Allow running Velocity without any servers.

This is a niche setup, however if your network is 100% dynamically configured, this is a handy feature to have available.

To support this functionality, a new PlayerChooseInitialServerEvent event was added to allow the initial server to connect to be changed as desired.

api/src/main/java/com/velocitypowered/api/event/player/PlayerChooseInitialServerEvent.java

@@ -0,0 +1,51 @@
+package com.velocitypowered.api.event.player;
+
+import com.google.common.base.Preconditions;
+import com.velocitypowered.api.proxy.Player;
+import com.velocitypowered.api.proxy.server.RegisteredServer;
+import java.util.Optional;
+import org.checkerframework.checker.nullness.qual.Nullable;
+
+/**
+ * Fired when a player has finished connecting to the proxy and we need to choose the first server
+ * to connect to.
+ */
+public class PlayerChooseInitialServerEvent {
+
+  private final Player player;
+  private @Nullable RegisteredServer initialServer;
+
+  /**
+   * Constructs a PlayerChooseInitialServerEvent.
+   * @param player the player that was connected
+   * @param initialServer the initial server selected, may be {@code null}
+   */
+  public PlayerChooseInitialServerEvent(Player player, @Nullable RegisteredServer initialServer) {
+    this.player = Preconditions.checkNotNull(player, "player");
+    this.initialServer = initialServer;
+  }
+
+  public Player getPlayer() {
+    return player;
+  }
+
+  public Optional<RegisteredServer> getInitialServer() {
+    return Optional.ofNullable(initialServer);
+  }
+
+  /**
+   * Sets the new initial server.
+   * @param server the initial server the player should connect to
+   */
+  public void setInitialServer(RegisteredServer server) {
+    this.initialServer = server;
+  }
+
+  @Override
+  public String toString() {
+    return "PlayerChooseInitialServerEvent{"
+        + "player=" + player
+        + ", initialServer=" + initialServer
+        + '}';
+  }
+}

proxy/src/main/java/com/velocitypowered/proxy/config/VelocityConfiguration.java

@@ -192,12 +192,7 @@ public class VelocityConfiguration extends AnnotatedConfig implements ProxyConfi
     }
 
     if (servers.getServers().isEmpty()) {
-      logger.error("You have no servers configured. :(");
+      logger.warn("You don't have any servers configured.");
-      valid = false;
-    } else {
-      if (servers.getAttemptConnectionOrder().isEmpty()) {
-        logger.error("No fallback servers are configured!");
-        valid = false;
     }
 
     for (Map.Entry<String, String> entry : servers.getServers().entrySet()) {
@@ -230,7 +225,6 @@ public class VelocityConfiguration extends AnnotatedConfig implements ProxyConfi
         }
       }
     }
-    }
 
     try {
       getMotdComponent();

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

@@ -417,17 +417,7 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player {
       return;
     }
 
-    if (connectedServer == null) {
+    boolean kickedFromCurrent = connectedServer == null || connectedServer.getServer().equals(rs);
-      Optional<RegisteredServer> nextServer = getNextServerToTry(rs);
-      if (nextServer.isPresent()) {
-        // There can't be any connection in flight now.
-        resetInFlightConnection();
-        createConnectionRequest(nextServer.get()).fireAndForget();
-      } else {
-        disconnect(friendlyReason);
-      }
-    } else {
-      boolean kickedFromCurrent = connectedServer.getServer().equals(rs);
     ServerKickResult result;
     if (kickedFromCurrent) {
       Optional<RegisteredServer> next = getNextServerToTry(rs);
@@ -444,7 +434,6 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player {
         !kickedFromCurrent, result);
     handleKickEvent(originalEvent, friendlyReason);
   }
-  }
 
   private void handleKickEvent(KickedFromServerEvent originalEvent, Component friendlyReason) {
     server.getEventManager().fire(originalEvent)

