Merge bitcoin/bitcoin#27577: p2p: give seednodes time before falling back to fixed seeds

3077812 net: Give seednodes time before falling back to fixed seeds (Martin Zumsande)

Pull request description:

  `-seednode` is an alternative bootstrap mechanism - when choosing it, we make a `AddrFetch` connection to the specified peer, gather addresses from them, and then disconnect. Presumably, if users specify a seednode they prefer addresses from that node over fixed seeds.

  However, when disabling dns seeds and specifiying `-seednode`, `CConnman::ProcessAddrFetch()`  immediately removes the entry from `m_addr_fetches` (before the seednode could give us addresses) - and once `m_addr_fetches`  is empty, `ThreadOpenConnections` will add fixed seeds, resulting in a "race" between the fixed seeds and seednodes filling up AddrMan.

  This PR suggests to check for any provided `-seednode` arg instead of using the size of `m_addr_fetches`, thus delaying the querying of fixed seeds for 1 minute when specifying any seednode (as we already do for `addnode` peers).
  That way, we actually give the seednodes a chance for  to provide us with addresses before falling back to fixed seeds.

  This can be tested with `bitcoind -debug=net -dnsseed=0 -seednode=(...)` on a node without `peers.dat` and observing the debug log.

ACKs for top commit:
  ajtowns:
    utACK 3077812
  achow101:
    ACK 3077812
  dergoegge:
    Code review ACK 3077812
  sr-gi:
    ACK [3077812](bitcoin/bitcoin@3077812) with a tiny nit, feel free to ignore it

Tree-SHA512: 96446eb34c0805f10ee158a00a3001a07029e795ac40ad5638228d426e30e9bb836c64ac05d145f2f9ab23ec5a528f3a416e3d52ecfdfb0b813bd4b1ebab3c01

Author: achow101

src/net.cpp

@@ -1637,6 +1637,7 @@ void CConnman::ThreadOpenConnections(const std::vector<std::string> connect)
     auto next_extra_block_relay = GetExponentialRand(start, EXTRA_BLOCK_RELAY_ONLY_PEER_INTERVAL);
     const bool dnsseed = gArgs.GetBoolArg("-dnsseed", DEFAULT_DNSSEED);
     bool add_fixed_seeds = gArgs.GetBoolArg("-fixedseeds", DEFAULT_FIXEDSEEDS);
+    const bool use_seednodes{gArgs.IsArgSet("-seednode")};
 
     if (!add_fixed_seeds) {
         LogPrintf("Fixed seeds are disabled\n");
@@ -1666,12 +1667,12 @@ void CConnman::ThreadOpenConnections(const std::vector<std::string> connect)
                 LogPrintf("Adding fixed seeds as 60 seconds have passed and addrman is empty for at least one reachable network\n");
             }
 
-            // Checking !dnsseed is cheaper before locking 2 mutexes.
-            if (!add_fixed_seeds_now && !dnsseed) {
-                LOCK2(m_addr_fetches_mutex, m_added_nodes_mutex);
-                if (m_addr_fetches.empty() && m_added_nodes.empty()) {
+            // Perform cheap checks before locking a mutex.
+            else if (!dnsseed && !use_seednodes) {
+                LOCK(m_added_nodes_mutex);
+                if (m_added_nodes.empty()) {
                     add_fixed_seeds_now = true;
-                    LogPrintf("Adding fixed seeds as -dnsseed=0 (or IPv4/IPv6 connections are disabled via -onlynet), -addnode is not provided and all -seednode(s) attempted\n");
+                    LogPrintf("Adding fixed seeds as -dnsseed=0 (or IPv4/IPv6 connections are disabled via -onlynet) and neither -addnode nor -seednode are provided\n");
                 }
             }
 

test/functional/feature_config_args.py

@@ -253,7 +253,7 @@ def test_seed_peers(self):
         with self.nodes[0].assert_debug_log(expected_msgs=[
                 "Loaded 0 addresses from peers.dat",
                 "DNS seeding disabled",
-                "Adding fixed seeds as -dnsseed=0 (or IPv4/IPv6 connections are disabled via -onlynet), -addnode is not provided and all -seednode(s) attempted\n",
+                "Adding fixed seeds as -dnsseed=0 (or IPv4/IPv6 connections are disabled via -onlynet) and neither -addnode nor -seednode are provided\n",
         ]):
             self.start_node(0, extra_args=['-dnsseed=0', '-fixedseeds=1'])
         assert time.time() - start < 60