lnwire: add SetOptFromMap and AddOpt

This gives us easier optional tlv field handling, which we will use for
the following message definitions.

lnwire/custom_records.go

@@ -263,6 +263,32 @@ func DecodeRecordsP2P(r *bytes.Reader,
 	return tlvStream.DecodeWithParsedTypesP2P(r)
 }
 
+// AddOpt appends a record producer for the given optional record to producers
+// when the optional is set, leaving producers unchanged otherwise.
+func AddOpt[T tlv.TlvType, V any](producers *[]tlv.RecordProducer,
+	opt tlv.OptionalRecordT[T, V]) {
+
+	opt.WhenSome(
+		func(r tlv.RecordT[T, V]) {
+			*producers = append(*producers, &r)
+		},
+	)
+}
+
+// SetOptFromMap marks target as Some(record) when record's TLV type appeared
+// on the wire (i.e., is a key in the decoded TypeMap).
+//
+// The caller must have passed record to the underlying Stream before decoding;
+// otherwise record.Val will not have been populated, and wrapping it as Some
+// would yield a zero-valued field.
+func SetOptFromMap[T tlv.TlvType, V any](typeMap tlv.TypeMap,
+	target *tlv.OptionalRecordT[T, V], record tlv.RecordT[T, V]) {
+
+	if _, ok := typeMap[record.TlvType()]; ok {
+		*target = tlv.SomeRecordT(record)
+	}
+}
+
 // AssertUniqueTypes asserts that the given records have unique types.
 func AssertUniqueTypes(r []tlv.Record) error {
 	seen := make(fn.Set[tlv.Type], len(r))

lnwire/custom_records_test.go

@@ -249,3 +249,46 @@ func TestCustomRecordsMergedCopy(t *testing.T) {
 		})
 	}
 }
+
+// TestAddOptAppendsOnlyWhenSet checks that AddOpt is a no-op for an empty
+// optional and appends a producer when the optional is populated.
+func TestAddOptAppendsOnlyWhenSet(t *testing.T) {
+	t.Parallel()
+
+	var producers []tlv.RecordProducer
+
+	emptyOpt := tlv.OptionalRecordT[tlv.TlvType1, uint16]{}
+	AddOpt(&producers, emptyOpt)
+	require.Empty(t, producers)
+
+	setOpt := tlv.SomeRecordT(
+		tlv.NewPrimitiveRecord[tlv.TlvType1, uint16](42),
+	)
+	AddOpt(&producers, setOpt)
+	require.Len(t, producers, 1)
+
+	rec := producers[0].Record()
+	require.Equal(t, tlv.Type(1), rec.Type())
+}
+
+// TestSetOptFromMapUsesTypeMapPresence verifies that SetOptFromMap populates
+// only when the TLV type is present in the TypeMap.
+func TestSetOptFromMapUsesTypeMapPresence(t *testing.T) {
+	t.Parallel()
+
+	present := tlv.TypeMap{tlv.Type(1): nil}
+	missing := tlv.TypeMap{}
+
+	var target tlv.OptionalRecordT[tlv.TlvType1, uint16]
+	SetOptFromMap(
+		missing, &target,
+		tlv.NewPrimitiveRecord[tlv.TlvType1, uint16](7),
+	)
+	require.True(t, target.IsNone())
+
+	SetOptFromMap(
+		present, &target,
+		tlv.NewPrimitiveRecord[tlv.TlvType1, uint16](7),
+	)
+	require.True(t, target.IsSome())
+}