From 6255b2a907c42ad00067e9e0338e5f2e1808b264 Mon Sep 17 00:00:00 2001
From: catbref <misc-github@talk2dom.com>
Date: Sat, 19 Mar 2022 13:16:32 +0000
Subject: [PATCH] Networking work-in-progress:

When node has reached max connections, Network will ignore pending incoming connections by:
1. not calling accept()
2. de-registering OP_ACCEPT 'interest op' on the listen socket's channel

When a peer disconnects, Network might re-register OP_ACCEPT interest op on listen socket.
---
 src/main/java/org/qortal/network/Network.java | 34 ++++++++++++++-----
 1 file changed, 25 insertions(+), 9 deletions(-)

diff --git a/src/main/java/org/qortal/network/Network.java b/src/main/java/org/qortal/network/Network.java
index 20aa10d1..44960ccc 100644
--- a/src/main/java/org/qortal/network/Network.java
+++ b/src/main/java/org/qortal/network/Network.java
@@ -43,7 +43,7 @@ public class Network {
     private static final Logger LOGGER = LogManager.getLogger(Network.class);
     private static Network instance;
 
-    private static final int LISTEN_BACKLOG = 10;
+    private static final int LISTEN_BACKLOG = 5;
     /**
      * How long before retrying after a connection failure, in milliseconds.
      */
@@ -122,6 +122,7 @@ public class Network {
     private final ExecuteProduceConsume networkEPC;
     private Selector channelSelector;
     private ServerSocketChannel serverChannel;
+    private SelectionKey serverSelectionKey;
     private Iterator<SelectionKey> channelIterator = null;
 
     // volatile because value is updated inside any one of the EPC threads
@@ -170,7 +171,7 @@ public class Network {
             serverChannel.configureBlocking(false);
             serverChannel.setOption(StandardSocketOptions.SO_REUSEADDR, true);
             serverChannel.bind(endpoint, LISTEN_BACKLOG);
-            serverChannel.register(channelSelector, SelectionKey.OP_ACCEPT);
+            serverSelectionKey = serverChannel.register(channelSelector, SelectionKey.OP_ACCEPT);
         } catch (UnknownHostException e) {
             LOGGER.error("Can't bind listen socket to address {}", Settings.getInstance().getBindAddress());
             throw new IOException("Can't bind listen socket to address", e);
@@ -657,6 +658,15 @@ public class Network {
         SocketChannel socketChannel;
 
         try {
+            if (getImmutableConnectedPeers().size() >= maxPeers) {
+                // We have enough peers
+                if (serverSelectionKey.interestOps() != 0) {
+                    LOGGER.debug("Ignoring pending incoming connections because the server is full");
+                    serverSelectionKey.interestOps(0);
+                }
+                return;
+            }
+
             socketChannel = serverSocketChannel.accept();
         } catch (IOException e) {
             return;
@@ -688,13 +698,6 @@ public class Network {
                 return;
             }
 
-            if (getImmutableConnectedPeers().size() >= maxPeers) {
-                // We have enough peers
-                LOGGER.debug("Connection discarded from peer {} because the server is full", address);
-                socketChannel.close();
-                return;
-            }
-
             LOGGER.debug("Connection accepted from peer {}", address);
 
             newPeer = new Peer(socketChannel, channelSelector);
@@ -783,6 +786,10 @@ public class Network {
     }
 
     private boolean connectPeer(Peer newPeer) throws InterruptedException {
+        // NOT CORRECT:
+        if (getImmutableConnectedPeers().size() >= minOutboundPeers)
+            return false;
+
         SocketChannel socketChannel = newPeer.connect(this.channelSelector);
         if (socketChannel == null) {
             return false;
@@ -866,6 +873,15 @@ public class Network {
         }
 
         this.removeConnectedPeer(peer);
+
+        if (getImmutableConnectedPeers().size() < maxPeers - 1 && (serverSelectionKey.interestOps() & SelectionKey.OP_ACCEPT) == 0) {
+            try {
+                LOGGER.debug("Re-enabling accepting incoming connections because the server is not longer full");
+                serverSelectionKey.interestOps(SelectionKey.OP_ACCEPT);
+            } catch (CancelledKeyException e) {
+                LOGGER.error("Failed to re-enable accepting of incoming connections: {}", e.getMessage());
+            }
+        }
     }
 
     public void peerMisbehaved(Peer peer) {