srml: system: prune block hash mapping (#3062)

* srml: system: prune block number to hash mapping

* srml: system: add test for block hash mapping pruning

* node: bump runtime version

* srml: system: wrap long line

* srml: system: use parameter type for block hash count

* srml: system: prune block hash mapping before storage root calculation

* srml: system: keep the genesis hash in block number map

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

@@ -111,6 +111,10 @@ pub fn native_version() -> NativeVersion {
 	}
 }
 
+parameter_types! {
+	pub const BlockHashCount: BlockNumber = 250;
+}
+
 impl system::Trait for Runtime {
 	/// The identifier used to distinguish between accounts.
 	type AccountId = AccountId;
@@ -130,6 +134,8 @@ impl system::Trait for Runtime {
 	type Event = Event;
 	/// The ubiquitous origin type.
 	type Origin = Origin;
+	/// Maximum number of block number to block hash mappings to keep (oldest pruned first).
+	type BlockHashCount = BlockHashCount;
 }
 
 impl aura::Trait for Runtime {

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

@@ -71,7 +71,7 @@ mod tests {
 
 	use runtime_io::with_externalities;
 	use primitives::{H256, Blake2Hasher};
-	use support::{impl_outer_origin, assert_ok};
+	use support::{impl_outer_origin, assert_ok, parameter_types};
 	use runtime_primitives::{traits::{BlakeTwo256, IdentityLookup}, testing::Header};
 
 	impl_outer_origin! {
@@ -83,6 +83,9 @@ mod tests {
 	// configuration traits of modules we want to use.
 	#[derive(Clone, Eq, PartialEq)]
 	pub struct Test;
+	parameter_types! {
+		pub const BlockHashCount: u64 = 250;
+	}
 	impl system::Trait for Test {
 		type Origin = Origin;
 		type Index = u64;
@@ -93,6 +96,7 @@ mod tests {
 		type Lookup = IdentityLookup<Self::AccountId>;
 		type Header = Header;
 		type Event = ();
+		type BlockHashCount = BlockHashCount;
 	}
 	impl Trait for Test {
 		type Event = ();