net: move CConnman-specific parts away from ThreadI2PAcceptIncoming()

CConnman-specific or in other words, Bitcoin P2P specific. Now
the `ThreadI2PAcceptIncoming()` method is protocol agnostic and
can be moved to `SockMan`.

src/common/sockman.cpp

@@ -125,3 +125,5 @@ void SockMan::StopListening()
 {
     m_listen.clear();
 }
+
+void SockMan::EventI2PStatus(const CService&, I2PStatus) {}

src/common/sockman.h

@@ -28,6 +28,23 @@ public:
      */
     using Id = int64_t;
 
+    /**
+     * Possible status changes that can be passed to `EventI2PStatus()`.
+     */
+    enum class I2PStatus : uint8_t {
+        /// The listen succeeded and we are now listening for incoming I2P connections.
+        START_LISTENING,
+
+        /// The listen failed and now we are not listening (even if START_LISTENING was signaled before).
+        STOP_LISTENING,
+    };
+
+    virtual ~SockMan() = default;
+
+    //
+    // Non-virtual functions, to be reused by children classes.
+    //
+
     /**
      * Bind to a new address:port, start listening and add the listen socket to `m_listen`.
      * @param[in] to Where to bind.
@@ -62,6 +79,23 @@ public:
 
 private:
 
+    //
+    // Pure virtual functions must be implemented by children classes.
+    //
+
+    //
+    // Non-pure virtual functions can be overridden by children classes or left
+    // alone to use the default implementation from SockMan.
+    //
+
+    /**
+     * Be notified of a change in the state of the I2P connectivity.
+     * The default behavior, implemented by `SockMan`, is to ignore this event.
+     * @param[in] addr The address we started or stopped listening on.
+     * @param[in] new_status New status.
+     */
+    virtual void EventI2PStatus(const CService& addr, I2PStatus new_status);
+
     /**
      * The id to assign to the next created connection. Used to generate ids of connections.
      */

src/net.cpp

@@ -3054,13 +3054,30 @@ void CConnman::ThreadMessageHandler()
     }
 }
 
+void CConnman::EventI2PStatus(const CService& addr, SockMan::I2PStatus new_status)
+{
+    switch (new_status) {
+    case SockMan::I2PStatus::START_LISTENING:
+        if (!m_i2p_advertising_listen_addr) {
+            AddLocal(addr, LOCAL_MANUAL);
+            m_i2p_advertising_listen_addr = true;
+        }
+        break;
+    case SockMan::I2PStatus::STOP_LISTENING:
+        if (m_i2p_advertising_listen_addr && addr.IsValid()) {
+            RemoveLocal(addr);
+            m_i2p_advertising_listen_addr = false;
+        }
+        break;
+    }
+}
+
 void CConnman::ThreadI2PAcceptIncoming()
 {
     static constexpr auto err_wait_begin = 1s;
     static constexpr auto err_wait_cap = 5min;
     auto err_wait = err_wait_begin;
 
-    bool advertising_listen_addr = false;
     i2p::Connection conn;
 
     auto SleepOnFailure = [&]() {
@@ -3073,18 +3090,12 @@ void CConnman::ThreadI2PAcceptIncoming()
     while (!interruptNet) {
 
         if (!m_i2p_sam_session->Listen(conn)) {
-            if (advertising_listen_addr && conn.me.IsValid()) {
-                RemoveLocal(conn.me);
-                advertising_listen_addr = false;
-            }
+            EventI2PStatus(conn.me, SockMan::I2PStatus::STOP_LISTENING);
             SleepOnFailure();
             continue;
         }
 
-        if (!advertising_listen_addr) {
-            AddLocal(conn.me, LOCAL_MANUAL);
-            advertising_listen_addr = true;
-        }
+        EventI2PStatus(conn.me, SockMan::I2PStatus::START_LISTENING);
 
         if (!m_i2p_sam_session->Accept(conn)) {
             SleepOnFailure();

src/net.h

@@ -1286,6 +1286,12 @@ private:
     void ProcessAddrFetch() EXCLUSIVE_LOCKS_REQUIRED(!m_addr_fetches_mutex, !m_unused_i2p_sessions_mutex);
     void ThreadOpenConnections(std::vector<std::string> connect, Span<const std::string> seed_nodes) EXCLUSIVE_LOCKS_REQUIRED(!m_addr_fetches_mutex, !m_added_nodes_mutex, !m_nodes_mutex, !m_unused_i2p_sessions_mutex, !m_reconnections_mutex);
     void ThreadMessageHandler() EXCLUSIVE_LOCKS_REQUIRED(!mutexMsgProc);
+
+    /// Whether we are currently advertising our I2P address (via `AddLocal()`).
+    bool m_i2p_advertising_listen_addr{false};
+
+    virtual void EventI2PStatus(const CService& addr, SockMan::I2PStatus new_status) override;
+
     void ThreadI2PAcceptIncoming();
 
     /**