Fetch HolderCommitmentPoint::current_point on read

When reading HolderCommitmentPoint, attempt to fetch the current point
if it wasn't serialized. This allows channels to be spliced without
first needing to have the HolderCommitmentPoint advanced. Don't fail if
it can't be fetch synchronously as the channel can still be spliced once
it is advanced.

Author: jkczyz

lightning/src/ln/channel.rs

@@ -13837,11 +13837,26 @@ where
 		// If we're restoring this channel for the first time after an upgrade, then we require that the
 		// signer be available so that we can immediately populate the next commitment point. Channel
 		// restoration will fail if this is not possible.
-		let holder_commitment_point =
+		let holder_commitment_point = {
+			let current_point = holder_commitment_point_current_opt.or_else(|| {
+				if holder_commitment_next_transaction_number == INITIAL_COMMITMENT_NUMBER {
+					None
+				} else {
+					// If the current point is not available then splicing can't be initiated
+					// until the next point is advanced and becomes the current point.
+					holder_signer
+						.get_per_commitment_point(
+							holder_commitment_next_transaction_number + 1,
+							&secp_ctx,
+						)
+						.ok()
+				}
+			});
+
 			match (holder_commitment_point_next_opt, holder_commitment_point_pending_next_opt) {
 				(Some(next_point), pending_next_point) => HolderCommitmentPoint {
 					next_transaction_number: holder_commitment_next_transaction_number,
-					current_point: holder_commitment_point_current_opt,
+					current_point,
 					next_point,
 					pending_next_point,
 				},
@@ -13861,12 +13876,13 @@ where
 						);
 					HolderCommitmentPoint {
 						next_transaction_number: holder_commitment_next_transaction_number,
-						current_point: holder_commitment_point_current_opt,
+						current_point,
 						next_point,
 						pending_next_point: Some(pending_next_point),
 					}
 				},
-			};
+			}
+		};
 
 		Ok(FundedChannel {
 			funding: FundingScope {