introduce well known key for fetching committed para head

primitives/src/v4/mod.rs

@@ -187,6 +187,23 @@ pub mod well_known_keys {
 	pub const ACTIVE_CONFIG: &[u8] =
 		&hex!["06de3d8a54d27e44a9d5ce189618f22db4b49d95320d9021994c850f25b8e385"];
 
+	/// Hash of the committed head data for a given registered para.
+	///
+	/// The storage entry stores wrapped `HeadData(Vec<u8>)`.
+	pub fn para_head(para_id: Id) -> Vec<u8> {
+		let prefix = hex!["cd710b30bd2eab0352ddcc26417aa1941b3c252fcb29d88eff4f3de5de4476c3"];
+
+		para_id.using_encoded(|para_id: &[u8]| {
+			prefix
+				.as_ref()
+				.iter()
+				.chain(twox_64(para_id).iter())
+				.chain(para_id.iter())
+				.cloned()
+				.collect()
+		})
+	}
+
 	/// The upward message dispatch queue for the given para id.
 	///
 	/// The storage entry stores a tuple of two values:

runtime/parachains/src/paras/tests.rs

@@ -1700,6 +1700,21 @@ fn verify_upgrade_restriction_signal_is_externally_accessible() {
 	});
 }
 
+#[test]
+fn verify_para_head_is_externally_accessible() {
+	use primitives::well_known_keys;
+
+	let a = ParaId::from(2020);
+	let expected_head_data = HeadData(vec![0, 1, 2, 3]);
+
+	new_test_ext(Default::default()).execute_with(|| {
+		Heads::<Test>::insert(&a, expected_head_data.clone());
+		let encoded = sp_io::storage::get(&well_known_keys::para_head(a)).unwrap();
+		let head_data = HeadData::decode(&mut encoded.as_ref());
+		assert_eq!(head_data, Ok(expected_head_data));
+	});
+}
+
 #[test]
 fn parakind_encodes_decodes_to_bool_scale() {
 	let chain_kind = ParaKind::Parachain.encode();