Don't create empty holding for MPT or trust line issuer (#5877)

- Adds a check to the MPToken creation invariant to ensure none are created for the issuer.
- `addEmptyHolding()` will return success without doing anything for these scenarios. There is nothing to do, as with XRP.

---------

Co-authored-by: Ed Hennis <ed@ripple.com>

src/libxrpl/ledger/View.cpp

@@ -1406,8 +1406,8 @@ addEmptyHolding(
     Issue const& issue,
     beast::Journal journal)
 {
-    // Every account can hold XRP.
-    if (issue.native())
+    // Every account can hold XRP. An issuer can issue directly.
+    if (issue.native() || accountID == issue.getIssuer())
         return tesSUCCESS;
 
     auto const& issuerId = issue.getIssuer();
@@ -1468,6 +1468,8 @@ addEmptyHolding(
         return tefINTERNAL;  // LCOV_EXCL_LINE
     if (view.peek(keylet::mptoken(mptID, accountID)))
         return tecDUPLICATE;
+    if (accountID == mptIssue.getIssuer())
+        return tesSUCCESS;
 
     return authorizeMPToken(view, priorBalance, mptID, accountID, journal);
 }

src/test/app/Loan_test.cpp

@@ -2827,6 +2827,46 @@ class Loan_test : public beast::unit_test::suite
         pass();
     }
 
+    void
+    testIssuerLoan()
+    {
+        testcase << "Issuer Loan";
+
+        using namespace jtx;
+        using namespace loan;
+        Account const issuer("issuer");
+        Account const borrower = issuer;
+        Account const lender("lender");
+        Env env(*this);
+
+        env.fund(XRP(1'000), issuer, lender);
+
+        std::int64_t constexpr issuerBalance = 10'000'000;
+        MPTTester asset(
+            {.env = env,
+             .issuer = issuer,
+             .holders = {lender},
+             .pay = issuerBalance});
+
+        auto const broker = createVaultAndBroker(env, asset, lender, 200);
+        auto const loanSetFee = fee(env.current()->fees().base * 2);
+        // Create Loan
+        env(set(borrower, broker.brokerID, 200),
+            sig(sfCounterpartySignature, lender),
+            loanSetFee);
+        env.close();
+        // Issuer should not create MPToken
+        BEAST_EXPECT(!env.le(keylet::mptoken(asset.issuanceID(), issuer)));
+        // Issuer "borrowed" 200, OutstandingAmount decreased by 200
+        BEAST_EXPECT(env.balance(issuer, asset) == asset(-issuerBalance + 200));
+        // Pay Loan
+        auto const loanKeylet = keylet::loan(broker.brokerID, 1);
+        env(pay(borrower, loanKeylet.key, asset(200)));
+        env.close();
+        // Issuer "re-payed" 200, OutstandingAmount increased by 200
+        BEAST_EXPECT(env.balance(issuer, asset) == asset(-issuerBalance));
+    }
+
     void
     testInvalidLoanDelete()
     {
@@ -3049,6 +3089,7 @@ public:
     void
     run() override
     {
+        testIssuerLoan();
         testDisabled();
         testSelfLoan();
         testLifecycle();

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

@@ -1479,7 +1479,12 @@ ValidMPTIssuance::visitEntry(
         if (isDelete)
             mptokensDeleted_++;
         else if (!before)
+        {
             mptokensCreated_++;
+            MPTIssue const mptIssue{after->at(sfMPTokenIssuanceID)};
+            if (mptIssue.getIssuer() == after->at(sfAccount))
+                mptCreatedByIssuer_ = true;
+        }
     }
 }
 
@@ -1493,6 +1498,25 @@ ValidMPTIssuance::finalize(
 {
     if (result == tesSUCCESS)
     {
+        auto const& rules = view.rules();
+        [[maybe_unused]]
+        bool enforceCreatedByIssuer = rules.enabled(featureSingleAssetVault) ||
+            rules.enabled(featureLendingProtocol);
+        if (mptCreatedByIssuer_)
+        {
+            JLOG(j.fatal())
+                << "Invariant failed: MPToken created for the MPT issuer";
+            // The comment above starting with "assert(enforce)" explains this
+            // assert.
+            XRPL_ASSERT_PARTS(
+                enforceCreatedByIssuer,
+                "ripple::ValidMPTIssuance::finalize",
+                "no issuer MPToken");
+            if (enforceCreatedByIssuer)
+                return false;
+        }
+
+        auto const txnType = tx.getTxnType();
         if (hasPrivilege(tx, createMPTIssuance))
         {
             if (mptIssuancesCreated_ == 0)
@@ -1540,7 +1564,7 @@ ValidMPTIssuance::finalize(
         // ttESCROW_FINISH may authorize an MPT, but it can't have the
         // mayAuthorizeMPT privilege, because that may cause
         // non-amendment-gated side effects.
-        bool const enforceEscrowFinish = (tx.getTxnType() == ttESCROW_FINISH) &&
+        bool const enforceEscrowFinish = (txnType == ttESCROW_FINISH) &&
             (view.rules().enabled(featureSingleAssetVault) ||
              lendingProtocolEnabled);
         if (hasPrivilege(tx, mustAuthorizeMPT | mayAuthorizeMPT) ||
@@ -1591,7 +1615,7 @@ ValidMPTIssuance::finalize(
 
             return true;
         }
-        if (tx.getTxnType() == ttESCROW_FINISH)
+        if (txnType == ttESCROW_FINISH)
         {
             // ttESCROW_FINISH may authorize an MPT, but it can't have the
             // mayAuthorizeMPT privilege, because that may cause

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

@@ -576,6 +576,9 @@ class ValidMPTIssuance
 
     std::uint32_t mptokensCreated_ = 0;
     std::uint32_t mptokensDeleted_ = 0;
+    // non-MPT transactions may attempt to create
+    // MPToken by an issuer
+    bool mptCreatedByIssuer_ = false;
 
 public:
     void