Add Loan Broker and Loan ledger objects:

- Also add new SFields, Keylet functions, and an Invariant to verify no
  illegal field modification

include/xrpl/protocol/Indexes.h

@@ -339,6 +339,24 @@ vault(uint256 const& vaultKey)
     return {ltVAULT, vaultKey};
 }
 
+Keylet
+loanbroker(AccountID const& owner, std::uint32_t seq) noexcept;
+
+inline Keylet
+loanbroker(uint256 const& vaultKey)
+{
+    return {ltLOAN_BROKER, vaultKey};
+}
+
+Keylet
+loan(AccountID const& owner, uint256 loanBrokerID, std::uint32_t seq) noexcept;
+
+inline Keylet
+loan(uint256 const& vaultKey)
+{
+    return {ltLOAN, vaultKey};
+}
+
 Keylet
 permissionedDomain(AccountID const& account, std::uint32_t seq) noexcept;
 

include/xrpl/protocol/STLedgerEntry.h

@@ -36,6 +36,8 @@ class STLedgerEntry final : public STObject, public CountedObject<STLedgerEntry>
 public:
     using pointer = std::shared_ptr<STLedgerEntry>;
     using ref = const std::shared_ptr<STLedgerEntry>&;
+    using const_pointer = std::shared_ptr<STLedgerEntry const>;
+    using const_ref = const std::shared_ptr<STLedgerEntry const>&;
 
     /** Create an empty object with the given key and type. */
     explicit STLedgerEntry(Keylet const& k);

include/xrpl/protocol/detail/ledger_entries.macro

@@ -167,6 +167,7 @@ LEDGER_ENTRY(ltACCOUNT_ROOT, 0x0061, AccountRoot, account, ({
     {sfFirstNFTokenSequence, soeOPTIONAL},
     {sfAMMID,                soeOPTIONAL}, // pseudo-account designator
     {sfVaultID,              soeOPTIONAL}, // pseudo-account designator
+    {sfLoanBrokerID,         soeOPTIONAL}, // pseudo-account designator
 }))
 
 /** A ledger object which contains a list of object identifiers.
@@ -485,6 +486,60 @@ LEDGER_ENTRY(ltVAULT, 0x0083, Vault, vault, ({
     // no PermissionedDomainID ever (use MPTIssuance.sfDomainID)
 }))
 
+/** A ledger object representing a loan broker
+
+    \sa keylet::loanbroker
+ */
+LEDGER_ENTRY(ltLOAN_BROKER, 0x0084, LoanBroker, loan_broker, ({
+    {sfPreviousTxnID,        soeREQUIRED},
+    {sfPreviousTxnLgrSeq,    soeREQUIRED},
+    {sfSequence,             soeREQUIRED},
+    {sfOwnerNode,            soeREQUIRED},
+    {sfVaultNode,            soeREQUIRED},
+    {sfVaultID,              soeREQUIRED},
+    {sfAccount,              soeREQUIRED},
+    {sfOwner,                soeREQUIRED},
+    {sfData,                 soeDEFAULT},
+    {sfManagementFeeRate,    soeDEFAULT},
+    {sfOwnerCount,           soeREQUIRED},
+    {sfDebtTotal,            soeREQUIRED},
+    {sfDebtMaximum,          soeREQUIRED},
+    {sfCoverAvailable,       soeREQUIRED},
+    {sfCoverRateMinimum,     soeREQUIRED},
+    {sfCoverRateLiquidation, soeREQUIRED},
+}))
+
+/** A ledger object representing a loan between a Borrower and a Loan Broker
+
+    \sa keylet::loan
+ */
+LEDGER_ENTRY(ltLOAN, 0x0085, Loan, loan, ({
+    {sfPreviousTxnID,        soeREQUIRED},
+    {sfPreviousTxnLgrSeq,    soeREQUIRED},
+    {sfSequence,             soeREQUIRED},
+    {sfOwnerNode,            soeREQUIRED},
+    {sfLoanBrokerNode,       soeREQUIRED},
+    {sfLoanBrokerID,         soeREQUIRED},
+    {sfBorrower,             soeREQUIRED},
+    {sfLoanOriginationFee,   soeREQUIRED},
+    {sfLoanServiceFee,       soeREQUIRED},
+    {sfLatePaymentFee,       soeREQUIRED},
+    {sfClosePaymentFee,      soeREQUIRED},
+    {sfOverpaymentFee,       soeREQUIRED},
+    {sfInterestRate,         soeREQUIRED},
+    {sfLateInterestRate,     soeREQUIRED},
+    {sfCloseInterestRate,    soeREQUIRED},
+    {sfOverpaymentInterestRate, soeREQUIRED},
+    {sfStartDate,            soeREQUIRED},
+    {sfPaymentInterval,      soeREQUIRED},
+    {sfGracePeriod,          soeREQUIRED},
+    {sfPreviousPaymentDate,  soeREQUIRED},
+    {sfNextPaymentDueDate,   soeREQUIRED},
+    {sfPaymentRemaining,     soeREQUIRED},
+    {sfAssetAvailable,       soeREQUIRED},
+    {sfPrincipalOutstanding, soeREQUIRED},
+}))
+
 #undef EXPAND
 #undef LEDGER_ENTRY_DUPLICATE
 

