net: fix race condition in self-connect detection

Initiating an outbound network connection currently involves the
following steps after the socket connection is established (see
 `CConnman::OpenNetworkConnection` method):
    1. set up node state
    2. queue VERSION message
    3. add new node to vector `m_nodes`

If we connect to ourself, it can happen that the sent VERSION message
(step 2) is received and processed locally *before* the node object
is added to the connection manager's `m_nodes` vector (step 3). In this
case, the self-connect remains undiscovered, as the detection doesn't
find the outbound peer in `m_nodes` yet (see `CConnman::CheckIncomingNonce`).

Fix this by swapping the order of 2. and 3., by taking the `PushNodeVersion`
call out of `InitializeNode` and doing that in the `SendMessages` method
instead, which is only called for `CNode` instances in `m_nodes`.

Thanks go to vasild, mzumsande, dergoegge and sipa for suggestions on
how to fix this.

src/net.h

@@ -991,7 +991,7 @@ class NetEventsInterface
     /** Mutex for anything that is only accessed via the msg processing thread */
     static Mutex g_msgproc_mutex;
 
-    /** Initialize a peer (setup state, queue any initial messages) */
+    /** Initialize a peer (setup state) */
     virtual void InitializeNode(CNode& node, ServiceFlags our_services) = 0;
 
     /** Handle removal of a peer (clear state) */

src/net_processing.cpp

@@ -243,6 +243,9 @@ struct Peer {
      * Most peers use headers-first syncing, which doesn't use this mechanism */
     uint256 m_continuation_block GUARDED_BY(m_block_inv_mutex) {};
 
+    /** Set to true once initial VERSION message was sent (only relevant for outbound peers). */
+    bool m_outbound_version_message_sent GUARDED_BY(NetEventsInterface::g_msgproc_mutex){false};
+
     /** This peer's reported block height when we connected */
     std::atomic<int> m_starting_height{-1};
 
@@ -1677,9 +1680,6 @@ void PeerManagerImpl::InitializeNode(CNode& node, ServiceFlags our_services)
         LOCK(m_peer_mutex);
         m_peer_map.emplace_hint(m_peer_map.end(), nodeid, peer);
     }
-    if (!node.IsInboundConn()) {
-        PushNodeVersion(node, *peer);
-    }
 }
 
 void PeerManagerImpl::ReattemptInitialBroadcast(CScheduler& scheduler)
@@ -5326,6 +5326,10 @@ bool PeerManagerImpl::ProcessMessages(CNode* pfrom, std::atomic<bool>& interrupt
     PeerRef peer = GetPeerRef(pfrom->GetId());
     if (peer == nullptr) return false;
 
+    // For outbound connections, ensure that the initial VERSION message
+    // has been sent first before processing any incoming messages
+    if (!pfrom->IsInboundConn() && !peer->m_outbound_version_message_sent) return false;
+
     {
         LOCK(peer->m_getdata_requests_mutex);
         if (!peer->m_getdata_requests.empty()) {
@@ -5817,6 +5821,12 @@ bool PeerManagerImpl::SendMessages(CNode* pto)
     // disconnect misbehaving peers even before the version handshake is complete.
     if (MaybeDiscourageAndDisconnect(*pto, *peer)) return true;
 
+    // Initiate version handshake for outbound connections
+    if (!pto->IsInboundConn() && !peer->m_outbound_version_message_sent) {
+        PushNodeVersion(*pto, *peer);
+        peer->m_outbound_version_message_sent = true;
+    }
+
     // Don't send anything until the version handshake is complete
     if (!pto->fSuccessfullyConnected || pto->fDisconnect)
         return true;

src/test/util/net.cpp

@@ -28,7 +28,8 @@ void ConnmanTestMsg::Handshake(CNode& node,
     auto& connman{*this};
 
     peerman.InitializeNode(node, local_services);
-    FlushSendBuffer(node); // Drop the version message added by InitializeNode.
+    peerman.SendMessages(&node);
+    FlushSendBuffer(node); // Drop the version message added by SendMessages.
 
     CSerializedNetMsg msg_version{
         NetMsg::Make(NetMsgType::VERSION,