Revert "Remove the shortage code completely"

This reverts commit 165478b929.

src/test/app/Loan_test.cpp

@@ -2758,6 +2758,15 @@ protected:
                             state.paymentRemaining,
                             broker.params.managementFeeRate);
 
+#if LOANFILLSHORTAGE
+                    BEAST_EXPECT(
+                        paymentComponents.trackedValueDelta ==
+                            roundedPeriodicPayment ||
+                        (paymentComponents.specialCase ==
+                             detail::PaymentSpecialCase::final &&
+                         paymentComponents.trackedValueDelta <
+                             roundedPeriodicPayment));
+#else
                     BEAST_EXPECTS(
                         paymentComponents.specialCase ==
                                 detail::PaymentSpecialCase::final ||
@@ -2767,6 +2776,7 @@ protected:
                             to_string(paymentComponents.trackedValueDelta) +
                             ", periodic payment: " +
                             to_string(roundedPeriodicPayment));
+#endif
 
                     xrpl::LoanState const nextTrueState =
                         computeTheoreticalLoanState(

src/xrpld/app/misc/detail/LendingHelpers.cpp

@@ -1057,18 +1057,42 @@ computePaymentComponents(
             "xrpl::detail::computePaymentComponents",
             "excess non-negative");
     };
-    // Helper to reduce deltas when they collectively exceed a limit.
-    // Order matters: we prefer to reduce interest first (most flexible),
-    // then management fee, then principal (least flexible).
+#if LOANFILLSHORTAGE
+    auto giveTo =
+        [](Number& component, Number& shortage, Number const& maximum) {
+            if (shortage > beast::zero)
+            {
+                // Put as much of the shortage as we can into the provided part
+                // and the total
+                auto part = std::min(maximum - component, shortage);
+                component += part;
+                shortage -= part;
+            }
+            // If the shortage goes negative, we put too much, which should be
+            // impossible
+            XRPL_ASSERT_PARTS(
+                shortage >= beast::zero,
+                "ripple::detail::computePaymentComponents",
+                "excess non-negative");
+        };
+#endif
     auto addressExcess = [&takeFrom](LoanStateDeltas& deltas, Number& excess) {
         // This order is based on where errors are the least problematic
         takeFrom(deltas.interest, excess);
         takeFrom(deltas.managementFee, excess);
         takeFrom(deltas.principal, excess);
     };
-
-    // Check if deltas exceed the total outstanding value. This should never
-    // happen due to earlier caps, but handle it defensively.
+#if LOANFILLSHORTAGE
+    auto addressShortage = [&giveTo](
+                               LoanStateDeltas& deltas,
+                               Number& shortage,
+                               LoanState const& current) {
+        giveTo(deltas.interestDueDelta, shortage, current.interestDue);
+        giveTo(
+            deltas.managementFeeDueDelta, shortage, current.managementFeeDue);
+        giveTo(deltas.principalDelta, shortage, current.principalOutstanding);
+    };
+#endif
     Number totalOverpayment =
         deltas.total() - currentLedgerState.valueOutstanding;
 
@@ -1097,14 +1121,44 @@ computePaymentComponents(
         addressExcess(deltas, excess);
         shortage = -excess;
     }
+#if LOANFILLSHORTAGE
+    else if (shortage > beast::zero && totalOverpayment < beast::zero)
+    {
+        // If there's a shortage, and there's room in the loan itself, we can
+        // top up the parts to make the payment correct.
+        shortage = std::min(-totalOverpayment, shortage);
+        addressShortage(deltas, shortage, currentLedgerState);
+    }
 
-    // At this point, shortage >= 0 means we're paying less than the full
-    // periodic payment (due to rounding or component caps).
-    // shortage < 0 would mean we're trying to pay more than allowed (bug).
+    // The shortage should never be negative, which indicates that the parts are
+    // trying to take more than the whole payment. The shortage should not be
+    // positive, either, which indicates that we're not going to take the whole
+    // payment amount. Only the last payment should be allowed to have a
+    // shortage, and that's handled in a special case above.
+    XRPL_ASSERT_PARTS(
+        shortage == beast::zero,
+        "ripple::detail::computePaymentComponents",
+        "no shortage or excess");
+#else
+    // The shortage should never be negative, which indicates that the
+    // parts are trying to take more than the whole payment. The
+    // shortage may be positive, which indicates that we're not going to
+    // take the whole payment amount.
     XRPL_ASSERT_PARTS(
         shortage >= beast::zero,
         "xrpl::detail::computePaymentComponents",
         "no shortage or excess");
+#endif
+#if LOANCOMPLETE
+    /*
+    // This used to be part of the above assert. It will eventually be removed
+    // if proved accurate
+    ||
+        (shortage > beast::zero &&
+         ((asset.integral() && shortage < 3) ||
+          (scale - shortage.exponent() > 14)))
+          */
+#endif
 
     // Final validation that all components are valid
     XRPL_ASSERT_PARTS(