Relay-parent digest logs for parachains (#2552)

* add digest item for relay-parent to primitives

* add a relay-parent-storage-root digest as a workaround

* more docs

* deposit log in pallet-parachain-system

* even more docs

* fix duplicate imports after botched mertge

* fix hyperlinks in docs

* clean up match

Co-authored-by: Bastian Köcher <git@kchr.de>

* improve docs

* fix typo

* add number to the digest item

---------

Co-authored-by: Bastian Köcher <git@kchr.de>

cumulus/pallets/parachain-system/src/lib.rs

@@ -385,6 +385,16 @@ pub mod pallet {
 			)
 			.expect("Invalid relay chain state proof");
 
+			// Deposit a log indicating the relay-parent storage root.
+			// TODO: remove this in favor of the relay-parent's hash after
+			// https://github.com/paritytech/cumulus/issues/303
+			frame_system::Pallet::<T>::deposit_log(
+				cumulus_primitives_core::rpsr_digest::relay_parent_storage_root_item(
+					vfp.relay_parent_storage_root,
+					vfp.relay_parent_number,
+				),
+			);
+
 			// initialization logic: we know that this runs exactly once every block,
 			// which means we can put the initialization logic here to remove the
 			// sequencing problem.

cumulus/pallets/parachain-system/src/tests.rs

@@ -1006,3 +1006,18 @@ fn upgrade_version_checks_should_work() {
 		});
 	}
 }
+
+#[test]
+fn deposits_relay_parent_storage_root() {
+	BlockTests::new().add_with_post_test(
+		123,
+		|| {},
+		|| {
+			let digest = System::digest();
+			assert!(cumulus_primitives_core::rpsr_digest::extract_relay_parent_storage_root(
+				&digest
+			)
+			.is_some());
+		},
+	);
+}