WIP

Signed-off-by: JCW <a1q123456@users.noreply.github.com>

include/xrpl/beast/utility/Journal.h

@@ -22,6 +22,7 @@
 
 #include <xrpl/beast/utility/instrumentation.h>
 
+#include <thread>
 #include <atomic>
 #include <charconv>
 #include <cstring>
@@ -34,6 +35,164 @@
 #include <string_view>
 #include <utility>
 
+namespace beast {
+
+class StringBufferPool {
+public:
+    // ----- Empty index marker -----
+    static constexpr std::uint32_t kEmptyIdx = std::numeric_limits<std::uint32_t>::max();
+
+    // ----- Single-word CAS target: {tag | idx} with pack/unpack -----
+    struct Head {
+        std::uint32_t tag;
+        std::uint32_t idx; // kEmptyIdx means empty
+
+        static std::uint64_t pack(Head h) noexcept {
+            return (std::uint64_t(h.tag) << 32) | h.idx;
+        }
+        static Head unpack(std::uint64_t v) noexcept {
+            return Head{ std::uint32_t(v >> 32), std::uint32_t(v) };
+        }
+    };
+
+    // ----- Internal node -----
+    struct Node {
+        std::uint32_t next_idx{kEmptyIdx};
+        std::uint32_t self_idx{kEmptyIdx};
+        std::string   buf{};
+    };
+    static_assert(std::is_standard_layout_v<Node>, "Node must be standard layout");
+
+    // ----- User-facing move-only RAII handle -----
+    class Handle {
+    public:
+        Handle() = default;
+        Handle(Handle&& other) noexcept
+            : owner_(other.owner_), node_(other.node_) {
+            other.owner_ = nullptr; other.node_ = nullptr;
+        }
+        Handle& operator=(Handle&& other) noexcept {
+            if (this != &other) {
+                // Return current if still held
+                if (owner_ && node_) owner_->give_back(std::move(*this));
+                owner_ = other.owner_;
+                node_  = other.node_;
+                other.owner_ = nullptr;
+                other.node_  = nullptr;
+            }
+            return *this;
+        }
+
+        Handle(const Handle&) = delete;
+        Handle& operator=(const Handle&) = delete;
+
+        ~Handle() noexcept {
+            if (owner_ && node_) owner_->give_back(std::move(*this));
+        }
+
+        bool valid() const noexcept { return node_ != nullptr; }
+        std::string&       string()       noexcept { return node_->buf; }
+        const std::string& string() const noexcept { return node_->buf; }
+
+    private:
+        friend class StringBufferPool;
+        Handle(StringBufferPool* owner, Node* n) : owner_(owner), node_(n) {}
+
+        StringBufferPool* owner_ = nullptr;
+        Node* node_ = nullptr;
+    };
+
+    explicit StringBufferPool(std::uint32_t grow_by = 20)
+        : grow_by_(grow_by), head_(Head::pack({0, kEmptyIdx})) {}
+
+    // Rent a buffer; grows on demand. Returns move-only RAII handle.
+    Handle rent() {
+        for (;;) {
+            std::uint64_t old64 = head_.load(std::memory_order_acquire);
+            Head old = Head::unpack(old64);
+            if (old.idx == kEmptyIdx) { grow_(); continue; }      // rare slow path
+
+            Node& n = nodes_[old.idx];
+            std::uint32_t next = n.next_idx;
+
+            Head neu{ std::uint32_t(old.tag + 1), next };
+            if (head_.compare_exchange_weak(old64, Head::pack(neu),
+                                            std::memory_order_acq_rel,
+                                            std::memory_order_acquire)) {
+                return {this, &n};
+            }
+        }
+    }
+
+private:
+    // Only the pool/handle can call this
+    void give_back(Handle&& h) noexcept {
+        Node* node = h.node_;
+        if (!node) return; // already invalid
+        const std::uint32_t idx = node->self_idx;
+
+        node->buf.clear();
+
+        for (;;) {
+            std::uint64_t old64 = head_.load(std::memory_order_acquire);
+            Head old = Head::unpack(old64);
+
+            node->next_idx = old.idx;
+
+            Head neu{ std::uint32_t(old.tag + 1), idx };
+            if (head_.compare_exchange_weak(old64, Head::pack(neu),
+                                            std::memory_order_acq_rel,
+                                            std::memory_order_acquire)) {
+                // Invalidate handle (prevents double return)
+                h.owner_ = nullptr;
+                h.node_  = nullptr;
+                return;
+            }
+        }
+    }
+
+    void grow_() {
+        if (Head::unpack(head_.load(std::memory_order_acquire)).idx != kEmptyIdx) return;
+        std::scoped_lock lk(grow_mu_);
+        if (Head::unpack(head_.load(std::memory_order_acquire)).idx != kEmptyIdx) return;
+
+        const std::uint32_t base = static_cast<std::uint32_t>(nodes_.size());
+        nodes_.resize(base + grow_by_); // indices [base .. base+grow_by_-1]
+
+        // Init nodes and local chain
+        for (std::uint32_t i = 0; i < grow_by_; ++i) {
+            std::uint32_t idx = base + i;
+            Node& n = nodes_[idx];
+            n.self_idx = idx;
+            n.next_idx = (i + 1 < grow_by_) ? (idx + 1) : kEmptyIdx;
+        }
+
+        // Splice chain onto global head: [base .. base+grow_by_-1]
+        const std::uint32_t chain_head = base;
+        const std::uint32_t chain_tail = base + grow_by_ - 1;
+
+        for (;;) {
+            std::uint64_t old64 = head_.load(std::memory_order_acquire);
+            Head old = Head::unpack(old64);
+
+            nodes_[chain_tail].next_idx = old.idx; // tail -> old head
+            Head neu{ std::uint32_t(old.tag + 1), chain_head };
+
+            if (head_.compare_exchange_weak(old64, Head::pack(neu),
+                                            std::memory_order_acq_rel,
+                                            std::memory_order_acquire)) {
+                break;
+            }
+        }
+    }
+
+    const std::uint32_t        grow_by_;
+    std::atomic<std::uint64_t> head_;     // single 64-bit CAS (Head packed)
+    std::mutex                 grow_mu_;  // only during growth
+    std::deque<Node>           nodes_;    // stable storage for nodes/strings
+};
+} // namespace beast
+
 namespace ripple::log {
 template <typename T>
 class LogParameter
@@ -181,10 +340,10 @@ public:
         buffer_.append(str);
     }
 