include/xrpl/protocol/detail/sfields.macro

@@ -59,6 +59,13 @@ TYPED_SFIELD(sfHookEmitCount,            UINT16,    18)
 TYPED_SFIELD(sfHookExecutionIndex,       UINT16,    19)
 TYPED_SFIELD(sfHookApiVersion,           UINT16,    20)
 TYPED_SFIELD(sfLedgerFixType,            UINT16,    21)
+TYPED_SFIELD(sfManagementFeeRate,        UINT16,    22)
+TYPED_SFIELD(sfCoverRateMinimum,         UINT16,    23)
+TYPED_SFIELD(sfCoverRateLiquidation,     UINT16,    24)
+TYPED_SFIELD(sfInterestRate,             UINT16,    25)
+TYPED_SFIELD(sfLateInterestRate,         UINT16,    26)
+TYPED_SFIELD(sfCloseInterestRate,        UINT16,    27)
+TYPED_SFIELD(sfOverpaymentInterestRate,  UINT16,    28)
 
 // 32-bit integers (common)
 TYPED_SFIELD(sfNetworkID,                UINT32,     1)
@@ -113,6 +120,12 @@ TYPED_SFIELD(sfEmitGeneration,           UINT32,    46)
 TYPED_SFIELD(sfVoteWeight,               UINT32,    48)
 TYPED_SFIELD(sfFirstNFTokenSequence,     UINT32,    50)
 TYPED_SFIELD(sfOracleDocumentID,         UINT32,    51)
+TYPED_SFIELD(sfStartDate,                UINT32,    52)
+TYPED_SFIELD(sfPaymentInterval,          UINT32,    53)
+TYPED_SFIELD(sfGracePeriod,              UINT32,    54)
+TYPED_SFIELD(sfPreviousPaymentDate,      UINT32,    55)
+TYPED_SFIELD(sfNextPaymentDueDate,       UINT32,    56)
+TYPED_SFIELD(sfPaymentRemaining,         UINT32,    57)
 
 // 64-bit integers (common)
 TYPED_SFIELD(sfIndexNext,                UINT64,     1)
@@ -143,6 +156,8 @@ TYPED_SFIELD(sfOutstandingAmount,        UINT64,    25, SField::sMD_BaseTen|SFie
 TYPED_SFIELD(sfMPTAmount,                UINT64,    26, SField::sMD_BaseTen|SField::sMD_Default)
 TYPED_SFIELD(sfIssuerNode,               UINT64,    27)
 TYPED_SFIELD(sfSubjectNode,              UINT64,    28)
+TYPED_SFIELD(sfVaultNode,                UINT64,    29)
+TYPED_SFIELD(sfLoanBrokerNode,           UINT64,    30)
 
 // 128-bit
 TYPED_SFIELD(sfEmailHash,                UINT128,    1)
@@ -194,6 +209,7 @@ TYPED_SFIELD(sfHookNamespace,            UINT256,   32)
 TYPED_SFIELD(sfHookSetTxnID,             UINT256,   33)
 TYPED_SFIELD(sfDomainID,                 UINT256,   34)
 TYPED_SFIELD(sfVaultID,                  UINT256,   35)
+TYPED_SFIELD(sfLoanBrokerID,             UINT256,   35)
 
