impl(crate::config_preludes::TestDefaultConfig)]
 impl crate::Config for Test {
-	type RuntimeEvent = RuntimeEvent;
+	#[cfg(feature = "runtime-benchmarks")]
+	type Migrations = crate::mock_helpers::MockedMigrations;
+	#[cfg(not(feature = "runtime-benchmarks"))]
 	type Migrations = MockedMigrations;
-	type CursorMaxLen = ConstU32<65_536>;
-	type IdentifierMaxLen = ConstU32<256>;
 	type MigrationStatusHandler = MockedMigrationStatusHandler;
 	type FailedMigrationHandler = MockedFailedMigrationHandler;
-	type MaxServiceWeight = MaxServiceWeight;
-	type WeightInfo = ();
 }
 
 frame_support::parameter_types! {

substrate/frame/migrations/src/mock_helpers.rs

@@ -79,7 +79,7 @@ impl SteppedMigrations for MockedMigrations {
 
 		let mut count: u32 =
 			cursor.as_ref().and_then(|c| Decode::decode(&mut &c[..]).ok()).unwrap_or(0);
-		log::debug!("MockedMigration: Step {}", count);
+		log::debug!("MockedMigration: Step {count} vs max {steps}");
 		if count != steps || matches!(kind, TimeoutAfter) {
 			count += 1;
 			return Some(Ok(Some(count.encode())))