From f68b3326dfd431014646085b2d14a70234666ad8 Mon Sep 17 00:00:00 2001
From: Ed Hennis <ed@ripple.com>
Date: Tue, 31 Mar 2026 21:02:28 -0400
Subject: [PATCH] Review feedback from @tapanito

- Return a default value in LoanPay balanceScale if the exponent list is
  empty.
- Amendment gate the "roundedAmount" change in overpayment.
- Improve comments and logging.
documentation
---
 .../tx/transactors/lending/LendingHelpers.cpp | 11 +++++++++-
 .../tx/transactors/lending/LoanPay.cpp        | 21 +++++++++++++------
 2 files changed, 25 insertions(+), 7 deletions(-)

diff --git a/src/libxrpl/tx/transactors/lending/LendingHelpers.cpp b/src/libxrpl/tx/transactors/lending/LendingHelpers.cpp
index 3fa42037fa..2405cee21d 100644
--- a/src/libxrpl/tx/transactors/lending/LendingHelpers.cpp
+++ b/src/libxrpl/tx/transactors/lending/LendingHelpers.cpp
@@ -1824,7 +1824,16 @@ loanMakePayment(
 
     // -------------------------------------------------------------
     // overpayment handling
-    auto const roundedAmount = roundToAsset(asset, amount, loanScale, Number::towards_zero);
+    //
+    // If the "fixSecurity3_1_3" amendment is enabled, truncate "amount",
+    // at the loan scale. If the raw value is used, the overpayment
+    // amount could be meaningless dust. Trying to process such a small
+    // amount will, at best, waste time when all the result values round
+    // to zero. At worst, it can cause logical errors with tiny amounts
+    // of interest that don't add up correctly.
+    auto const roundedAmount = view.rules().enabled(fixSecurity3_1_3)
+        ? roundToAsset(asset, amount, loanScale, Number::towards_zero)
+        : amount;
     if (paymentType == LoanPaymentType::overpayment && loan->isFlag(lsfLoanOverpayment) &&
         paymentRemainingProxy > 0 && totalPaid < roundedAmount &&
         numPayments < loanMaximumPaymentsPerTransaction)
diff --git a/src/libxrpl/tx/transactors/lending/LoanPay.cpp b/src/libxrpl/tx/transactors/lending/LoanPay.cpp
index a59c95066e..a45448bdf1 100644
--- a/src/libxrpl/tx/transactors/lending/LoanPay.cpp
+++ b/src/libxrpl/tx/transactors/lending/LoanPay.cpp
@@ -515,10 +515,10 @@ LoanPay::doApply()
         // something really weird happened. That should be flat out impossible.
         //
         // LCOV_EXCL_START
-        JLOG(j_.warn()) << "LoanPay: Vault assets changed unexpectedly after rounding: "  //
-                        << "Before: " << assetsTotalBefore                                //
-                        << ", After: " << assetsTotalAfter                                //
-                        << ", ValueChange: " << paymentParts->valueChange;
+        JLOG(j_.fatal()) << "LoanPay: Vault assets changed unexpectedly after rounding: "  //
+                         << "Before: " << assetsTotalBefore                                //
+                         << ", After: " << assetsTotalAfter                                //
+                         << ", ValueChange: " << paymentParts->valueChange;
         return tecINTERNAL;
         // LCOV_EXCL_STOP
     }
@@ -642,8 +642,12 @@ LoanPay::doApply()
               j_,
               SpendableHandling::shFULL_BALANCE);
     auto const balanceScale = [&]() {
-        // Find the maximum exponent of all the non-zero balances, before and after.
-        // This is so ugly.
+        // Find a reasonable scale to use for the balance comparisons.
+        //
+        // First find the minimum and maximum exponent of all the non-zero balances, before and
+        // after. If min and max are equal, use that value. If they are not, use "max + 1" to reduce
+        // rounding discrepancies without making the result meaningless. Cap the scale at
+        // STAmount::cMaxOffset, just in case the numbers are all very large.
         std::vector<int> exponents;
 
         for (auto const& a : {
@@ -659,6 +663,11 @@ LoanPay::doApply()
             if (a != beast::zero)
                 exponents.push_back(a.exponent());
         }
+        if (exponents.empty())
+        {
+            UNREACHABLE("xrpl::LoanPay::doApply : all zeroes");
+            return 0;
+        }
         auto const [minItr, maxItr] = std::minmax_element(exponents.begin(), exponents.end());
         auto const min = *minItr;
         auto const max = *maxItr;