Fuzz fetching InvoiceRequestFields from VerifiedInvoiceRequests

This should allow us to reach the panic from two commits ago from
the fuzzer.

Author: TheBlueMatt

fuzz/src/invoice_request_deser.rs

@@ -134,6 +134,12 @@ fn build_response<T: secp256k1::Signing + secp256k1::Verification>(
 	)
 	.unwrap();
 
+	if let Ok(verified) = invoice_request.clone().verify_using_metadata(&expanded_key, secp_ctx) {
+		// Previously we had a panic where we'd truncate the payer note possibly cutting a Unicode
+		// character in two here, so try to fetch fields if we can validate.
+		let _ = verified.fields();
+	}
+
 	let payment_hash = PaymentHash([42; 32]);
 	invoice_request.respond_with(vec![payment_path], payment_hash)?.build()
 }

lightning/src/offers/invoice_request.rs

@@ -989,7 +989,12 @@ impl VerifiedInvoiceRequest {
 		InvoiceWithDerivedSigningPubkeyBuilder
 	);
 
-	pub(crate) fn fields(&self) -> InvoiceRequestFields {
+	/// Fetch the [`InvoiceRequestFields`] for this verified invoice.
+	///
+	/// These are fields which we expect to be useful when receiving a payment for this invoice
+	/// request, and include the returned [`InvoiceRequestFields`] in the
+	/// [`PaymentContext::Bolt12Offer`].
+	pub fn fields(&self) -> InvoiceRequestFields {
 		let InvoiceRequestContents {
 			payer_signing_pubkey,
 			inner: InvoiceRequestContentsWithoutPayerSigningPubkey { quantity, payer_note, .. },
@@ -1404,8 +1409,13 @@ pub struct InvoiceRequestFields {
 }
 
 /// The maximum number of characters included in [`InvoiceRequestFields::payer_note_truncated`].
+#[cfg(not(fuzzing))]
 pub const PAYER_NOTE_LIMIT: usize = 512;
 
+/// The maximum number of characters included in [`InvoiceRequestFields::payer_note_truncated`].
+#[cfg(fuzzing)]
+pub const PAYER_NOTE_LIMIT: usize = 8;
+
 impl Writeable for InvoiceRequestFields {
 	fn write<W: Writer>(&self, writer: &mut W) -> Result<(), io::Error> {
 		write_tlv_fields!(writer, {