Implement missing writabilityChanged() and add backlog logging with BACKPRESSURE_LOG to all writabilityChanged() implementations. (#1745)

proxy/src/main/java/com/velocitypowered/proxy/connection/backend/ConfigSessionHandler.java

@@ -60,6 +60,7 @@ import com.velocitypowered.proxy.protocol.packet.config.TagsUpdatePacket;
 import com.velocitypowered.proxy.protocol.util.PluginMessageUtil;
 import io.netty.buffer.ByteBufUtil;
 import io.netty.buffer.Unpooled;
+import io.netty.channel.Channel;
 import java.io.IOException;
 import java.net.InetSocketAddress;
 import java.util.concurrent.CompletableFuture;
@@ -72,6 +73,9 @@ import org.apache.logging.log4j.Logger;
  * 1.20.2+ switching. Yes, some of this is exceptionally stupid.
  */
 public class ConfigSessionHandler implements MinecraftSessionHandler {
+  private static final boolean BACKPRESSURE_LOG =
+      Boolean.getBoolean("velocity.log-server-backpressure");
+
   private static final Logger logger = LogManager.getLogger(ConfigSessionHandler.class);
   private final VelocityServer server;
   private final VelocityServerConnection serverConn;
@@ -382,6 +386,22 @@ public class ConfigSessionHandler implements MinecraftSessionHandler {
     serverConn.getPlayer().getConnection().write(packet);
   }
 
+  @Override
+  public void writabilityChanged() {
+    Channel serverChan = serverConn.ensureConnected().getChannel();
+    boolean writable = serverChan.isWritable();
+
+    if (BACKPRESSURE_LOG) {
+      if (writable) {
+        logger.info("{} is writable, will auto-read player connection data", this.serverConn);
+      } else {
+        logger.info("{} is not writable, not auto-reading player connection data", this.serverConn);
+      }
+    }
+
+    serverConn.getPlayer().getConnection().setAutoReading(writable);
+  }
+
   private void switchFailure(Throwable cause) {
     logger.error("Unable to switch to new server {} for {}", serverConn.getServerInfo().getName(),
         serverConn.getPlayer().getUsername(), cause);

proxy/src/main/java/com/velocitypowered/proxy/connection/client/ClientConfigSessionHandler.java

@@ -60,6 +60,8 @@ import org.apache.logging.log4j.Logger;
  * Handles the client config stage.
  */
 public class ClientConfigSessionHandler implements MinecraftSessionHandler {
+  private static final boolean BACKPRESSURE_LOG =
+      Boolean.getBoolean("velocity.log-server-backpressure");
 
   private static final Logger logger = LogManager.getLogger(ClientConfigSessionHandler.class);
   private final VelocityServer server;
@@ -268,6 +270,33 @@ public class ClientConfigSessionHandler implements MinecraftSessionHandler {
     player.disconnect(Component.translatable("velocity.error.player-connection-error", NamedTextColor.RED));
   }
 
+  @Override
+  public void writabilityChanged() {
+    final boolean writable = player.getConnection().getChannel().isWritable();
+
+    if (BACKPRESSURE_LOG) {
+      if (writable) {
+        logger.info("{} is writable, will auto-read backend connection data", player);
+      } else {
+        logger.info("{} is not writable, not auto-reading backend connection data", player);
+      }
+    }
+
+    if (!writable) {
+      // Flush pending packets to free up memory. Schedule on a future event loop invocation
+      // to avoid disabling auto-read while the flush resolves backpressure.
+      player.getConnection().eventLoop().execute(() -> player.getConnection().flush());
+    }
+
+    final VelocityServerConnection serverConn = player.getConnectionInFlightOrConnectedServer();
+    if (serverConn != null) {
+      final MinecraftConnection smc = serverConn.getConnection();
+      if (smc != null) {
+        smc.setAutoReading(writable);
+      }
+    }
+  }
+
   /**
    * Calls the {@link PlayerConfigurationEvent}.
    * For 1.20.5+ backends this is done when the client responds to

proxy/src/main/java/com/velocitypowered/proxy/connection/client/ClientPlaySessionHandler.java

@@ -98,6 +98,8 @@ import org.checkerframework.checker.nullness.qual.Nullable;
  * center that joins backend servers with players.
  */
 public class ClientPlaySessionHandler implements MinecraftSessionHandler {
+  private static final boolean BACKPRESSURE_LOG =
+      Boolean.getBoolean("velocity.log-server-backpressure");
 
   private static final Logger logger = LogManager.getLogger(ClientPlaySessionHandler.class);
 
@@ -505,6 +507,14 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
   public void writabilityChanged() {
     boolean writable = player.getConnection().getChannel().isWritable();
 
+    if (BACKPRESSURE_LOG) {
+      if (writable) {
+        logger.info("{} is writable, will auto-read backend connection data", player);
+      } else {
+        logger.info("{} is not writable, not auto-reading backend connection data", player);
+      }
+    }
+
     if (!writable) {
       // We might have packets queued from the server, so flush them now to free up memory. Make
       // sure to do it on a future invocation of the event loop, otherwise while the issue will