Fix some minor bugs in Lending Protocol (#6101)

- add nodiscard to unimpairLoan, and check result in LoanPay
- add a check to verify that issuer exists
- improve LoanManage error code for dust amounts

src/test/app/Loan_test.cpp

@@ -589,7 +589,7 @@ protected:
                 auto const unrealizedLoss = vaultSle->at(sfLossUnrealized) +
                     state.totalValue - state.managementFeeOutstanding;
 
-                if (unrealizedLoss > assetsUnavailable)
+                if (!BEAST_EXPECT(unrealizedLoss <= assetsUnavailable))
                 {
                     return false;
                 }

src/xrpld/app/tx/detail/LoanBrokerCoverClawback.cpp

@@ -287,6 +287,14 @@ LoanBrokerCoverClawback::preclaim(PreclaimContext const& ctx)
     // Check if the vault asset issuer has the correct flags
     auto const sleIssuer =
         ctx.view.read(keylet::account(vaultAsset.getIssuer()));
+    if (!sleIssuer)
+    {
+        // LCOV_EXCL_START
+        JLOG(ctx.j.fatal()) << "Issuer account does not exist.";
+        return tefBAD_LEDGER;
+        // LCOV_EXCL_STOP
+    }
+
     return std::visit(
         [&]<typename T>(T const&) {
             return preclaimHelper<T>(ctx, *sleIssuer, clawAmount);

src/xrpld/app/tx/detail/LoanManage.cpp

@@ -223,11 +223,13 @@ LoanManage::defaultLoan(
         }
         if (*vaultAvailableProxy > *vaultTotalProxy)
         {
-            JLOG(j.warn()) << "Vault assets available must not be greater "
-                              "than assets outstanding. Available: "
-                           << *vaultAvailableProxy
-                           << ", Total: " << *vaultTotalProxy;
-            return tecLIMIT_EXCEEDED;
+            // LCOV_EXCL_START
+            JLOG(j.fatal())
+                << "Vault assets available must not be greater "
+                   "than assets outstanding. Available: "
+                << *vaultAvailableProxy << ", Total: " << *vaultTotalProxy;
+            return tecINTERNAL;
+            // LCOV_EXCL_STOP
         }
 
         // The loss has been realized
@@ -329,7 +331,7 @@ LoanManage::impairLoan(
     return tesSUCCESS;
 }
 
-TER
+[[nodiscard]] TER
 LoanManage::unimpairLoan(
     ApplyView& view,
     SLE::ref loanSle,

src/xrpld/app/tx/detail/LoanManage.h

@@ -48,7 +48,7 @@ public:
 
     /** Helper function that might be needed by other transactors
      */
-    static TER
+    [[nodiscard]] static TER
     unimpairLoan(
         ApplyView& view,
         SLE::ref loanSle,

src/xrpld/app/tx/detail/LoanPay.cpp

@@ -305,7 +305,12 @@ LoanPay::doApply()
     // change will be discarded.
     if (loanSle->isFlag(lsfLoanImpaired))
     {
-        LoanManage::unimpairLoan(view, loanSle, vaultSle, j_);
+        if (auto const ret =
+                LoanManage::unimpairLoan(view, loanSle, vaultSle, asset, j_))
+        {
+            JLOG(j_.fatal()) << "Failed to unimpair loan before payment.";
+            return ret;  // LCOV_EXCL_LINE
+        }
     }
 
     LoanPaymentType const paymentType = [&tx]() {
@@ -449,6 +454,10 @@ LoanPay::doApply()
         if (*assetsAvailableProxy > *assetsTotalProxy)
         {
             // LCOV_EXCL_START
+            JLOG(j_.fatal())
+                << "Vault assets available must not be greater "
+                   "than assets outstanding. Available: "
+                << *assetsAvailableProxy << ", Total: " << *assetsTotalProxy;
             return tecINTERNAL;
             // LCOV_EXCL_STOP
         }