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

@@ -1,24 +1,21 @@
 package com.velocitypowered.proxy.connection.client;
 
 import static com.google.common.net.UrlEscapers.urlFormParameterEscaper;
-import static com.velocitypowered.api.network.ProtocolVersion.MINECRAFT_1_13;
 import static com.velocitypowered.api.network.ProtocolVersion.MINECRAFT_1_8;
 import static com.velocitypowered.proxy.VelocityServer.GSON;
 import static com.velocitypowered.proxy.connection.VelocityConstants.EMPTY_BYTE_ARRAY;
-import static com.velocitypowered.proxy.connection.VelocityConstants.VELOCITY_IP_FORWARDING_CHANNEL;
 import static com.velocitypowered.proxy.util.EncryptionUtils.decryptRsa;
 import static com.velocitypowered.proxy.util.EncryptionUtils.generateServerId;
 import static org.asynchttpclient.Dsl.asyncHttpClient;
-import static org.asynchttpclient.Dsl.config;
 
 import com.google.common.base.Preconditions;
-import com.google.common.net.UrlEscapers;
 import com.velocitypowered.api.event.connection.LoginEvent;
 import com.velocitypowered.api.event.connection.PostLoginEvent;
 import com.velocitypowered.api.event.connection.PreLoginEvent;
 import com.velocitypowered.api.event.connection.PreLoginEvent.PreLoginComponentResult;
 import com.velocitypowered.api.event.permission.PermissionsSetupEvent;
 import com.velocitypowered.api.event.player.GameProfileRequestEvent;
+import com.velocitypowered.api.event.player.PlayerChooseInitialServerEvent;
 import com.velocitypowered.api.proxy.server.RegisteredServer;
 import com.velocitypowered.api.util.GameProfile;
 import com.velocitypowered.proxy.VelocityServer;
@@ -29,17 +26,12 @@ import com.velocitypowered.proxy.protocol.StateRegistry;
 import com.velocitypowered.proxy.protocol.packet.Disconnect;
 import com.velocitypowered.proxy.protocol.packet.EncryptionRequest;
 import com.velocitypowered.proxy.protocol.packet.EncryptionResponse;
-import com.velocitypowered.proxy.protocol.packet.LoginPluginMessage;
-import com.velocitypowered.proxy.protocol.packet.LoginPluginResponse;
 import com.velocitypowered.proxy.protocol.packet.ServerLogin;
 import com.velocitypowered.proxy.protocol.packet.ServerLoginSuccess;
 import com.velocitypowered.proxy.protocol.packet.SetCompression;
 import com.velocitypowered.proxy.util.VelocityMessages;
 import io.netty.buffer.ByteBuf;
-import io.netty.buffer.Unpooled;
 import java.net.InetSocketAddress;
-import java.net.MalformedURLException;
-import java.net.URL;
 import java.security.GeneralSecurityException;
 import java.security.KeyPair;
 import java.util.Arrays;
@@ -50,7 +42,6 @@ import java.util.concurrent.ThreadLocalRandom;
 import net.kyori.text.Component;
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
-import org.asynchttpclient.Dsl;
 import org.asynchttpclient.ListenableFuture;
 import org.asynchttpclient.Response;
 import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
@@ -231,12 +222,6 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
   }
 
   private void finishLogin(ConnectedPlayer player) {
-    Optional<RegisteredServer> toTry = player.getNextServerToTry();
-    if (!toTry.isPresent()) {
-      player.disconnect(VelocityMessages.NO_AVAILABLE_SERVERS);
-      return;
-    }
-
     int threshold = server.getConfiguration().getCompressionThreshold();
     if (threshold >= 0 && mcConnection.getProtocolVersion().compareTo(MINECRAFT_1_8) >= 0) {
       mcConnection.write(new SetCompression(threshold));
@@ -269,11 +254,27 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
 
             mcConnection.setSessionHandler(new InitialConnectSessionHandler(player));
             server.getEventManager().fire(new PostLoginEvent(player))
-                .thenRun(() -> player.createConnectionRequest(toTry.get()).fireAndForget());
+                .thenRun(() -> connectToInitialServer(player));
           }
         }, mcConnection.eventLoop());
   }
 
+  private void connectToInitialServer(ConnectedPlayer player) {
+    Optional<RegisteredServer> initialFromConfig = player.getNextServerToTry();
+    PlayerChooseInitialServerEvent event = new PlayerChooseInitialServerEvent(player,
+        initialFromConfig.orElse(null));
+
+    server.getEventManager().fire(event)
+        .thenRunAsync(() -> {
+          Optional<RegisteredServer> toTry = event.getInitialServer();
+          if (!toTry.isPresent()) {
+            player.disconnect(VelocityMessages.NO_AVAILABLE_SERVERS);
+            return;
+          }
+          player.createConnectionRequest(toTry.get()).fireAndForget();
+        }, mcConnection.eventLoop());
+  }
+
   @Override
   public void handleUnknown(ByteBuf buf) {
     mcConnection.close();