set the first_slot to zero and fallback to ideal duration for one round

pallets/parachain-staking/src/lib.rs

@@ -468,8 +468,13 @@ pub mod pallet {
 				weight = weight.saturating_add(T::DbWeight::get().reads_writes(1, 0));
 
 				// Compute round duration in slots
-				let round_duration = (current_slot.saturating_sub(round.first_slot))
-					.saturating_mul(T::SlotDuration::get());
+				let round_duration = if round.first_slot == 0 {
+					// If the first slot is zero, we fallback to the ideal duration
+					(round.length as u64).saturating_mul(T::BlockTime::get())
+				} else {
+					(current_slot.saturating_sub(round.first_slot))
+						.saturating_mul(T::SlotDuration::get())
+				};
 
 				// mutate round
 				round.update(n, current_slot);

pallets/parachain-staking/src/migrations.rs

@@ -64,13 +64,12 @@ where
 		// Read round
 		let mut round = crate::Round::<T>::get();
 
-		// Compute theoretical `first_slot``
-		round.first_slot = compute_theoretical_first_slot(
-			<frame_system::Pallet<T>>::block_number(),
-			round.first,
-			u64::from(T::SlotProvider::get()),
-			T::BlockTime::get(),
-		);
+		// Force the `first_slot` to zero
+		// We can't compute the theoretical first slot because we don't have access to the
+		// relay slot
+		// To handle that, we added a hack in pallet staking that fallback to the ideal round
+		// duration if `first_slot` is zero.
+		round.first_slot = 0;
 
 		// Apply the migration (write new Round value)
 		crate::Round::<T>::put(round);