paras: Add runtime events for PVF pre-checking (#4683)

In this PR, paras module emit runtime events on certain PVF pre-checking
related conditions.

Specifically, there are 3 new events in the paras module:

1. PvfCheckStarted
2. PvfCheckAccepted
3. PvfCheckRejected

All of those have identifiers for the parachain that triggered the PVF
pre-checking and the validation code that goes through the pre-checking.

The mechanics of those are as follows. Each time a new PVF is added, be
it due to onboarding or upgrading, the `PvfCheckStarted` will be
triggered. If another parachain triggers a pre-checking process for the
validation code which is already being pre-checked, another
`PvfCheckStarted` event will be triggered with the corresponding para
id.

When the PVF pre-checking voting for a PVF was finished, several
`PvfCheckAccepted/Rejected` events will be triggered: one for each para id that
was subscribed to this check (i.e. was a "cause" for it).

If the PVF pre-checking is disabled, then one can still expect these
events to be fired. Since insta PVF approval is syncronous, the
`PvfCheckStarted` will be followed by the `PvfCheckAccepted` with the
same validation code and para id.

If somebody is interested in following validation code changes for a PVF
of a parachain, they would need to subscribe to those events. I did not
supply the topics for the events, since I am not sure if that's needed
or will be used, but they can be added later if needed.

polkadot/runtime/parachains/src/paras/tests.rs

@@ -113,6 +113,49 @@ fn check_code_is_not_stored(validation_code: &ValidationCode) {
 	assert!(!<Paras as Store>::CodeByHash::contains_key(validation_code.hash()));
 }
 
+/// An utility for checking that certain events were deposited.
+struct EventValidator {
+	events:
+		Vec<frame_system::EventRecord<<Test as frame_system::Config>::Event, primitives::v1::Hash>>,
+}
+
+impl EventValidator {
+	fn new() -> Self {
+		Self { events: Vec::new() }
+	}
+
+	fn started(&mut self, code: &ValidationCode, id: ParaId) -> &mut Self {
+		self.events.push(frame_system::EventRecord {
+			phase: frame_system::Phase::Initialization,
+			event: Event::PvfCheckStarted(code.hash(), id).into(),
+			topics: vec![],
+		});
+		self
+	}
+
+	fn rejected(&mut self, code: &ValidationCode, id: ParaId) -> &mut Self {
+		self.events.push(frame_system::EventRecord {
+			phase: frame_system::Phase::Initialization,
+			event: Event::PvfCheckRejected(code.hash(), id).into(),
+			topics: vec![],
+		});
+		self
+	}
+
+	fn accepted(&mut self, code: &ValidationCode, id: ParaId) -> &mut Self {
+		self.events.push(frame_system::EventRecord {
+			phase: frame_system::Phase::Initialization,
+			event: Event::PvfCheckAccepted(code.hash(), id).into(),
+			topics: vec![],
+		});
+		self
+	}
+
+	fn check(&self) {
+		assert_eq!(&frame_system::Pallet::<Test>::events(), &self.events);
+	}
+}
+
 #[test]
 fn para_past_code_pruning_works_correctly() {
 	let mut past_code = ParaPastCodeMeta::default();
@@ -1048,6 +1091,14 @@ fn pvf_check_coalescing_onboarding_and_upgrade() {
 			<Paras as Store>::FutureCodeUpgrades::get(&a),
 			Some(RELAY_PARENT + validation_upgrade_delay),
 		);
+
+		// Verify that the required events were emitted.
+		EventValidator::new()
+			.started(&validation_code, b)
+			.started(&validation_code, a)
+			.accepted(&validation_code, b)
+			.accepted(&validation_code, a)
+			.check();
 	});
 }
 
@@ -1157,6 +1208,9 @@ fn pvf_check_upgrade_reject() {
 		assert!(<Paras as Store>::PvfActiveVoteMap::get(&new_code.hash()).is_none());
 		assert!(Paras::pvfs_require_precheck().is_empty());
 		assert!(<Paras as Store>::FutureCodeHash::get(&a).is_none());
+
+		// Verify that the required events were emitted.
+		EventValidator::new().started(&new_code, a).rejected(&new_code, a).check();
 	});
 }
 
@@ -1398,6 +1452,12 @@ fn add_trusted_validation_code_insta_approval() {
 			<Paras as Store>::FutureCodeUpgrades::get(&para_id),
 			Some(1 + validation_upgrade_delay)
 		);
+
+		// Verify that the required events were emitted.
+		EventValidator::new()
+			.started(&validation_code, para_id)
+			.accepted(&validation_code, para_id)
+			.check();
 	});
 }