docs: improves migration path and adds function to reuse update_due c…

…alculation

prdoc/pr_7723.prdoc

@@ -11,8 +11,9 @@ migrations:
   runtime:
     - reference: pallet-bounties
       description: |
-        Migrating `BountyUpdatePeriod` from `BlockNumber` to `Option<BlockNumber>` where
-        if set to (), bounties will no longer expire.
+        Migrating `BountyUpdatePeriod` from `BlockNumber` to `Option<BlockNumber>`.
+        To preserve the existing behavior, wrap the current value in `Some(BountyUpdatePeriod)`.
+        if set to None, bounties will no longer expire. 
 
 crates:
 - name: pallet-bounties

substrate/frame/bounties/src/lib.rs

@@ -586,10 +586,7 @@ pub mod pallet {
 						T::Currency::reserve(curator, deposit)?;
 						bounty.curator_deposit = deposit;
 
-						let update_due = Self::treasury_block_number().saturating_add(
-							T::BountyUpdatePeriod::get()
-								.unwrap_or(BlockNumberFor::<T, I>::max_value()),
-						);
+						let update_due = Self::calculate_update_due();
 						bounty.status =
 							BountyStatus::Active { curator: curator.clone(), update_due };
 
@@ -830,11 +827,7 @@ pub mod pallet {
 				match bounty.status {
 					BountyStatus::Active { ref curator, ref mut update_due } => {
 						ensure!(*curator == signer, Error::<T, I>::RequireCurator);
-						*update_due = (Self::treasury_block_number().saturating_add(
-							T::BountyUpdatePeriod::get()
-								.unwrap_or(BlockNumberFor::<T, I>::max_value()),
-						))
-						.max(*update_due);
+						*update_due = Self::calculate_update_due().max(*update_due);
 					},
 					_ => return Err(Error::<T, I>::UnexpectedStatus.into()),
 				}
@@ -965,6 +958,10 @@ impl<T: Config<I>, I: 'static> Pallet<T, I> {
 		deposit
 	}
 
+	pub fn calculate_update_due() -> BlockNumberFor<T, I> {
+		Self::treasury_block_number().saturating_add(T::BountyUpdatePeriod::get().unwrap_or(BlockNumberFor::<T, I>::max_value()))
+	}
+
 	/// The account ID of the treasury pot.
 	///
 	/// This actually does computation. If you need to keep using it, then make sure you cache the