-    [[nodiscard]] std::string_view
+    void
     finish()
     {
-        return std::string_view{buffer_.c_str(), buffer_.size() - 1};
+        buffer_.pop_back();
     }
 
 private:
@@ -323,18 +482,25 @@ public:
 
     class Sink;
 
+    using MessagePoolNode = lockfree::queue<std::string>::Node*;
+
     class JsonLogContext
     {
-        std::string buffer_;
+        MessagePoolNode messageBuffer_;
         detail::SimpleJsonWriter messageParamsWriter_;
         bool hasMessageParams_ = false;
 
     public:
-        JsonLogContext() : messageParamsWriter_(buffer_)
+        explicit JsonLogContext()
+            : messageBuffer_(rentFromPool())
+            , messageParamsWriter_(messageBuffer_->data)
         {
-            buffer_.reserve(1024 * 5);
+            messageBuffer_->data.reserve(1024 * 5);
         }
 
+        MessagePoolNode
+        messageBuffer() { return messageBuffer_; }
+
         void
         startMessageParams()
         {
@@ -379,6 +545,7 @@ private:
     static std::shared_mutex globalLogAttributesMutex_;
     static bool jsonLogsEnabled_;
 
+    static lockfree::queue<std::string> messagePool_;
     static thread_local JsonLogContext currentJsonLogContext_;
 
     // Invariant: m_sink always points to a valid Sink
@@ -389,12 +556,26 @@ private:
         std::source_location location,
         severities::Severity severity) const;
 
-    static std::string_view
+    static MessagePoolNode
     formatLog(std::string const& message);
 
 public:
     //--------------------------------------------------------------------------
 
+    static MessagePoolNode
+    rentFromPool()
+    {
+        auto node = messagePool_.pop();
+        if (!node)
+        {
+            node = new lockfree::queue<std::string>::Node();
+        }
+        return node;
+    }
+
+    static void
+    returnMessageNode(MessagePoolNode node) { messagePool_.push(node); }
+
     static void
     enableStructuredJournal();
 
@@ -444,7 +625,7 @@ public:
             level is below the current threshold().
         */
         virtual void
-        write(Severity level, std::string_view text) = 0;
+        write(Severity level, std::string_view text, MessagePoolNode owner = nullptr) = 0;
 
         /** Bypass filter and write text to the sink at the specified severity.
          * Always write the message, but maintain the same formatting as if
@@ -454,7 +635,7 @@ public:
          * @param text Text to write to sink.
          */
         virtual void
-        writeAlways(Severity level, std::string_view text) = 0;
+        writeAlways(Severity level, std::string_view text, MessagePoolNode owner = nullptr) = 0;
 
     private:
         Severity thresh_;