 // number (common)
 TYPED_SFIELD(sfNumber,                   NUMBER,     1)
@@ -201,6 +217,15 @@ TYPED_SFIELD(sfAssetsAvailable,          NUMBER,     2)
 TYPED_SFIELD(sfAssetsMaximum,            NUMBER,     3)
 TYPED_SFIELD(sfAssetsTotal,              NUMBER,     4)
 TYPED_SFIELD(sfLossUnrealized,           NUMBER,     5)
+TYPED_SFIELD(sfDebtTotal,                NUMBER,     6)
+TYPED_SFIELD(sfDebtMaximum,              NUMBER,     7)
+TYPED_SFIELD(sfCoverAvailable,           NUMBER,     8)
+TYPED_SFIELD(sfLoanOriginationFee,       NUMBER,     9)
+TYPED_SFIELD(sfLoanServiceFee,           NUMBER,     9)
+TYPED_SFIELD(sfLatePaymentFee,           NUMBER,     10)
+TYPED_SFIELD(sfClosePaymentFee,          NUMBER,     11)
+TYPED_SFIELD(sfOverpaymentFee,           NUMBER,     12)
+TYPED_SFIELD(sfPrincipalOutstanding,     NUMBER,     13)
 
 // currency amount (common)
 TYPED_SFIELD(sfAmount,                   AMOUNT,     1)
@@ -295,6 +320,7 @@ TYPED_SFIELD(sfAttestationRewardAccount, ACCOUNT,   21)
 TYPED_SFIELD(sfLockingChainDoor,         ACCOUNT,   22)
 TYPED_SFIELD(sfIssuingChainDoor,         ACCOUNT,   23)
 TYPED_SFIELD(sfSubject,                  ACCOUNT,   24)
+TYPED_SFIELD(sfBorrower,                 ACCOUNT,   25)
 
 // vector of 256-bit
 TYPED_SFIELD(sfIndexes,                  VECTOR256,  1, SField::sMD_Never)

src/libxrpl/protocol/Indexes.cpp

@@ -95,6 +95,8 @@ enum class LedgerNameSpace : std::uint16_t {
     CREDENTIAL = 'D',
     PERMISSIONED_DOMAIN = 'm',
     VAULT = 'V',
+    LOAN_BROKER = 'l',  // lower-case L
+    LOAN = 'L',
 
     // No longer used or supported. Left here to reserve the space
     // to avoid accidental reuse.
@@ -550,6 +552,18 @@ vault(AccountID const& owner, std::uint32_t seq) noexcept
     return vault(indexHash(LedgerNameSpace::VAULT, owner, seq));
 }
 
+Keylet
+loanbroker(AccountID const& owner, std::uint32_t seq) noexcept
+{
+    return loanbroker(indexHash(LedgerNameSpace::LOAN_BROKER, owner, seq));
+}
+
+Keylet
+loan(AccountID const& owner, uint256 loanBrokerID, std::uint32_t seq) noexcept
+{
+    return loan(indexHash(LedgerNameSpace::LOAN, loanBrokerID, seq));
+}
+
 Keylet
 permissionedDomain(AccountID const& account, std::uint32_t seq) noexcept
 {

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

@@ -1578,4 +1578,101 @@ ValidPermissionedDomain::finalize(
         (sleStatus_[1] ? check(*sleStatus_[1], j) : true);
 }
 
