From a6fe3596f1f1876bccd387b34085aaf1e50232d4 Mon Sep 17 00:00:00 2001
From: Eric Voskuil <eric@voskuil.org>
Date: Fri, 11 Sep 2026 16:56:54 -0400
Subject: [PATCH] Cache notified txs in tx_out protocol for relay.

---
 .../protocol_transaction_out_106.hpp          | 20 +++++++
 .../protocol_transaction_out_106.cpp          | 55 ++++++++++++++++++-
 2 files changed, 74 insertions(+), 1 deletion(-)

diff --git a/include/bitcoin/node/protocols/protocol_transaction_out_106.hpp b/include/bitcoin/node/protocols/protocol_transaction_out_106.hpp
index 05dcf659..192ae5f0 100644
--- a/include/bitcoin/node/protocols/protocol_transaction_out_106.hpp
+++ b/include/bitcoin/node/protocols/protocol_transaction_out_106.hpp
@@ -36,6 +36,7 @@ class BCN_API protocol_transaction_out_106
         const network::channel::ptr& channel) NOEXCEPT
       : node::protocol_peer(session, channel),
         node_witness_(session->node_settings().provide_witness),
+        broadcast_(maximum_retained),
         network::tracker<protocol_transaction_out_106>(session->log)
     {
     }
@@ -70,9 +71,28 @@ class BCN_API protocol_transaction_out_106
 
     virtual bool announce(const system::hash_digest& hash) NOEXCEPT;
 
+    /// Retain a broadcast tx, pending request by the peer.
+    virtual void retain(const system::hash_digest& hash,
+        const system::chain::transaction::cptr& tx) NOEXCEPT;
+
+    /// Obtain and drop a retained tx, or nullptr.
+    virtual system::chain::transaction::cptr release(
+        const system::hash_digest& hash) NOEXCEPT;
+
 private:
+    using retained_t = std::pair<system::hash_digest,
+        system::chain::transaction::cptr>;
+    using retained_txs = boost::circular_buffer<retained_t>;
+
+    static constexpr size_t maximum_retained = 42;
+
+    retained_txs::iterator find(const system::hash_digest& hash) NOEXCEPT;
+
     // These are thread safe.
     const bool node_witness_;
+
+    // This is protected by strand.
+    retained_txs broadcast_;
 };
 
 } // namespace node
diff --git a/src/protocols/protocol_transaction_out_106.cpp b/src/protocols/protocol_transaction_out_106.cpp
index 7d0d8e57..ca0faabb 100644
--- a/src/protocols/protocol_transaction_out_106.cpp
+++ b/src/protocols/protocol_transaction_out_106.cpp
@@ -18,6 +18,7 @@
  */
 #include <bitcoin/node/protocols/protocol_transaction_out_106.hpp>
 
+#include <algorithm>
 #include <bitcoin/node/define.hpp>
 
 namespace libbitcoin {
@@ -116,7 +117,13 @@ bool protocol_transaction_out_106::handle_broadcast_transaction(const code& ec,
     if (sender == identifier())
         return true;
 
-    return announce(message->transaction_ptr->hash(false));
+    const auto& tx = message->transaction_ptr;
+    const auto hash = tx->hash(false);
+    if (was_announced(hash))
+        return true;
+
+    retain(hash, tx);
+    return announce(hash);
 }
 
 bool protocol_transaction_out_106::announce(const hash_digest& hash) NOEXCEPT
@@ -139,6 +146,45 @@ bool protocol_transaction_out_106::announce(const hash_digest& hash) NOEXCEPT
     return true;
 }
 
+// Retention.
+// ----------------------------------------------------------------------------
+
+void protocol_transaction_out_106::retain(const hash_digest& hash,
+    const chain::transaction::cptr& tx) NOEXCEPT
+{
+    BC_ASSERT(stranded());
+
+    if (find(hash) != broadcast_.end())
+        return;
+
+    broadcast_.push_back({ hash, tx });
+}
+
+chain::transaction::cptr protocol_transaction_out_106::release(
+    const hash_digest& hash) NOEXCEPT
+{
+    BC_ASSERT(stranded());
+
+    const auto it = find(hash);
+    if (it == broadcast_.end())
+        return {};
+
+    const auto tx = it->second;
+    broadcast_.erase(it);
+    return tx;
+}
+
+protocol_transaction_out_106::retained_txs::iterator
+protocol_transaction_out_106::find(const hash_digest& hash) NOEXCEPT
+{
+    const auto match = [&hash](const auto& entry) NOEXCEPT
+    {
+        return entry.first == hash;
+    };
+
+    return std::find_if(broadcast_.begin(), broadcast_.end(), match);
+}
+
 // Inbound (get_data).
 // ----------------------------------------------------------------------------
 
@@ -192,6 +238,13 @@ void protocol_transaction_out_106::send_transaction(const code& ec,
         return;
     }
 
+    // A broadcast tx is not archived, so is served from retention, once.
+    if (const auto retained = release(item.hash))
+    {
+        SEND(transaction{ retained }, send_transaction, _1, add1(index), message);
+        return;
+    }
+
     // Tx could be always queried with witness and therefore safely cached.
     // If can then be serialized according to channel configuration, however
     // that is currently fixed to witness as available in the object.