Add a LoanBroker invariant to compare CoverAvailable to balance

- Ensures that LoanBroker.CoverAvailable is never less than the
  pseudo-account balance for the Vault asset.

include/xrpl/protocol/STAmount.h

@@ -564,7 +564,7 @@ STAmount::clear()
 {
     // The -100 is used to allow 0 to sort less than a small positive values
     // which have a negative exponent.
-    mOffset = native() ? 0 : -100;
+    mOffset = integral() ? 0 : -100;
     mValue = 0;
     mIsNegative = false;
 }

src/libxrpl/protocol/STAmount.cpp

@@ -830,7 +830,7 @@ STAmount::isDefault() const
 void
 STAmount::canonicalize()
 {
-    if (native() || mAsset.holds<MPTIssue>())
+    if (integral())
     {
         // native and MPT currency amounts should always have an offset of zero
         // log(2^64,10) ~ 19.2
@@ -859,8 +859,10 @@ STAmount::canonicalize()
             };
             if (native())
                 set(XRPAmount{num});
-            else
+            else if (mAsset.holds<MPTIssue>())
                 set(MPTAmount{num});
+            else
+                Throw<std::runtime_error>("Unknown integral asset type");
             mOffset = 0;
         }
         else

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

@@ -2342,9 +2342,30 @@ ValidLoanBroker::visitEntry(
     std::shared_ptr<SLE const> const& before,
     std::shared_ptr<SLE const> const& after)
 {
-    if (after && after->getType() == ltLOAN_BROKER)
+    if (after)
     {
-        brokers_.emplace_back(before, after);
+        if (after->getType() == ltLOAN_BROKER)
+        {
+            auto& broker = brokers_[after->key()];
+            broker.brokerBefore = before;
+            broker.brokerAfter = after;
+        }
+        else if (
+            after->getType() == ltACCOUNT_ROOT &&
+            after->isFieldPresent(sfLoanBrokerID))
+        {
+            auto const& loanBrokerID = after->at(sfLoanBrokerID);
+            // create an entry if one doesn't already exist
+            auto& broker = brokers_[loanBrokerID];
+        }
+        else if (after->getType() == ltRIPPLE_STATE)
+        {
+            lines_.emplace_back(after);
+        }
+        else if (after->getType() == ltMPTOKEN)
+        {
+            mpts_.emplace_back(after);
+        }
     }
 }
 
@@ -2403,8 +2424,51 @@ ValidLoanBroker::finalize(
     // Loan Brokers will not exist on ledger if the Lending Protocol amendment
     // is not enabled, so there's no need to check it.
 
-    for (auto const& [before, after] : brokers_)
+    for (auto const& line : lines_)
     {
+        for (auto const& field : {&sfLowLimit, &sfHighLimit})
+        {
+            auto const account =
+                view.read(keylet::account(line->at(*field).getIssuer()));
+            // This Invariant doesn't know about the rules for Trust Lines, so
+            // if the account is missing, don't treat it as an error. This
+            // loop is only concerned with finding Broker pseudo-accounts
+            if (account && account->isFieldPresent(sfLoanBrokerID))
+            {
+                auto const& loanBrokerID = account->at(sfLoanBrokerID);
+                // create an entry if one doesn't already exist
+                auto& broker = brokers_[loanBrokerID];
+            }
+        }
+    }
+    for (auto const& mpt : mpts_)
+    {
+        auto const account = view.read(keylet::account(mpt->at(sfAccount)));
+        // This Invariant doesn't know about the rules for MPTokens, so
+        // if the account is missing, don't treat is as an error. This
+        // loop is only concerned with finding Broker pseudo-accounts
+        if (account && account->isFieldPresent(sfLoanBrokerID))
+        {
+            auto const& loanBrokerID = account->at(sfLoanBrokerID);
+            // create an entry if one doesn't already exist
+            auto& broker = brokers_[loanBrokerID];
+        }
+    }
+
+    for (auto const& [brokerID, broker] : brokers_)
+    {
+        auto const& after = broker.brokerAfter
+            ? broker.brokerAfter
+            : view.read(keylet::loanbroker(brokerID));
+
+        if (!after)
+        {
+            JLOG(j.fatal()) << "Invariant failed: Loan Broker missing";
+            return false;
+        }
+
+        auto const& before = broker.brokerBefore;
+
         // https://github.com/Tapanito/XRPL-Standards/blob/xls-66-lending-protocol/XLS-0066d-lending-protocol/README.md#3123-invariants
         // If `LoanBroker.OwnerCount = 0` the `DirectoryNode` will have at most
         // one node (the root), which will only hold entries for `RippleState`
@@ -2438,6 +2502,26 @@ ValidLoanBroker::finalize(
                 << "Invariant failed: Loan Broker cover available is negative";
             return false;
         }
+        auto const vault = view.read(keylet::vault(after->at(sfVaultID)));
+        if (!vault)
+        {
+            JLOG(j.fatal())
+                << "Invariant failed: Loan Broker vault ID is invalid";
+            return false;
+        }
+        auto const& vaultAsset = vault->at(sfAsset);
+        if (after->at(sfCoverAvailable) < accountHolds(
+                                              view,
+                                              after->at(sfAccount),
+                                              vaultAsset,
+                                              FreezeHandling::fhIGNORE_FREEZE,
+                                              AuthHandling::ahIGNORE_AUTH,
+                                              j))
+        {
+            JLOG(j.fatal()) << "Invariant failed: Loan Broker cover available "
+                               "is less than pseudo-account asset balance";
+            return false;
+        }
     }
     return true;
 }

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

@@ -763,9 +763,25 @@ public:
  */
 class ValidLoanBroker
 {
-    // Pair is <before, after>. After is used for most of the checks, except
+    // Not all of these elements will necessarily be populated. Remaining items
+    // will be looked up as needed.
+    struct BrokerInfo
+    {
+        SLE::const_pointer brokerBefore = nullptr;
+        // After is used for most of the checks, except
         // those that check changed values.
-    std::vector<std::pair<SLE::const_pointer, SLE::const_pointer>> brokers_;
+        SLE::const_pointer brokerAfter = nullptr;
+    };
+    // Collect all the LoanBrokers found directly or indirectly through
+    // pseudo-accounts. Key is the brokerID / index. It will be used to find the
+    // LoanBroker object if brokerBefore and brokerAfter are nullptr
+    std::map<uint256, BrokerInfo> brokers_;
+    // Collect all the modified trust lines. Their high and low accounts will be
+    // loaded to look for LoanBroker pseudo-accounts.
+    std::vector<SLE::const_pointer> lines_;
+    // Collect all the modified MPTokens. Their accounts will be loaded to look
+    // for LoanBroker pseudo-accounts.
+    std::vector<SLE::const_pointer> mpts_;
 
     bool
     goodZeroDirectory(