backing: Remove redundant erasure encoding (#7469)

* Remove redundant erasure encoding

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* Review feedback

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* fix comments

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

---------

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

polkadot/node/subsystem-types/src/messages.rs

@@ -462,9 +462,10 @@ pub enum AvailabilityStoreMessage {
 		tx: oneshot::Sender<Result<(), ()>>,
 	},
 
-	/// Store a `AvailableData` and all of its chunks in the AV store.
+	/// Computes and checks the erasure root of `AvailableData` before storing all of its chunks in
+	/// the AV store.
 	///
-	/// Return `Ok(())` if the store operation succeeded, `Err(())` if it failed.
+	/// Return `Ok(())` if the store operation succeeded, `Err(StoreAvailableData)` if it failed.
 	StoreAvailableData {
 		/// A hash of the candidate this `available_data` belongs to.
 		candidate_hash: CandidateHash,
@@ -472,11 +473,21 @@ pub enum AvailabilityStoreMessage {
 		n_validators: u32,
 		/// The `AvailableData` itself.
 		available_data: AvailableData,
+		/// Erasure root we expect to get after chunking.
+		expected_erasure_root: Hash,
 		/// Sending side of the channel to send result to.
-		tx: oneshot::Sender<Result<(), ()>>,
+		tx: oneshot::Sender<Result<(), StoreAvailableDataError>>,
 	},
 }
 
+/// The error result type of a [`AvailabilityStoreMessage::StoreAvailableData`] request.
+#[derive(Error, Debug, Clone, PartialEq, Eq)]
+#[allow(missing_docs)]
+pub enum StoreAvailableDataError {
+	#[error("The computed erasure root did not match expected one")]
+	InvalidErasureRoot,
+}
+
 /// A response channel for the result of a chain API request.
 pub type ChainApiResponseChannel<T> = oneshot::Sender<Result<T, crate::errors::ChainApiError>>;