1 files changed, 51 insertions, 8 deletions

diff --git a/src/net.cpp b/src/net.cpp
index 4cbc43e4d8..eb2c38dd10 100644
--- a/src/net.cpp
+++ b/src/net.cpp
@@ -42,6 +42,9 @@
 // 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
@@ -60,6 +63,7 @@
 
 namespace {
     const int MAX_OUTBOUND_CONNECTIONS = 8;
+    const int MAX_FEELER_CONNECTIONS = 1;
 
     struct ListenSocket {
         SOCKET socket;
@@ -1016,7 +1020,8 @@ 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))
@@ -1609,6 +1614,9 @@ 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();
@@ -1646,13 +1654,36 @@ 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))
@@ -1688,8 +1719,17 @@ void ThreadOpenConnections()
             break;
         }
 
-        if (addrConnect.IsValid())
-            OpenNetworkConnection(addrConnect, (int)setConnected.size() >= std::min(nMaxConnections - 1, 2), &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, (int)setConnected.size() >= std::min(nMaxConnections - 1, 2), &grant, NULL, false, fFeeler);
+        }
     }
 }
 
@@ -1771,7 +1811,7 @@ void ThreadOpenAddedConnections()
 }
 
 // if successful, this moves the passed grant to the constructed node
-bool OpenNetworkConnection(const CAddress& addrConnect, bool fCountFailure, CSemaphoreGrant *grantOutbound, const char *pszDest, bool fOneShot)
+bool OpenNetworkConnection(const CAddress& addrConnect, bool fCountFailure, CSemaphoreGrant *grantOutbound, const char *pszDest, bool fOneShot, bool fFeeler)
 {
     //
     // Initiate outbound network connection
@@ -1795,6 +1835,8 @@ bool OpenNetworkConnection(const CAddress& addrConnect, bool fCountFailure, CSem
     pnode->fNetworkNode = true;
     if (fOneShot)
         pnode->fOneShot = true;
+    if (fFeeler)
+        pnode->fFeeler = true;
 
     return true;
 }
@@ -2054,7 +2096,7 @@ 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);
     }
 
@@ -2096,7 +2138,7 @@ 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 (fAddressesInitialized)
@@ -2437,6 +2479,7 @@ 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 = false;
     fSuccessfullyConnected = false;