lnwire: modify PackRecords to prepend new records

As is, we can't use PackRecords when some data may already be stored in the ExtraData field. To allow this use case, we add a failing test, then modify `PackRecords` to append the existing raw bytes (prepend the new records).

Note that this doesn't 100% solve the problem, as for the stream to be cannonical we need unique IDs/types and also for them to be in ascending order.

lnwire/extra_bytes.go

@@ -71,7 +71,7 @@ func (e *ExtraOpaqueData) PackRecords(recordProducers ...tlv.RecordProducer) err
  return err
  }
 
- *e = ExtraOpaqueData(extraBytesWriter.Bytes())
+ *e = append(extraBytesWriter.Bytes(), *e...)
 
  return nil
 }

lnwire/extra_bytes_test.go

@@ -151,3 +151,25 @@ func TestExtraOpaqueDataPackUnpackRecords(t *testing.T) {
  t.Fatalf("type2 not found in typeMap")
  }
 }
+
+// TestPackRecordsPrepend tests that if an ExtraOpaqueData instance already a
+// set of opaque bytes, then any records passed in are prepended to the
+// existing bytes.
+func TestPackRecordsPrepend(t *testing.T) {
+ t.Parallel()
+
+ chanTypeRecord := tlv.NewPrimitiveRecord[tlv.TlvType1](uint8(2))
+
+ // Create some opaque data that is already pre-populated with some
+ // bytes.
+ existingBytes := bytes.Repeat([]byte{1}, 10)
+ extraBytes := ExtraOpaqueData(existingBytes)
+
+ // Now we'll attempt to pack the records into the existing bytes.
+ err := extraBytes.PackRecords(&chanTypeRecord)
+ require.NoError(t, err)
+
+ // After we've packed the records, the existing bytes should be at the
+ // very end.
+ require.True(t, bytes.HasSuffix(extraBytes[:], existingBytes))
+}