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diff --git a/src/rpc/zmq_pub.h b/src/rpc/zmq_pub.h
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+// Copyright (c) 2020, The Monero Project
+//
+// All rights reserved.
+ //
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+//
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+//
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#pragma once
+
+#include <array>
+#include <boost/thread/mutex.hpp>
+#include <boost/utility/string_ref.hpp>
+#include <cstdint>
+#include <deque>
+#include <memory>
+#include <vector>
+
+#include "cryptonote_basic/fwd.h"
+#include "net/zmq.h"
+#include "span.h"
+
+namespace cryptonote { namespace listener
+{
+/*! \brief Sends ZMQ PUB messages on cryptonote events
+
+    Clients must ensure that all transaction(s) are notified before any blocks
+    they are contained in, and must ensure that each block is notified in chain
+    order. An external lock **must** be held by clients during the entire
+    txpool check and notification sequence and (a possibly second) lock is held
+    during the entire block check and notification sequence. Otherwise, events
+    could be sent in a different order than processed. */
+class zmq_pub
+{
+  /* Each socket has its own internal queue. So we can only use one socket, else
+     the messages being published are not guaranteed to be in the same order
+     pushed. */
+
+    net::zmq::socket relay_;
+    std::deque<std::vector<txpool_event>> txes_;
+    std::array<std::size_t, 2> chain_subs_;
+    std::array<std::size_t, 2> txpool_subs_;
+    boost::mutex sync_; //!< Synchronizes counts in `*_subs_` arrays.
+
+  public:
+    //! \return Name of ZMQ_PAIR endpoint for pub notifications
+    static constexpr const char* relay_endpoint() noexcept { return "inproc://pub_relay"; }
+
+    explicit zmq_pub(void* context);
+
+    zmq_pub(const zmq_pub&) = delete;
+    zmq_pub(zmq_pub&&) = delete;
+
+    ~zmq_pub();
+
+    zmq_pub& operator=(const zmq_pub&) = delete;
+    zmq_pub& operator=(zmq_pub&&) = delete;
+
+    //! Process a client subscription request (from XPUB sockets). Thread-safe.
+    bool sub_request(const boost::string_ref message);
+
+    /*! Forward ZMQ messages sent to `relay` via `send_chain_main` or
+      `send_txpool_add` to `pub`. Used by `ZmqServer`. */
+    bool relay_to_pub(void* relay, void* pub);
+
+    /*! Send a `ZMQ_PUB` notification for a change to the main chain.
+        Thread-safe.
+        \return Number of ZMQ messages sent to relay. */
+    std::size_t send_chain_main(std::uint64_t height, epee::span<const cryptonote::block> blocks);
+
+    /*! Send a `ZMQ_PUB` notification for new tx(es) being added to the local
+        pool. Thread-safe.
+        \return Number of ZMQ messages sent to relay. */
+    std::size_t send_txpool_add(std::vector<cryptonote::txpool_event> txes);
+
+    //! Callable for `send_chain_main` with weak ownership to `zmq_pub` object.
+    struct chain_main
+    {
+      std::weak_ptr<zmq_pub> self_;
+      void operator()(std::uint64_t height, epee::span<const cryptonote::block> blocks) const;
+    };
+
+    //! Callable for `send_txpool_add` with weak ownership to `zmq_pub` object.
+    struct txpool_add
+    {
+      std::weak_ptr<zmq_pub> self_;
+      void operator()(std::vector<cryptonote::txpool_event> txes) const;
+    };
+  };
+}}