Follow-up on #1068 (#1567)

* BestParaHead small changes

Signed-off-by: Serban Iorga <serban@parity.io>

* Renamings

Signed-off-by: Serban Iorga <serban@parity.io>

* Use ParaInfo in parachains loop

Signed-off-by: Serban Iorga <serban@parity.io>

* Define StorageMapKeyProvider

Signed-off-by: Serban Iorga <serban@parity.io>

* CR fixes

Signed-off-by: Serban Iorga <serban@parity.io>

bridges/primitives/parachains/src/lib.rs

@@ -22,7 +22,7 @@ use bp_polkadot_core::{
 	parachains::{ParaHash, ParaHead, ParaId},
 	BlockNumber as RelayBlockNumber,
 };
-use bp_runtime::StorageDoubleMapKeyProvider;
+use bp_runtime::{StorageDoubleMapKeyProvider, StorageMapKeyProvider};
 use codec::{Decode, Encode};
 use frame_support::{Blake2_128Concat, RuntimeDebug, Twox64Concat};
 use scale_info::TypeInfo;
@@ -44,6 +44,15 @@ pub struct BestParaHeadHash {
 	pub head_hash: ParaHash,
 }
 
+/// Best known parachain head as it is stored in the runtime storage.
+#[derive(Decode, Encode, PartialEq, RuntimeDebug, TypeInfo)]
+pub struct ParaInfo {
+	/// Best known parachain head hash.
+	pub best_head_hash: BestParaHeadHash,
+	/// Current ring buffer position for this parachain.
+	pub next_imported_hash_position: u32,
+}
+
 /// Returns runtime storage key of given parachain head at the source chain.
 ///
 /// The head is stored by the `paras` pallet in the `Heads` map.
@@ -54,18 +63,16 @@ pub fn parachain_head_storage_key_at_source(
 	bp_runtime::storage_map_final_key::<Twox64Concat>(paras_pallet_name, "Heads", &para_id.encode())
 }
 
-/// Returns runtime storage key of best known parachain head at the target chain.
+/// Can be use to access the runtime storage key of the parachains info at the target chain.
 ///
-/// The head is stored by the `pallet-bridge-parachains` pallet in the `BestParaHeads` map.
-pub fn best_parachain_head_hash_storage_key_at_target(
-	bridge_parachains_pallet_name: &str,
-	para_id: ParaId,
-) -> StorageKey {
-	bp_runtime::storage_map_final_key::<Blake2_128Concat>(
-		bridge_parachains_pallet_name,
-		"BestParaHeads",
-		&para_id.encode(),
-	)
+/// The info is stored by the `pallet-bridge-parachains` pallet in the `ParasInfo` map.
+pub struct ParasInfoKeyProvider;
+impl StorageMapKeyProvider for ParasInfoKeyProvider {
+	const MAP_NAME: &'static str = "ParasInfo";
+
+	type Hasher = Blake2_128Concat;
+	type Key = ParaId;
+	type Value = ParaInfo;
 }
 
 /// Can be use to access the runtime storage key of the parachain head at the target chain.

bridges/primitives/runtime/src/lib.rs

@@ -285,20 +285,42 @@ pub fn storage_value_key(pallet_prefix: &str, value_name: &str) -> StorageKey {
 	StorageKey(final_key)
 }
 
-/// Can be use to access the runtime storage key of a `StorageDoubleMap`.
-pub trait StorageDoubleMapKeyProvider {
-	// The name of the variable that holds the `StorageDoubleMap`
+/// Can be use to access the runtime storage key of a `StorageMap`.
+pub trait StorageMapKeyProvider {
+	/// The name of the variable that holds the `StorageMap`.
 	const MAP_NAME: &'static str;
 
-	// The same as `StorageDoubleMap::Hasher1`
+	/// The same as `StorageMap::Hasher1`.
+	type Hasher: StorageHasher;
+	/// The same as `StorageMap::Key1`.
+	type Key: FullCodec;
+	/// The same as `StorageMap::Value`.
+	type Value: FullCodec;
+
+	/// This is a copy of the
+	/// `frame_support::storage::generator::StorageMap::storage_map_final_key`.
+	///
+	/// We're using it because to call `storage_map_final_key` directly, we need access
+	/// to the runtime and pallet instance, which (sometimes) is impossible.
+	fn final_key(pallet_prefix: &str, key: &Self::Key) -> StorageKey {
+		storage_map_final_key::<Self::Hasher>(pallet_prefix, Self::MAP_NAME, &key.encode())
+	}
+}
+
+/// Can be use to access the runtime storage key of a `StorageDoubleMap`.
+pub trait StorageDoubleMapKeyProvider {
+	/// The name of the variable that holds the `StorageDoubleMap`.
+	const MAP_NAME: &'static str;
+
+	/// The same as `StorageDoubleMap::Hasher1`.
 	type Hasher1: StorageHasher;
-	// The same as `StorageDoubleMap::Key1`
+	/// The same as `StorageDoubleMap::Key1`.
 	type Key1: FullCodec;
-	// The same as `StorageDoubleMap::Hasher2`
+	/// The same as `StorageDoubleMap::Hasher2`.
 	type Hasher2: StorageHasher;
-	// The same as `StorageDoubleMap::Key2`
+	/// The same as `StorageDoubleMap::Key2`.
 	type Key2: FullCodec;
-	// The same as `StorageDoubleMap::Value`
+	/// The same as `StorageDoubleMap::Value`.
 	type Value: FullCodec;
 
 	/// This is a copy of the