Backport Bitcoin PR #8282: net: Feeler connections to increase online addrs in the tried table.

src/main.cpp

@@ -5284,6 +5284,12 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
 
     if (strCommand == NetMsgType::VERSION)
     {
+        // Feeler connections exist only to verify if address is online.
+        if (pfrom->fFeeler) {
+            assert(pfrom->fInbound == false);
+            pfrom->fDisconnect = true;
+        }
+
         // Each connection can only send one version message
         if (pfrom->nVersion != 0)
         {

src/net.cpp

@@ -48,6 +48,9 @@
 // Dump addresses to peers.dat and banlist.dat every 15 minutes (900s)
 #define DUMP_ADDRESSES_INTERVAL 900
 
+// We add a random period time (0 to 1 seconds) to feeler connections to prevent synchronization.
+#define FEELER_SLEEP_WINDOW 1
+
 #if !defined(HAVE_MSG_NOSIGNAL) && !defined(MSG_NOSIGNAL)
 #define MSG_NOSIGNAL 0
 #endif
@@ -67,6 +70,7 @@
 namespace {
     const int MAX_OUTBOUND_CONNECTIONS = 8;
     const int MAX_OUTBOUND_MASTERNODE_CONNECTIONS = 20;
+    const int MAX_FEELER_CONNECTIONS = 1;
 
     struct ListenSocket {
         SOCKET socket;
@@ -1053,7 +1057,8 @@ static void AcceptConnection(const ListenSocket& hListenSocket) {
     SOCKET hSocket = accept(hListenSocket.socket, (struct sockaddr*)&sockaddr, &len);
     CAddress addr;
     int nInbound = 0;
-    int nMaxInbound = nMaxConnections - MAX_OUTBOUND_CONNECTIONS;
+    int nMaxInbound = nMaxConnections - (MAX_OUTBOUND_CONNECTIONS + MAX_FEELER_CONNECTIONS);
+    assert(nMaxInbound > 0);
 
     if (hSocket != INVALID_SOCKET)
         if (!addr.SetSockAddr((const struct sockaddr*)&sockaddr))
@@ -1627,6 +1632,9 @@ void ThreadOpenConnections()
 
     // Initiate network connections
     int64_t nStart = GetTime();
+
+    // Minimum time before next feeler connection (in microseconds).
+    int64_t nNextFeeler = PoissonNextSend(nStart*1000*1000, FEELER_INTERVAL);
     while (true)
     {
         ProcessOneShot();
@@ -1664,13 +1672,36 @@ void ThreadOpenConnections()
                 }
             }
         }
+        assert(nOutbound <= (MAX_OUTBOUND_CONNECTIONS + MAX_FEELER_CONNECTIONS));
 
-        int64_t nANow = GetAdjustedTime();
+        // Feeler Connections
+        //
+        // Design goals:
+        //  * Increase the number of connectable addresses in the tried table.
+        //
+        // Method:
+        //  * Choose a random address from new and attempt to connect to it if we can connect 
+        //    successfully it is added to tried.
+        //  * Start attempting feeler connections only after node finishes making outbound 
+        //    connections.
+        //  * Only make a feeler connection once every few minutes.
+        //
+        bool fFeeler = false;
+        if (nOutbound >= MAX_OUTBOUND_CONNECTIONS) {
+            int64_t nTime = GetTimeMicros(); // The current time right now (in microseconds).
+            if (nTime > nNextFeeler) {
+                nNextFeeler = PoissonNextSend(nTime, FEELER_INTERVAL);
+                fFeeler = true;
+            } else {
+                continue;
+            }
+        }
 
+        int64_t nANow = GetAdjustedTime();
         int nTries = 0;
         while (true)
         {
-            CAddrInfo addr = addrman.Select();
+            CAddrInfo addr = addrman.Select(fFeeler);
 
             // if we selected an invalid address, restart
             if (!addr.IsValid() || setConnected.count(addr.GetGroup()) || IsLocal(addr))
@@ -1702,8 +1733,17 @@ void ThreadOpenConnections()
             break;
         }
 
-        if (addrConnect.IsValid())
-            OpenNetworkConnection(addrConnect, &grant);
+        if (addrConnect.IsValid()) {
+
+            if (fFeeler) {
+                // Add small amount of random noise before connection to avoid synchronization.
+                int randsleep = GetRandInt(FEELER_SLEEP_WINDOW * 1000);
+                MilliSleep(randsleep);
+                LogPrint("net", "Making feeler connection to %s\n", addrConnect.ToString());
+            }
+
+            OpenNetworkConnection(addrConnect, &grant, NULL, false, fFeeler);
+        }
     }
 }
 
