Revert "Remove the shortage code completely"

This reverts commit 165478b929.

src/test/app/Loan_test.cpp

@@ -2674,9 +2674,19 @@ protected:
                             state.paymentRemaining,
                             broker.params.managementFeeRate);
 
+#if LOANFILLSHORTAGE
+                    BEAST_EXPECT(
+                        paymentComponents.trackedValueDelta ==
+                            roundedPeriodicPayment ||
+                        (paymentComponents.specialCase ==
+                             detail::PaymentSpecialCase::final &&
+                         paymentComponents.trackedValueDelta <
+                             roundedPeriodicPayment));
+#else
                     BEAST_EXPECT(
                         paymentComponents.trackedValueDelta <=
                         roundedPeriodicPayment);
+#endif
 
                     ripple::LoanState const nextTrueState =
                         calculateRawLoanState(

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

@@ -1215,12 +1215,42 @@ computePaymentComponents(
             "ripple::detail::computePaymentComponents",
             "excess non-negative");
     };
+#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](LoanDeltas& deltas, Number& excess) {
         // This order is based on where errors are the least problematic
         takeFrom(deltas.interestDueDelta, excess);
         takeFrom(deltas.managementFeeDueDelta, excess);
         takeFrom(deltas.principalDelta, excess);
     };
+#if LOANFILLSHORTAGE
+    auto addressShortage = [&giveTo](
+                               LoanDeltas& deltas,
+                               Number& shortage,
+                               LoanState const& current) {
+        giveTo(deltas.interestDueDelta, shortage, current.interestDue);
+        giveTo(deltas.principalDelta, shortage, current.principalOutstanding);
+        giveTo(
+            deltas.managementFeeDueDelta, shortage, current.managementFeeDue);
+    };
+#endif
     Number totalOverpayment =
         deltas.valueDelta() - currentLedgerState.valueOutstanding;
     if (totalOverpayment > beast::zero)
@@ -1249,6 +1279,25 @@ computePaymentComponents(
 
         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);
+    }
+
+    // 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
@@ -1257,6 +1306,7 @@ computePaymentComponents(
         shortage >= beast::zero,
         "ripple::detail::computePaymentComponents",
         "no shortage or excess");
+#endif
 #if LOANCOMPLETE
     /*
     // This used to be part of the above assert. It will eventually be removed