feature: Cancellable connection establish event

Author: kezz

api/src/main/java/com/velocitypowered/api/event/connection/ConnectionEstablishEvent.java

@@ -0,0 +1,46 @@
+/*
+ * Copyright (C) 2018-2021 Velocity Contributors
+ *
+ * The Velocity API is licensed under the terms of the MIT License. For more details,
+ * reference the LICENSE file in the api top-level directory.
+ */
+
+package com.velocitypowered.api.event.connection;
+
+import com.google.common.base.Preconditions;
+import com.velocitypowered.api.event.ResultedEvent;
+import com.velocitypowered.api.event.annotation.AwaitingEvent;
+import com.velocitypowered.api.proxy.InboundConnection;
+import org.jetbrains.annotations.NotNull;
+
+/**
+ * Event called when a connection is initially established with the proxy.
+ * Velocity will wait for this event to fire before continuing with the connection.
+ */
+@AwaitingEvent
+public class ConnectionEstablishEvent implements ResultedEvent<ResultedEvent.GenericResult> {
+  private final InboundConnection connection;
+  private GenericResult result = GenericResult.allowed();
+
+  public ConnectionEstablishEvent(final @NotNull InboundConnection connection) {
+    this.connection = Preconditions.checkNotNull(connection, "connection");
+  }
+
+  /**
+   * Returns the inbound connection that is being established.
+   * @return the connection
+   */
+  public @NotNull InboundConnection getConnection() {
+    return this.connection;
+  }
+
+  @Override
+  public GenericResult getResult() {
+    return this.result;
+  }
+
+  @Override
+  public void setResult(final @NotNull GenericResult result) {
+    this.result = Preconditions.checkNotNull(result, "result");
+  }
+}

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

@@ -19,6 +19,7 @@
 
 import com.google.common.annotations.VisibleForTesting;
 import com.google.common.base.Preconditions;
+import com.velocitypowered.api.event.connection.ConnectionEstablishEvent;
 import com.velocitypowered.api.event.connection.ConnectionHandshakeEvent;
 import com.velocitypowered.api.network.ProtocolVersion;
 import com.velocitypowered.proxy.VelocityServer;
@@ -85,6 +86,13 @@ public boolean handle(LegacyHandshake packet) {
   public boolean handle(Handshake handshake) {
     InitialInboundConnection ic = new InitialInboundConnection(connection,
         cleanVhost(handshake.getServerAddress()), handshake);
+
+    ConnectionEstablishEvent event = server.getEventManager().fire(new ConnectionEstablishEvent(ic)).join();
+    if (!event.getResult().isAllowed()) {
+      connection.close(true);
+      return true;
+    }
+
     StateRegistry nextState = getStateForProtocol(handshake.getNextStatus());
     if (nextState == null) {
       LOGGER.error("{} provided invalid protocol {}", ic, handshake.getNextStatus());