invoices: force MPP payload inclusion for non-keysend payments

In this commit, we move to start rejecting any normal payments that
aren't keysend, if they don't also include the MPP invoice payload. With
this change, we require that some sort of e2e secret (either the payment
addr or the keysend pre-image) is present in a payload before we'll
accept the payment.

The second portion of the commit also updates all current tests in the
package. We kept the base `TestSettleInvoice` test in-tact as it still
exercises some useful behavior. However, we've removed all cases that
allow an overpayment, as the new MPP logic doesn't allow overpayment for
various reasons. In addition to this, some of the returned errors are
slightly different, tho the actual behavior is equivalent.

invoices/update.go

@@ -90,6 +90,9 @@ func updateInvoice(ctx *invoiceUpdateCtx, inv *channeldb.Invoice) (
 		}
 	}
 
+	// If no MPP payload was provided, then we expect this to be a keysend,
+	// or a payment to an invoice created before we started to require the
+	// MPP payload.
 	if ctx.mpp == nil {
 		return updateLegacy(ctx, inv)
 	}
@@ -206,6 +209,10 @@ func updateMpp(ctx *invoiceUpdateCtx,
 
 // updateLegacy is a callback for DB.UpdateInvoice that contains the invoice
 // settlement logic for legacy payments.
+//
+// NOTE: This function is only kept in place in order to be able to handle key
+// send payments and any invoices we created in the past that are valid and
+// still had the optional mpp bit set.
 func updateLegacy(ctx *invoiceUpdateCtx,
 	inv *channeldb.Invoice) (*channeldb.InvoiceUpdateDesc, HtlcResolution, error) {
 
@@ -223,8 +230,20 @@ func updateLegacy(ctx *invoiceUpdateCtx,
 		return nil, ctx.failRes(ResultAmountTooLow), nil
 	}
 
-	// TODO(joostjager): Check invoice mpp required feature
-	// bit when feature becomes mandatory.
+	// If the invoice had the required feature bit set at this point, then
+	// if we're in this method it means that the remote party didn't supply
+	// the expected payload. However if this is a keysend payment, then
+	// we'll permit it to pass.
+	_, isKeySend := ctx.customRecords[record.KeySendType]
+	invoiceFeatures := inv.Terms.Features
+	paymentAddrRequired := invoiceFeatures.RequiresFeature(
+		lnwire.PaymentAddrRequired,
+	)
+	if !isKeySend && paymentAddrRequired {
+		log.Warnf("Payment to pay_hash=%v doesn't include MPP "+
+			"payload, rejecting", ctx.hash)
+		return nil, ctx.failRes(ResultAddressMismatch), nil
+	}
 
 	// Don't allow settling the invoice with an old style
 	// htlc if we are already in the process of gathering an