Mock epoch index and randomness in SproofBuilder (for testing) (#1594)

* minimal babe randomness in sproof builder

* fix indexing

* fix index and clean comment

* use current para block for epoch and randomness

* epoch is para block randomness is relay block

* fix

* make more configurable but keep defaults

* fix export

* fix

* flatten

* better field names

* Update primitives/parachain-inherent/src/mock.rs

* Update primitives/parachain-inherent/src/mock.rs

* FMT

* FIX

* fmt

* Update primitives/parachain-inherent/src/mock.rs

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

primitives/parachain-inherent/src/mock.rs

@@ -42,7 +42,7 @@ use cumulus_test_relay_sproof_builder::RelayStateSproofBuilder;
 /// in addition to the messages themselves, you must provide some information about
 /// your parachain's configuration in order to mock the MQC heads properly.
 /// See [`MockXcmConfig`] for more information
-pub struct MockValidationDataInherentDataProvider {
+pub struct MockValidationDataInherentDataProvider<R = ()> {
 	/// The current block number of the local block chain (the parachain)
 	pub current_para_block: u32,
 	/// The relay block in which this parachain appeared to start. This will be the relay block
@@ -51,6 +51,11 @@ pub struct MockValidationDataInherentDataProvider {
 	/// The number of relay blocks that elapses between each parablock. Probably set this to 1 or 2
 	/// to simulate optimistic or realistic relay chain behavior.
 	pub relay_blocks_per_para_block: u32,
+	/// Number of parachain blocks per relay chain epoch
+	/// Mock epoch is computed by dividing `current_para_block` by this value.
+	pub para_blocks_per_relay_epoch: u32,
+	/// Function to mock BABE one epoch ago randomness
+	pub relay_randomness_config: R,
 	/// XCM messages and associated configuration information.
 	pub xcm_config: MockXcmConfig,
 	/// Inbound downward XCM messages to be injected into the block.
@@ -59,6 +64,20 @@ pub struct MockValidationDataInherentDataProvider {
 	pub raw_horizontal_messages: Vec<(ParaId, Vec<u8>)>,
 }
 
+pub trait GenerateRandomness<I> {
+	fn generate_randomness(&self, input: I) -> relay_chain::Hash;
+}
+
+impl GenerateRandomness<u64> for () {
+	/// Default implementation uses relay epoch as randomness value
+	/// A more seemingly random implementation may hash the relay epoch instead
+	fn generate_randomness(&self, input: u64) -> relay_chain::Hash {
+		let mut mock_randomness: [u8; 32] = [0u8; 32];
+		mock_randomness[..8].copy_from_slice(&input.to_be_bytes());
+		mock_randomness.into()
+	}
+}
+
 /// Parameters for how the Mock inherent data provider should inject XCM messages.
 /// In addition to the messages themselves, some information about the parachain's
 /// configuration is also required so that the MQC heads can be read out of the
@@ -130,7 +149,9 @@ impl MockXcmConfig {
 }
 
 #[async_trait::async_trait]
-impl InherentDataProvider for MockValidationDataInherentDataProvider {
+impl<R: Send + Sync + GenerateRandomness<u64>> InherentDataProvider
+	for MockValidationDataInherentDataProvider<R>
+{
 	fn provide_inherent_data(
 		&self,
 		inherent_data: &mut InherentData,
@@ -178,6 +199,17 @@ impl InherentDataProvider for MockValidationDataInherentDataProvider {
 			sproof_builder.upsert_inbound_channel(*para_id).mqc_head = Some(channel_mqc.head());
 		}
 
+		// Epoch is set equal to current para block / blocks per epoch
+		sproof_builder.current_epoch = if self.para_blocks_per_relay_epoch == 0 {
+			// do not divide by 0 => set epoch to para block number
+			self.current_para_block.into()
+		} else {
+			(self.current_para_block / self.para_blocks_per_relay_epoch).into()
+		};
+		// Randomness is set by randomness generator
+		sproof_builder.randomness =
+			self.relay_randomness_config.generate_randomness(self.current_para_block.into());
+
 		let (relay_parent_storage_root, proof) = sproof_builder.into_state_root_and_proof();
 
 		inherent_data.put_data(

test/relay-sproof-builder/src/lib.rs

@@ -44,6 +44,7 @@ pub struct RelayStateSproofBuilder {
 	pub hrmp_channels: BTreeMap<relay_chain::v2::HrmpChannelId, AbridgedHrmpChannel>,
 	pub current_slot: relay_chain::v2::Slot,
 	pub current_epoch: u64,
+	pub randomness: relay_chain::Hash,
 }
 
 impl Default for RelayStateSproofBuilder {
@@ -69,6 +70,7 @@ impl Default for RelayStateSproofBuilder {
 			hrmp_channels: BTreeMap::new(),
 			current_slot: 0.into(),
 			current_epoch: 0u64,
+			randomness: relay_chain::Hash::default(),
 		}
 	}
 }
@@ -156,6 +158,10 @@ impl RelayStateSproofBuilder {
 				insert(relay_chain::well_known_keys::hrmp_channels(channel), metadata.encode());
 			}
 			insert(relay_chain::well_known_keys::EPOCH_INDEX.to_vec(), self.current_epoch.encode());
+			insert(
+				relay_chain::well_known_keys::ONE_EPOCH_AGO_RANDOMNESS.to_vec(),
+				self.randomness.encode(),
+			);
 			insert(relay_chain::well_known_keys::CURRENT_SLOT.to_vec(), self.current_slot.encode());
 		}