p2p: fix fallback seed node usage

Those were added to the seed nodes list even when they had already
been added. Moreover, the current index was not reset after they
were added, typically causing previous seeds to be used, and some
of those fallback seeds to not be tried.

src/p2p/net_node.h

@@ -316,6 +316,7 @@ namespace nodetool
     std::list<epee::net_utils::network_address>   m_priority_peers;
     std::vector<epee::net_utils::network_address> m_exclusive_peers;
     std::vector<epee::net_utils::network_address> m_seed_nodes;
+    bool m_fallback_seed_nodes_added;
     std::list<nodetool::peerlist_entry> m_command_line_peers;
     uint64_t m_peer_livetime;
     //keep connections to initiate some interactions

src/p2p/net_node.inl

@@ -405,6 +405,7 @@ namespace nodetool
     bool res = handle_command_line(vm);
     CHECK_AND_ASSERT_MES(res, false, "Failed to handle command line");
 
+    m_fallback_seed_nodes_added = false;
     if (m_nettype == cryptonote::TESTNET)
     {
       memcpy(&m_network_id, &::config::testnet::NETWORK_ID, 16);
@@ -498,6 +499,7 @@ namespace nodetool
 
         for (const auto &peer: get_seed_nodes(cryptonote::MAINNET))
           full_addrs.insert(peer);
+        m_fallback_seed_nodes_added = true;
       }
     }
     }
@@ -1134,7 +1136,6 @@ namespace nodetool
 
       size_t try_count = 0;
       size_t current_index = crypto::rand<size_t>()%m_seed_nodes.size();
-      bool fallback_nodes_added = false;
       while(true)
       {
         if(m_net_server.is_stop_signal_sent())
@@ -1144,15 +1145,21 @@ namespace nodetool
           break;
         if(++try_count > m_seed_nodes.size())
         {
-          if (!fallback_nodes_added)
+          if (!m_fallback_seed_nodes_added)
           {
             MWARNING("Failed to connect to any of seed peers, trying fallback seeds");
+            current_index = m_seed_nodes.size();
             for (const auto &peer: get_seed_nodes(m_nettype))
             {
               MDEBUG("Fallback seed node: " << peer);
               append_net_address(m_seed_nodes, peer);
             }
-            fallback_nodes_added = true;
+            m_fallback_seed_nodes_added = true;
+            if (current_index == m_seed_nodes.size())
+            {
+              MWARNING("No fallback seeds, continuing without seeds");
+              break;
+            }
             // continue for another few cycles
           }
           else