multi: enforce routing.MinCLTVDelta=18 for invoices + chanupd

This commit clamps all user-chosen CLTVs in LND to be at least 18, which
is the new conservative value used in the sepc. This minimum is applied
uniformly to forwarding CLTV deltas (via channel updates) as well as
final CLTV deltas for new invoices.

routing/router.go

@@ -46,6 +46,27 @@ const (
 	// stats related to processing new channels, updates, or node
 	// announcements.
 	defaultStatInterval = time.Minute
+
+	// MinCLTVDelta is the minimum CLTV value accepted by LND for all
+	// timelock deltas. This includes both forwarding CLTV deltas set on
+	// channel updates, as well as final CLTV deltas used to create BOLT 11
+	// payment requests.
+	//
+	// NOTE: For payment requests, BOLT 11 stipulates that a final CLTV
+	// delta of 9 should be used when no value is decoded. This however
+	// leads to inflexiblity in upgrading this default parameter, since it
+	// can create inconsistencies around the assumed value between sender
+	// and receiver. Specifically, if the receiver assumes a higher value
+	// than the sender, the receiver will always see the received HTLCs as
+	// invalid due to their timelock not meeting the required delta.
+	//
+	// We skirt this by always setting an explicit CLTV delta when creating
+	// invoices. This allows LND nodes to freely update the minimum without
+	// creating incompatibilities during the upgrade process. For some time
+	// LND has used an explicit default final CLTV delta of 40 blocks for
+	// bitcoin (160 for litecoin), though we now clamp the lower end of this
+	// range for user-chosen deltas to 18 blocks to be conservative.
+	MinCLTVDelta = 18
 )
 
 var (