Request announcement_signatures retransmission

During channel reestablishment, announcement_signatures may need to be
retransmitted. The splicing spec allows doing so without retransmitting
splice_locked first, which could normally trigger retransmitting
announcement_signatures. Instead, my_current_funding_locked lets the
sender request retransmitting it.

Author: jkczyz

lightning/src/ln/channel.rs

@@ -11067,7 +11067,25 @@ where
 			.or_else(|| {
 				self.is_our_channel_ready().then(|| self.funding.get_funding_txid()).flatten()
 			})
-			.map(|txid| msgs::FundingLocked { txid, retransmit_flags: 0 })
+			.map(|txid| {
+				let mut funding_locked = msgs::FundingLocked { txid, retransmit_flags: 0 };
+
+				// - if `my_current_funding_locked` is included:
+				//   - if `announce_channel` is set for this channel:
+				//     - if it has not received `announcement_signatures` for that transaction:
+				//       - MUST set the `announcement_signatures` bit to `1` in `retransmit_flags`.
+				//     - otherwise:
+				//       - MUST set the `announcement_signatures` bit to `0` in `retransmit_flags`.
+				if self.context.config.announce_for_forwarding {
+					if self.funding.get_funding_txid() != Some(txid)
+						|| self.context.announcement_sigs.is_none()
+					{
+						funding_locked.retransmit(msgs::FundingLockedFlags::AnnouncementSignatures);
+					}
+				}
+
+				funding_locked
+			})
 	}
 
 	#[cfg(not(splicing))]

lightning/src/ln/msgs.rs

@@ -976,12 +976,24 @@ pub struct FundingLocked {
 
 	/// A bitfield indicating which messages should be retransmitted by the receiving node.
 	///
-	/// | Bit Position  | Name                      |
-	/// | ------------- | --------------------------|
-	/// | 0             | `announcement_signatures` |
+	/// See [`FundingLockedFlags`] for details.
 	pub retransmit_flags: u8,
 }
 
+impl FundingLocked {
+	/// Sets the bit in `retransmit_flags` for retransmitting the message corresponding to `flag`.
+	pub fn retransmit(&mut self, flag: FundingLockedFlags) {
+		self.retransmit_flags |= 1 << flag as u8;
+	}
+}
+
+/// Bit positions used in [`FundingLocked::retransmit_flags`] for requesting message retransmission.
+#[repr(u8)]
+pub enum FundingLockedFlags {
+	/// Retransmit `announcement_signatures`.
+	AnnouncementSignatures = 0,
+}
+
 /// An [`announcement_signatures`] message to be sent to or received from a peer.
 ///
 /// [`announcement_signatures`]: https://github.com/lightning/bolts/blob/master/07-routing-gossip.md#the-announcement_signatures-message