tapchannel: add HtlcIndex to allocation sort

This fixes a bug where the order of the HTLCs would not be deterministic
between two peers if they were identical due to them being shards of the
same MPP payment. By adding the HTLC index as a further sort argument,
the order is again stable and deterministic.

Author: guggero

tapchannel/allocation_sort.go

@@ -43,7 +43,7 @@ func (s sortableAllocationSlice) Swap(i, j int) {
 }
 
 // Less is a modified BIP69 output comparison, that sorts based on value, then
-// pkScript, then CLTV value.
+// pkScript, then CLTV value and finally HtlcIndex.
 //
 // NOTE: Part of the sort.Interface interface.
 func (s sortableAllocationSlice) Less(i, j int) bool {
@@ -60,5 +60,9 @@ func (s sortableAllocationSlice) Less(i, j int) bool {
 		return pkScriptCmp < 0
 	}
 
-	return allocI.CLTV < allocJ.CLTV
+	if allocI.CLTV < allocJ.CLTV {
+		return true
+	}
+
+	return allocI.HtlcIndex < allocJ.HtlcIndex
 }