discovery: fix bug that can lead to sending invalid chan_ann msgs

Initially in lnd, we didn't store the extra TLV data that could be
dangling off of gossip messages. This was fixed initially in lnd v0.5
with this PR: #1825.

Within the PR, we incorrect set the `ExtraOpaqueData` (extra TLV blob)
of the `ChannelAnnouncement` to the value stored in `edge`, which is
actually our channel update. As 6-ish years ago we didn't yet have
anything that used the TLV gossip fields, this went unnoticed.

Fast forward to 2024, we shipped an experimental version of inbounbd
fees. This starts to store additional data in the `ExtraOpaqueData`
field, the TLV for the inbound fee. Initially, everything is valid when
the first `ChannelAnnouncement` is sent, but as soon as a user attempts
to set an inbound fee policy, we'd incorrectly swap in that new
serialized TLV for the _channel announcement_:
841e243#diff-1eda595bbebe495bd74a6a0431c46b66cb4e8b53beb311067c010feac2665dcbR2560.

Since we're just trying to generate a new `channel_update`, we don't
also regenerate the signature for the `channel_announcement` message. As
a result, we end up storing a `channel_announcement` with an invalid sig
on disk, continuing to broadcast that to peers.

Author: Roasbeef

discovery/gossiper.go

@@ -2232,7 +2232,7 @@ func (d *AuthenticatedGossiper) updateChannel(info *models.ChannelEdgeInfo,
 			BitcoinKey1:     info.BitcoinKey1Bytes,
 			Features:        lnwire.NewRawFeatureVector(),
 			BitcoinKey2:     info.BitcoinKey2Bytes,
-			ExtraOpaqueData: edge.ExtraOpaqueData,
+			ExtraOpaqueData: info.ExtraOpaqueData,
 		}
 		chanAnn.NodeSig1, err = lnwire.NewSigFromECDSARawSignature(
 			info.AuthProof.NodeSig1Bytes,

docs/release-notes/release-notes-0.18.3.md

@@ -64,6 +64,10 @@ commitment when the channel was force closed.
   cause UpdateAddHTLC message with blinding point fields to not be re-forwarded
   correctly on restart.
 
+* [A bug has been fixed that could cause invalid channel
+  announcements](https://github.com/lightningnetwork/lnd/pull/9002) to be
+  generated if the inbound fee discount is used.
+
 # New Features
 ## Functional Enhancements
 ## RPC Additions