p2p: first addr self-announcement in separate msg

This makes sure the initial address self-announcement a node sends to
a peer happends in a separate P2P message. This has benefits for both
inbound and outbound connections:

For inbound connections from a peer to us, previously, we might send
the self-announcement along with our response to a GETADDR request.
However, the self-announcement might replace an address from the
GETADDR response. This isn't clean.

For outbound connections from us to a peer, previously, it could have
happend that we send the self-announcement along with other addresses.
Since shortly after connection open, the peer might only have one
rate-limiting token for us, and the addresses are shuffeld on arrival,
it's possible that the self-announcement gets rate-limited. However,
note that these rate-limitings seem to be rare in practice.

This is inspired by and based on https://github.com/bitcoin/bitcoin/pull/33699#issuecomment-3462287763

Co-Authored-By: Anthony Towns <aj@erisian.com.au>

src/net_processing.cpp

@@ -5328,8 +5328,21 @@ void PeerManagerImpl::MaybeSendAddr(CNode& node, Peer& peer, std::chrono::micros
         }
         if (std::optional<CService> local_service = GetLocalAddrForPeer(node)) {
             CAddress local_addr{*local_service, peer.m_our_services, Now<NodeSeconds>()};
+            if (peer.m_next_local_addr_send == 0us) {
+                // Send the initial self-announcement in its own message. This makes sure
+                // rate-limiting with limited start-tokens doesn't ignore it if the first
+                // message ends up containing multiple addresses.
+                std::vector<CAddress> self_announcement {local_addr};
+                if (peer.m_wants_addrv2) {
+                    MakeAndPushMessage(node, NetMsgType::ADDRV2, CAddress::V2_NETWORK(self_announcement));
+                } else {
+                    MakeAndPushMessage(node, NetMsgType::ADDR, CAddress::V1_NETWORK(self_announcement));
+                }
+            } else {
+                // All later self-announcements are sent together with the other addresses.
                 PushAddress(peer, local_addr);
             }
+        }
         peer.m_next_local_addr_send = current_time + m_rng.rand_exp_duration(AVG_LOCAL_ADDRESS_BROADCAST_INTERVAL);
     }
 

test/functional/p2p_addr_selfannouncement.py

@@ -5,6 +5,9 @@
 """
 Test that a node sends a self-announcement with its external IP to
 in- and outbound peers after connection open and again sometime later.
+
+Additionally, this checks that the first self-announcement arrives
+in its own message and that this message is the first we receive.
 """
 
 import time
@@ -42,6 +45,13 @@ class SelfAnnouncementReceiver(P2PInterface):
             self.addresses_received += 1
             if addr == self.expected:
                 self.self_announcements_received += 1
+                if self.self_announcements_received == 1:
+                    # If it's the first self-announcement, check that it is
+                    # in the first addr message we receive, and that this message
+                    # only contains one address. This also implies that it is
+                    # the first address we receive.
+                    assert_equal(self.addr_messages_received, 1)
+                    assert_equal(len(message.addrs), 1)
 
     def on_addrv2(self, message):
         assert (self.addrv2_test)
@@ -69,10 +79,10 @@ class AddrSelfAnnouncementTest(BitcoinTestFramework):
         self.self_announcement_test(outbound=True, addrv2=True)
 
     def inbound_connection_open_assertions(self, addr_receiver):
-        # We expect one self-announcement and multiple other addresses in
+        # In response to a GETADDR, we expect a message with the self-announcement
-        # response to a GETADDR in a single addr / addrv2 message.
+        # and an addr message containing the GETADDR response.
         assert_equal(addr_receiver.self_announcements_received, 1)
-        assert_equal(addr_receiver.addr_messages_received, 1)
+        assert_equal(addr_receiver.addr_messages_received, 2)
         assert_greater_than(addr_receiver.addresses_received, 1)
 
     def outbound_connection_open_assertions(self, addr_receiver):