net: Load fixed seeds from reachable networks for which we don't have addresses

Previously, we'd only load fixed seeds if we'd not
know any addresses at all. This change makes it possible
to change -onlynet abruptly, e.g. from -onlynet=onion to
-onlynet=i2p and still find peers.

src/net.cpp

@@ -1575,6 +1575,19 @@ int CConnman::GetExtraBlockRelayCount() const
     return std::max(block_relay_peers - m_max_outbound_block_relay, 0);
 }
 
+std::unordered_set<Network> CConnman::GetReachableEmptyNetworks() const
+{
+    std::unordered_set<Network> networks{};
+    for (int n = 0; n < NET_MAX; n++) {
+        enum Network net = (enum Network)n;
+        if (net == NET_UNROUTABLE || net == NET_INTERNAL) continue;
+        if (IsReachable(net) && addrman.Size(net, std::nullopt) == 0) {
+            networks.insert(net);
+        }
+    }
+    return networks;
+}
+
 void CConnman::ThreadOpenConnections(const std::vector<std::string> connect)
 {
     SetSyscallSandboxPolicy(SyscallSandboxPolicy::NET_OPEN_CONNECTION);
@@ -1624,7 +1637,8 @@ void CConnman::ThreadOpenConnections(const std::vector<std::string> connect)
         if (interruptNet)
             return;
 
-        if (add_fixed_seeds && addrman.size() == 0) {
+        const std::unordered_set<Network> fixed_seed_networks{GetReachableEmptyNetworks()};
+        if (add_fixed_seeds && !fixed_seed_networks.empty()) {
             // When the node starts with an empty peers.dat, there are a few other sources of peers before
             // we fallback on to fixed seeds: -dnsseed, -seednode, -addnode
             // If none of those are available, we fallback on to fixed seeds immediately, else we allow
@@ -1633,7 +1647,7 @@ void CConnman::ThreadOpenConnections(const std::vector<std::string> connect)
             // It is cheapest to check if enough time has passed first.
             if (GetTime<std::chrono::seconds>() > start + std::chrono::minutes{1}) {
                 add_fixed_seeds_now = true;
-                LogPrintf("Adding fixed seeds as 60 seconds have passed and addrman is empty\n");
+                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.
@@ -1650,14 +1664,12 @@ void CConnman::ThreadOpenConnections(const std::vector<std::string> connect)
                 // We will not make outgoing connections to peers that are unreachable
                 // (e.g. because of -onlynet configuration).
                 // Therefore, we do not add them to addrman in the first place.
-                // Note that if you change -onlynet setting from one network to another,
-                // peers.dat will contain only peers of unreachable networks and
-                // manual intervention will be needed (either delete peers.dat after
-                // configuration change or manually add some reachable peer using addnode),
-                // see <https://github.com/bitcoin/bitcoin/issues/26035> for details.
+                // In case previously unreachable networks become reachable
+                // (e.g. in case of -onlynet changes by the user), fixed seeds will
+                // be loaded only for networks for which we have no addressses.
                 seed_addrs.erase(std::remove_if(seed_addrs.begin(), seed_addrs.end(),
-                                               [](const CAddress& addr) { return !IsReachable(addr); }),
-                                seed_addrs.end());
+                                                [&fixed_seed_networks](const CAddress& addr) { return fixed_seed_networks.count(addr.GetNetwork()) == 0; }),
+                                 seed_addrs.end());
                 CNetAddr local;
                 local.SetInternal("fixedseeds");
                 addrman.Add(seed_addrs, local);

src/net.h

@@ -39,6 +39,7 @@
 #include <memory>
 #include <optional>
 #include <thread>
+#include <unordered_set>
 #include <vector>
 
 class AddrMan;
@@ -969,6 +970,12 @@ private:
     void RecordBytesRecv(uint64_t bytes);
     void RecordBytesSent(uint64_t bytes) EXCLUSIVE_LOCKS_REQUIRED(!m_total_bytes_sent_mutex);
 
+    /**
+     Return reachable networks for which we have no addresses in addrman and therefore
+     may require loading fixed seeds.
+     */
+    std::unordered_set<Network> GetReachableEmptyNetworks() const;
+
     /**
      * Return vector of current BLOCK_RELAY peers.
      */