style: Compare connection_ to nullptr explicitly

Resolve the PeerReservationTable.h conflict with develop's naming and
member-init changes. Use insertOrAssign in the new error text, and add the
direct includes that misc-include-cleaner needs for the new Throw, JLOG
and std::runtime_error uses.

.cspell.config.yaml

@@ -166,6 +166,7 @@ words:
   - legleux
   - levelization
   - levelized
+  - lgrdb
   - libpb
   - libxrpl
   - llection

include/xrpl/core/PeerReservationTable.h

@@ -95,6 +95,10 @@ public:
 private:
     beast::Journal mutable journal_;
     std::mutex mutable mutex_;
+    // Initialized to nullptr for safety. Set by load() during the second
+    // phase of ApplicationImp initialization. Methods that dereference
+    // this pointer must validate it first, since two-phase init means
+    // load() may not have been called yet.
     DatabaseCon* connection_{};
     std::unordered_set<PeerReservation, beast::Uhash<>, KeyEqual> table_;
 };

src/xrpld/app/rdb/backend/SQLiteDatabase.h

@@ -1,7 +1,9 @@
 #pragma once
 
+#include <xrpl/basics/Log.h>
 #include <xrpl/basics/RangeSet.h>
 #include <xrpl/basics/base_uint.h>
+#include <xrpl/basics/contract.h>
 #include <xrpl/beast/utility/Journal.h>
 #include <xrpl/protocol/ErrorCodes.h>
 #include <xrpl/protocol/LedgerHeader.h>
@@ -16,6 +18,7 @@
 #include <map>
 #include <memory>
 #include <optional>
+#include <stdexcept>
 #include <utility>
 #include <variant>
 #include <vector>
@@ -398,15 +401,6 @@ public:
     SQLiteDatabase&
     operator=(SQLiteDatabase&&) = delete;
 
-    /**
-     * @brief ledgerDbHasSpace Checks if the ledger database has available
-     *        space.
-     * @param config Config object.
-     * @return True if space is available.
-     */
-    bool
-    ledgerDbHasSpace(Config const& config);
-
     /**
      * @brief transactionDbHasSpace Checks if the transaction database has
      *        available space.
@@ -458,13 +452,27 @@ private:
     }
 
     /**
-     * @brief checkoutTransaction Checks out and returns node store ledger
+     * @brief checkoutLedger Checks out and returns node store ledger
      *        database.
      * @return Session to the node store ledger database.
+     * @throws std::runtime_error if ledger database is not available.
+     *
+     * @note Callers typically guard with existsLedger() before calling
+     *       this method. The explicit null check here provides
+     *       defense-in-depth so that safety does not depend solely on
+     *       an implicit caller contract. See PR #6029 for context on
+     *       the pattern of relying on config settings instead of
+     *       validating actual objects.
      */
     auto
     checkoutLedger()
     {
+        if (!ledgerDb_)
+        {
+            constexpr auto msg = "Ledger database is not available";
+            JLOG(j_.fatal()) << msg;
+            Throw<std::runtime_error>(msg);
+        }
         return ledgerDb_->checkoutDb();
     }
 
@@ -472,10 +480,23 @@ private:
      * @brief checkoutTransaction Checks out and returns the node store
      *        transaction database.
      * @return Session to the node store transaction database.
+     * @throws std::runtime_error if transaction database is not available.
+     *
+     * @note Callers typically guard with existsTransaction() and/or
+     *       useTxTables_ before calling this method. The explicit null
+     *       check here provides defense-in-depth so that safety does
+     *       not depend solely on an implicit caller contract or config
+     *       settings. See PR #6029 for context.
      */
     auto
     checkoutTransaction()
     {
+        if (!txdb_)
+        {
+            constexpr auto msg = "Transaction database is not available";
+            JLOG(j_.fatal()) << msg;
+            Throw<std::runtime_error>(msg);
+        }
         return txdb_->checkoutDb();
     }
 };

src/xrpld/app/rdb/backend/detail/SQLiteDatabase.cpp

@@ -559,18 +559,6 @@ SQLiteDatabase::SQLiteDatabase(SQLiteDatabase&& rhs) noexcept
     std::exchange(txdb_, std::move(rhs.txdb_));
 }
 
-bool
-SQLiteDatabase::ledgerDbHasSpace(Config const& config)
-{
-    if (existsLedger())
-    {
-        auto db = checkoutLedger();
-        return detail::dbHasSpace(*db, config, j_);
-    }
-
-    return true;
-}
-
 bool
 SQLiteDatabase::transactionDbHasSpace(Config const& config)
 {

src/xrpld/overlay/detail/PeerReservationTable.cpp

@@ -1,5 +1,6 @@
 #include <xrpl/core/PeerReservationTable.h>
 
+#include <xrpl/basics/contract.h>
 #include <xrpl/json/json_value.h>
 #include <xrpl/protocol/PublicKey.h>
 #include <xrpl/protocol/jss.h>
@@ -10,6 +11,7 @@
 #include <iterator>
 #include <mutex>
 #include <optional>
+#include <stdexcept>
 #include <string>
 #include <vector>
 
@@ -87,6 +89,15 @@ PeerReservationTable::insertOrAssign(PeerReservation const& reservation)
     }
     table_.insert(hint, reservation);
 
+    // connection_ is set by load() during two-phase init. Validate
+    // before dereferencing to guard against use-before-load or a reset
+    // connection. See PR #6029 for the general pattern discussion.
+    if (connection_ == nullptr)
+    {
+        Throw<std::runtime_error>(
+            "PeerReservationTable::insertOrAssign: database connection is "
+            "not available");
+    }
     auto db = connection_->checkoutDb();
     insertPeerReservation(*db, reservation.nodeId, reservation.description);
 
@@ -105,6 +116,14 @@ PeerReservationTable::erase(PublicKey const& nodeId)
     {
         previous = *it;
         table_.erase(it);
+        // Validate connection_ before dereferencing — see comment in
+        // insertOrAssign above.
+        if (connection_ == nullptr)
+        {
+            Throw<std::runtime_error>(
+                "PeerReservationTable::erase: database connection is not "
+                "available");
+        }
         auto db = connection_->checkoutDb();
         deletePeerReservation(*db, nodeId);
     }