net_processing: retry private broadcast

diff --git a/src/net_processing.cpp b/src/net_processing.cpp
index 2f539a16..512f99f0 100644
--- a/src/net_processing.cpp
+++ b/src/net_processing.cpp
@@ -566,6 +566,9 @@ private:
     /** Retrieve unbroadcast transactions from the mempool and reattempt sending to peers */
     void ReattemptInitialBroadcast(CScheduler& scheduler) EXCLUSIVE_LOCKS_REQUIRED(!m_peer_mutex);
 
+    /** Rebroadcast stale private transactions (already broadcast but not received back from the network). */
+    void ReattemptPrivateBroadcast(CScheduler& scheduler);
+
     /** Get a shared pointer to the Peer object.
      *  May return an empty shared_ptr if the Peer object can't be found. */
     PeerRef GetPeerRef(NodeId id) const EXCLUSIVE_LOCKS_REQUIRED(!m_peer_mutex);
@@ -1633,6 +1636,37 @@ void PeerManagerImpl::ReattemptInitialBroadcast(CScheduler& scheduler)
     scheduler.scheduleFromNow([&] { ReattemptInitialBroadcast(scheduler); }, delta);
 }
 
+void PeerManagerImpl::ReattemptPrivateBroadcast(CScheduler& scheduler)
+{
+    // Remove stale transactions that are no longer relevant (e.g. already in
+    // the mempool or mined) and count the remaining ones.
+    size_t num_for_rebroadcast{0};
+    const auto stale_txs = m_tx_for_private_broadcast.GetStale();
+    if (!stale_txs.empty()) {
+        LOCK(cs_main);
+        for (const auto& stale_tx : stale_txs) {
+            auto mempool_acceptable = m_chainman.ProcessTransaction(stale_tx, /*test_accept=*/true);
+            if (mempool_acceptable.m_result_type == MempoolAcceptResult::ResultType::VALID) {
+                LogDebug(BCLog::PRIVBROADCAST,
+                         "Reattempting broadcast of stale txid=%s wtxid=%s",
+                         stale_tx->GetHash().ToString(), stale_tx->GetWitnessHash().ToString());
+                ++num_for_rebroadcast;
+            } else {
+                LogInfo("[privatebroadcast] Giving up broadcast attempts for txid=%s wtxid=%s: %s",
+                        stale_tx->GetHash().ToString(), stale_tx->GetWitnessHash().ToString(),
+                        mempool_acceptable.m_state.ToString());
+                m_tx_for_private_broadcast.Remove(stale_tx);
+            }
+        }
+
+        // This could overshoot, but that is ok - we will open some private connections in vain.
+        m_connman.m_private_broadcast.NumToOpenAdd(num_for_rebroadcast);
+    }
+
+    const auto delta{2min + FastRandomContext().randrange<std::chrono::milliseconds>(1min)};
+    scheduler.scheduleFromNow([&] { ReattemptPrivateBroadcast(scheduler); }, delta);
+}
+
 void PeerManagerImpl::FinalizeNode(const CNode& node)
 {
     NodeId nodeid = node.GetId();
@@ -1971,6 +2005,10 @@ void PeerManagerImpl::StartScheduledTasks(CScheduler& scheduler)
     // schedule next run for 10-15 minutes in the future
     const auto delta = 10min + FastRandomContext().randrange<std::chrono::milliseconds>(5min);
     scheduler.scheduleFromNow([&] { ReattemptInitialBroadcast(scheduler); }, delta);
+
+    if (m_opts.private_broadcast) {
+        scheduler.scheduleFromNow([&] { ReattemptPrivateBroadcast(scheduler); }, 0min);
+    }
 }
 
 void PeerManagerImpl::ActiveTipChange(const CBlockIndex& new_tip, bool is_ibd)
diff --git a/src/net_processing.h b/src/net_processing.h
index ecfb6985..09f348c8 100644
--- a/src/net_processing.h
+++ b/src/net_processing.h
@@ -90,6 +90,8 @@ public:
         //! Number of headers sent in one getheaders message result (this is
         //! a test-only option).
         uint32_t max_headers_result{MAX_HEADERS_RESULTS};
+        //! Whether private broadcast is used for sending transactions.
+        bool private_broadcast{DEFAULT_PRIVATE_BROADCAST};
     };
 
     static std::unique_ptr<PeerManager> make(CConnman& connman, AddrMan& addrman,
diff --git a/src/node/peerman_args.cpp b/src/node/peerman_args.cpp
index 59dc592d..9745d69d 100644
--- a/src/node/peerman_args.cpp
+++ b/src/node/peerman_args.cpp
@@ -23,6 +23,8 @@ void ApplyArgsManOptions(const ArgsManager& argsman, PeerManager::Options& optio
     if (auto value{argsman.GetBoolArg("-capturemessages")}) options.capture_messages = *value;
 
     if (auto value{argsman.GetBoolArg("-blocksonly")}) options.ignore_incoming_txs = *value;
+
+    if (auto value{argsman.GetBoolArg("-privatebroadcast")}) options.private_broadcast = *value;
 }
 
 } // namespace node
diff --git a/src/private_broadcast.cpp b/src/private_broadcast.cpp
index 395b8dd0..c7c311c0 100644
--- a/src/private_broadcast.cpp
+++ b/src/private_broadcast.cpp
@@ -7,6 +7,10 @@
 
 #include <algorithm>
 
+/// If a transaction is not received back from the network for this duration
+/// after it is broadcast, then we consider it stale / for rebroadcasting.
+static constexpr auto STALE_DURATION{1min};
+
 bool PrivateBroadcast::Add(const CTransactionRef& tx)
     EXCLUSIVE_LOCKS_REQUIRED(!m_mutex)
 {
@@ -85,6 +89,21 @@ bool PrivateBroadcast::HavePendingTransactions()
     return !m_transactions.empty();
 }
 
+std::vector<CTransactionRef> PrivateBroadcast::GetStale() const
+    EXCLUSIVE_LOCKS_REQUIRED(!m_mutex)
+{
+    LOCK(m_mutex);
+    const auto stale_time{NodeClock::now() - STALE_DURATION};
+    std::vector<CTransactionRef> stale;
+    for (const auto& [tx, send_status] : m_transactions) {
+        const Priority p{DerivePriority(send_status)};
+        if (p.last_confirmed < stale_time) {
+            stale.push_back(tx);
+        }
+    }
+    return stale;
+}
+
 PrivateBroadcast::Priority PrivateBroadcast::DerivePriority(const std::vector<SendStatus>& sent_to)
 {
     Priority p;
diff --git a/src/private_broadcast.h b/src/private_broadcast.h
index 3a1c6e29..e88db6bb 100644
--- a/src/private_broadcast.h
+++ b/src/private_broadcast.h
@@ -89,6 +89,12 @@ public:
     bool HavePendingTransactions()
         EXCLUSIVE_LOCKS_REQUIRED(!m_mutex);
 
+    /**
+     * Get the transactions that have not been broadcast recently.
+     */
+    std::vector<CTransactionRef> GetStale() const
+        EXCLUSIVE_LOCKS_REQUIRED(!m_mutex);
+
 private:
     /// Status of a transaction sent to a given node.
     struct SendStatus {