offersrpc: store the paid BOLT 12 invoice re-encoded, and none above 32 KB

lnrpc/offersrpc/offers_server.go

@@ -674,7 +674,7 @@ func (s *Server) PayOffer(ctx context.Context,
 	// The invoice paid is the payment's request: kept with the payment,
 	// it is, with the preimage, the proof of what was paid and to whom
 	// (the issuer signed it), as a BOLT11 request is for those payments.
-	intent.PaymentRequest = []byte(bolt12)
+	intent.PaymentRequest = []byte(storedInvoice(bolt12))
 
 	hash := intent.Identifier()
 	preimage, rt, err := s.cfg.Deps.PayInvoice(ctx, intent)
@@ -815,3 +815,26 @@ func invoiceToRPC(inv *bolt12.Invoice) *InvoiceInfo {
 		SignatureValid: bolt12.VerifyInvoice(inv) == nil,
 	}
 }
+
+// MaxStoredInvoice bounds the BOLT 12 invoice kept as a payment's request.
+// A BOLT 11 request is at most about 7 KB; a BOLT 12 invoice its issuer pads
+// (unknown odd TLVs) could run to megabytes, and every listpayments page
+// holding it would carry them, past what a gRPC client accepts by default.
+const MaxStoredInvoice = 32 * 1024
+
+// storedInvoice is the invoice to keep with a payment: re-encoded from its
+// bytes (no '+' continuations or whitespace, the same signed TLVs, so it is
+// still the issuer's signature over what was paid), or nothing when that is
+// above MaxStoredInvoice or it does not decode.
+func storedInvoice(s string) string {
+	hrp, data, err := bolt12.Decode(strings.TrimSpace(s))
+	if err != nil {
+		return ""
+	}
+	enc, err := bolt12.Encode(hrp, data)
+	if err != nil || len(enc) > MaxStoredInvoice {
+		return ""
+	}
+
+	return enc
+}

lnrpc/offersrpc/offers_server_test.go

@@ -4,6 +4,7 @@
 package offersrpc
 
 import (
+	"bytes"
 	"context"
 	"errors"
 	"strings"
@@ -804,3 +805,22 @@ func fetchPayAndList(t *testing.T, chain [32]byte) {
 	})
 	requireCode(t, err, codes.InvalidArgument)
 }
+
+// TestStoredInvoice checks the invoice kept with a payment: the same bytes,
+// re-encoded without continuations, and nothing above MaxStoredInvoice.
+func TestStoredInvoice(t *testing.T) {
+	data := bytes.Repeat([]byte{0x42}, 600)
+	plain, err := bolt12.Encode(offers.InvoiceHRP, data)
+	require.NoError(t, err)
+
+	padded := plain[:20] + "+\n  " + plain[20:300] + "+ " + plain[300:]
+	require.Equal(t, plain, storedInvoice(" "+padded+"\n"))
+
+	big, err := bolt12.Encode(
+		offers.InvoiceHRP, make([]byte, MaxStoredInvoice),
+	)
+	require.NoError(t, err)
+	require.Empty(t, storedInvoice(big))
+
+	require.Empty(t, storedInvoice("not an invoice"))
+}