+//------------------------------------------------------------------------------
+
+void
+NoModifiedUnmodifiableFields::visitEntry(
+    bool isDelete,
+    std::shared_ptr<SLE const> const& before,
+    std::shared_ptr<SLE const> const& after)
+{
+    if (isDelete || !before)
+        // Creation and deletion are ignored
+        return;
+
+    auto const type = after->getType();
+    if (knownTypes_.contains(type))
+    {
+        changedEntries_.emplace(before, after);
+    }
+}
+
+bool
+NoModifiedUnmodifiableFields::finalize(
+    STTx const& tx,
+    TER const,
+    XRPAmount const,
+    ReadView const& view,
+    beast::Journal const& j)
+{
+    static auto const fieldChanged = [](auto const& before,
+                                        auto const& after,
+                                        auto const& field) {
+        return before->isFieldPresent(field) != after->isFieldPresent(field) ||
+            before->at(field) != after->at(field);
+    };
+    for (auto const& slePair : changedEntries_)
+    {
+        auto const& before = slePair.first;
+        auto const& after = slePair.second;
+        auto const type = after->getType();
+        bool bad = false;
+        [[maybe_unused]] bool enforce = false;
+        switch (type)
+        {
+            case ltLOAN_BROKER:
+                /*
+                 * We check this invariant regardless of lending protocol
+                 * amendment status, allowing for detection and logging of
+                 * potential issues even when the amendment is disabled.
+                 */
+                enforce = view.rules().enabled(featureLendingProtocol);
+                bad = fieldChanged(before, after, sfVaultID) ||
+                    fieldChanged(before, after, sfAccount) ||
+                    fieldChanged(before, after, sfOwner) ||
+                    fieldChanged(before, after, sfManagementFeeRate) ||
+                    fieldChanged(before, after, sfCoverRateMinimum) ||
+                    fieldChanged(before, after, sfCoverRateLiquidation);
+                break;
+            case ltLOAN:
+                /*
+                 * We check this invariant regardless of lending protocol
+                 * amendment status, allowing for detection and logging of
+                 * potential issues even when the amendment is disabled.
+                 */
+                enforce = view.rules().enabled(featureLendingProtocol);
+                bad = fieldChanged(before, after, sfLoanBrokerID) ||
+                    fieldChanged(before, after, sfBorrower) ||
+                    fieldChanged(before, after, sfLoanOriginationFee) ||
+                    fieldChanged(before, after, sfLoanServiceFee) ||
+                    fieldChanged(before, after, sfLatePaymentFee) ||
+                    fieldChanged(before, after, sfClosePaymentFee) ||
+                    fieldChanged(before, after, sfOverpaymentFee) ||
+                    fieldChanged(before, after, sfInterestRate) ||
+                    fieldChanged(before, after, sfLateInterestRate) ||
+                    fieldChanged(before, after, sfCloseInterestRate) ||
+                    fieldChanged(before, after, sfOverpaymentInterestRate) ||
+                    fieldChanged(before, after, sfPaymentInterval);
+                break;
+            default:
+                UNREACHABLE(
+                    "ripple::NoModifiedUnmodifiableFields::finalize : unknown "
+                    "SLE type");
+        }
+        XRPL_ASSERT(
+            !bad || enforce,
+            "ripple::NoModifiedUnmodifiableFields::finalize : no bad "
+            "changes or enforce invariant");
+        if (bad)
+        {
+            JLOG(j.fatal())
+                << "Invariant failed: changed an unchangable field for "
+                << tx.getTransactionID();
+            if (enforce)
+                return false;
+        }
+    }
+    return true;
+}
+
 }  // namespace ripple

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

@@ -618,6 +618,35 @@ public:
         beast::Journal const&);
 };
 
+/**
+ * @brief Invariants: Some fields are unmodifiable
+ *
+ * Check that any fields specified as unmodifiable are not modified when the
+ * object is modified. Creation and deletion are ignored.
+ *
+ */
+class NoModifiedUnmodifiableFields
+{
+    std::set<LedgerEntryType> const knownTypes_{ltLOAN_BROKER, ltLOAN};
+
+    std::set<std::pair<SLE::const_pointer, SLE::const_pointer>> changedEntries_;
+
+public:
+    void
+    visitEntry(
+        bool,
+        std::shared_ptr<SLE const> const&,
+        std::shared_ptr<SLE const> const&);
+
+    bool
+    finalize(
+        STTx const&,
+        TER const,
+        XRPAmount const,
+        ReadView const&,
+        beast::Journal const&);
+};
+
 // additional invariant checks can be declared above and then added to this
 // tuple
 using InvariantChecks = std::tuple<
@@ -637,7 +666,8 @@ using InvariantChecks = std::tuple<
     NFTokenCountTracking,
     ValidClawback,
     ValidMPTIssuance,
-    ValidPermissionedDomain>;
+    ValidPermissionedDomain,
+    NoModifiedUnmodifiableFields>;
 
 /**
  * @brief get a tuple of all invariant checks