pallet-referenda: Detect incorrect pre-image length (#3850)

There has been a case that a referenda failed because the length given
to `submit` was incorrect. The pallet can actually check the length if
the pre-image already exists to ensure that these kind of issues are not
happening again.

substrate/frame/referenda/src/lib.rs

@@ -424,6 +424,8 @@ pub mod pallet {
 		BadStatus,
 		/// The preimage does not exist.
 		PreimageNotExist,
+		/// The preimage is stored with a different length than the one provided.
+		PreimageStoredWithDifferentLength,
 	}
 
 	#[pallet::hooks]
@@ -462,6 +464,16 @@ pub mod pallet {
 			let proposal_origin = *proposal_origin;
 			let who = T::SubmitOrigin::ensure_origin(origin, &proposal_origin)?;
 
+			// If the pre-image is already stored, ensure that it has the same length as given in
+			// `proposal`.
+			if let (Some(preimage_len), Some(proposal_len)) =
+				(proposal.lookup_hash().and_then(|h| T::Preimages::len(&h)), proposal.lookup_len())
+			{
+				if preimage_len != proposal_len {
+					return Err(Error::<T, I>::PreimageStoredWithDifferentLength.into())
+				}
+			}
+
 			let track =
 				T::Tracks::track_for(&proposal_origin).map_err(|_| Error::<T, I>::NoTrack)?;
 			let submission_deposit = Self::take_deposit(who, T::SubmissionDeposit::get())?;