revert serverside behavior of keepalives

This patch intends to bump up the time that a client has to reply to the
server back to 30 seconds as per pre 1.12.2, which allowed clients
more than enough time to reply potentially allowing them to be less
tempermental due to lag spikes on the network thread, e.g. that caused
by plugins that are interacting with netty.

We also add a system property to allow people to tweak how long the server
will wait for a reply. There is a compromise here between lower and higher
values, lower values will mean that dead connections can be closed sooner,
whereas higher values will make this less sensitive to issues such as spikes
from networking or during connections flood of chunk packets on slower clients,
 at the cost of dead connections being kept open for longer.

paper-server/patches/sources/net/minecraft/server/network/ServerCommonPacketListenerImpl.java.patch

@@ -14,7 +14,7 @@
  import net.minecraft.network.DisconnectionDetails;
  import net.minecraft.network.PacketSendListener;
  import net.minecraft.network.chat.Component;
-@@ -22,22 +24,56 @@
+@@ -22,39 +24,87 @@
  import net.minecraft.network.protocol.common.ServerboundPongPacket;
  import net.minecraft.network.protocol.common.ServerboundResourcePackPacket;
  import net.minecraft.network.protocol.cookie.ServerboundCookieResponsePacket;
@@ -71,13 +71,16 @@
 -    protected final Connection connection;
 +    public final Connection connection; // Paper
      private final boolean transferred;
-     private long keepAliveTime;
+-    private long keepAliveTime;
++    private long keepAliveTime = Util.getMillis(); // Paper
      private boolean keepAlivePending;
-@@ -46,15 +82,28 @@
+     private long keepAliveChallenge;
+     private long closedListenerTime;
      private boolean closed = false;
      private int latency;
      private volatile boolean suspendFlushingOnServerThread = false;
 +    public final java.util.Map<java.util.UUID, net.kyori.adventure.resource.ResourcePackCallback> packCallbacks = new java.util.concurrent.ConcurrentHashMap<>(); // Paper - adventure resource pack callbacks
++    private static final long KEEPALIVE_LIMIT = Long.getLong("paper.playerconnection.keepalive", 30) * 1000; // Paper - provide property to set keepalive limit
  
 -    public ServerCommonPacketListenerImpl(MinecraftServer server, Connection connection, CommonListenerCookie clientData) {
 -        this.server = server;
@@ -107,7 +110,7 @@
      private void close() {
          if (!this.closed) {
              this.closedListenerTime = Util.getMillis();
-@@ -80,13 +129,18 @@
+@@ -80,13 +130,18 @@
  
      @Override
      public void handleKeepAlive(ServerboundKeepAlivePacket packet) {
@@ -127,7 +130,7 @@
          }
  
      }
-@@ -94,9 +148,57 @@
+@@ -94,8 +149,56 @@
      @Override
      public void handlePong(ServerboundPongPacket packet) {}
  
@@ -144,7 +147,7 @@
 +        PacketUtils.ensureRunningOnSameThread(packet, this, this.player.serverLevel());
 +        ResourceLocation identifier = packet.payload().type().id();
 +        ByteBuf payload = ((DiscardedPayload)packet.payload()).data();
- 
++
 +        if (identifier.equals(ServerCommonPacketListenerImpl.CUSTOM_REGISTER)) {
 +            try {
 +                String channels = payload.toString(com.google.common.base.Charsets.UTF_8);
@@ -182,11 +185,10 @@
 +        return (!this.player.joining && !this.connection.isConnected()) || this.processedDisconnect; // Paper - Fix duplication bugs
 +    }
 +    // CraftBukkit end
-+
+ 
      @Override
      public void handleResourcePackResponse(ServerboundResourcePackPacket packet) {
-         PacketUtils.ensureRunningOnSameThread(packet, this, (BlockableEventLoop) this.server);
+@@ -104,28 +207,55 @@
-@@ -104,11 +206,34 @@
              ServerCommonPacketListenerImpl.LOGGER.info("Disconnecting {} due to resource pack {} rejection", this.playerProfile().getName(), packet.id());
              this.disconnect((Component) Component.translatable("multiplayer.requiredTexturePrompt.disconnect"));
          }
@@ -221,16 +223,34 @@
          this.disconnect(ServerCommonPacketListenerImpl.DISCONNECT_UNEXPECTED_QUERY);
      }
  
-@@ -116,7 +241,7 @@
+     protected void keepConnectionAlive() {
          Profiler.get().push("keepAlive");
-         long i = Util.getMillis();
+-        long i = Util.getMillis();
++        // Paper start - give clients a longer time to respond to pings as per pre 1.12.2 timings
++        // This should effectively place the keepalive handling back to "as it was" before 1.12.2
++        long currentTime = Util.getMillis();
++        long elapsedTime = currentTime - this.keepAliveTime;
  
 -        if (!this.isSingleplayerOwner() && i - this.keepAliveTime >= 15000L) {
-+        if (!this.isSingleplayerOwner() && i - this.keepAliveTime >= 25000L) { // CraftBukkit
+-            if (this.keepAlivePending) {
-             if (this.keepAlivePending) {
++        if (!this.isSingleplayerOwner() && elapsedTime >= 15000L) { // Paper - use vanilla's 15000L between keep alive packets
++            if (this.keepAlivePending && !this.processedDisconnect && elapsedTime >= KEEPALIVE_LIMIT) { // Paper - check keepalive limit, don't fire if already disconnected
                  this.disconnect(ServerCommonPacketListenerImpl.TIMEOUT_DISCONNECTION_MESSAGE);
-             } else if (this.checkIfClosed(i)) {
+-            } else if (this.checkIfClosed(i)) {
-@@ -156,6 +281,14 @@
++            } else if (this.checkIfClosed(currentTime)) { // Paper
+                 this.keepAlivePending = true;
+-                this.keepAliveTime = i;
+-                this.keepAliveChallenge = i;
++                this.keepAliveTime = currentTime;
++                this.keepAliveChallenge = currentTime;
+                 this.send(new ClientboundKeepAlivePacket(this.keepAliveChallenge));
+             }
+         }
++        // Paper end - give clients a longer time to respond to pings as per pre 1.12.2 timings
+ 
+         Profiler.get().pop();
+     }
+@@ -156,6 +286,14 @@
      }
  
      public void send(Packet<?> packet, @Nullable PacketSendListener callbacks) {
@@ -245,7 +265,7 @@
          if (packet.isTerminal()) {
              this.close();
          }
-@@ -175,20 +308,72 @@
+@@ -175,20 +313,72 @@
          }
      }