@@ -1829,7 +1869,7 @@ void ThreadMnbRequestConnections()
 }
 
 // if successful, this moves the passed grant to the constructed node
-bool OpenNetworkConnection(const CAddress& addrConnect, CSemaphoreGrant *grantOutbound, const char *pszDest, bool fOneShot)
+bool OpenNetworkConnection(const CAddress& addrConnect, CSemaphoreGrant *grantOutbound, const char *pszDest, bool fOneShot, bool fFeeler)
 {
     //
     // Initiate outbound network connection
@@ -1852,6 +1892,8 @@ bool OpenNetworkConnection(const CAddress& addrConnect, CSemaphoreGrant *grantOu
         grantOutbound->MoveTo(pnode->grantOutbound);
     if (fOneShot)
         pnode->fOneShot = true;
+    if (fFeeler)
+        pnode->fFeeler = true;
 
     return true;
 }
@@ -2101,7 +2143,7 @@ void StartNode(boost::thread_group& threadGroup, CScheduler& scheduler)
 
     if (semOutbound == NULL) {
         // initialize semaphore
-        int nMaxOutbound = std::min(MAX_OUTBOUND_CONNECTIONS, nMaxConnections);
+        int nMaxOutbound = std::min((MAX_OUTBOUND_CONNECTIONS + MAX_FEELER_CONNECTIONS), nMaxConnections);
         semOutbound = new CSemaphore(nMaxOutbound);
     }
 
@@ -2151,7 +2193,7 @@ bool StopNode()
     LogPrintf("StopNode()\n");
     MapPort(false);
     if (semOutbound)
-        for (int i=0; i<MAX_OUTBOUND_CONNECTIONS; i++)
+        for (int i=0; i<(MAX_OUTBOUND_CONNECTIONS + MAX_FEELER_CONNECTIONS); i++)
             semOutbound->post();
 
     if (semMasternodeOutbound)
@@ -2554,6 +2596,7 @@ CNode::CNode(SOCKET hSocketIn, const CAddress& addrIn, const std::string& addrNa
     fWhitelisted = false;
     fOneShot = false;
     fClient = false; // set by version message
+    fFeeler = false;
     fInbound = fInboundIn;
     fNetworkNode = fNetworkNodeIn;
     fSuccessfullyConnected = false;

src/net.h

@@ -43,6 +43,8 @@ static const int PING_INTERVAL = 2 * 60;
 static const int TIMEOUT_INTERVAL = 20 * 60;
 /** Minimum time between warnings printed to log. */
 static const int WARNING_INTERVAL = 10 * 60;
+/** Run the feeler connection loop once every 2 minutes or 120 seconds. **/
+static const int FEELER_INTERVAL = 120;
 /** The maximum number of entries in an 'inv' protocol message */
 static const unsigned int MAX_INV_SZ = 50000;
 /** The maximum number of new addresses to accumulate before announcing. */
@@ -91,7 +93,7 @@ CNode* FindNode(const CService& ip);
 // fConnectToMasternode should be 'true' only if you want this node to allow to connect to itself
 // and/or you want it to be disconnected on CMasternodeMan::ProcessMasternodeConnections()
 CNode* ConnectNode(CAddress addrConnect, const char *pszDest = NULL, bool fConnectToMasternode = false);
-bool OpenNetworkConnection(const CAddress& addrConnect, CSemaphoreGrant *grantOutbound = NULL, const char *strDest = NULL, bool fOneShot = false);
+bool OpenNetworkConnection(const CAddress& addrConnect, CSemaphoreGrant *grantOutbound = NULL, const char *strDest = NULL, bool fOneShot = false, bool fFeeler = false);
 void MapPort(bool fUseUPnP);
 unsigned short GetListenPort();
 bool BindListenPort(const CService &bindAddr, std::string& strError, bool fWhitelisted = false);
@@ -357,6 +359,7 @@ class CNode
     // the network or wire types and the cleaned string used when displayed or logged.
     std::string strSubVer, cleanSubVer;
     bool fWhitelisted; // This peer can bypass DoS banning.
+    bool fFeeler; // If true this node is being used as a short lived feeler.
     bool fOneShot;
     bool fClient;
     bool fInbound;

src/test/net_tests.cpp

@@ -142,4 +142,26 @@ BOOST_AUTO_TEST_CASE(caddrdb_read_corrupted)
     BOOST_CHECK(addrman2.size() == 0);
 }
 
+BOOST_AUTO_TEST_CASE(cnode_simple_test)
+{
+    SOCKET hSocket = INVALID_SOCKET;
+
+    in_addr ipv4Addr;
+    ipv4Addr.s_addr = 0xa0b0c001;
+
+    CAddress addr = CAddress(CService(ipv4Addr, 7777), NODE_NETWORK);
+    std::string pszDest = "";
+    bool fInboundIn = false;
+
+    // Test that fFeeler is false by default.
+    CNode* pnode1 = new CNode(hSocket, addr, pszDest, fInboundIn);
+    BOOST_CHECK(pnode1->fInbound == false);
+    BOOST_CHECK(pnode1->fFeeler == false);
+
+    fInboundIn = true;
+    CNode* pnode2 = new CNode(hSocket, addr, pszDest, fInboundIn);
+    BOOST_CHECK(pnode2->fInbound == true);
+    BOOST_CHECK(pnode2->fFeeler == false);
+}
+
 BOOST_AUTO_TEST_SUITE_END()