fix missing server info in some events when kicked from current server (#703)

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

@@ -13,6 +13,7 @@ import com.velocitypowered.api.event.annotation.AwaitingEvent;
 import com.velocitypowered.api.proxy.ConnectionRequestBuilder;
 import com.velocitypowered.api.proxy.ConnectionRequestBuilder.Status;
 import com.velocitypowered.api.proxy.Player;
+import com.velocitypowered.api.proxy.ServerConnection;
 import com.velocitypowered.api.proxy.server.RegisteredServer;
 import java.util.Optional;
 import org.checkerframework.checker.nullness.qual.Nullable;
@@ -27,6 +28,7 @@ public final class ServerPreConnectEvent implements
 
   private final Player player;
   private final RegisteredServer originalServer;
+  private final RegisteredServer previousServer;
   private ServerResult result;
 
   /**
@@ -35,8 +37,21 @@ public final class ServerPreConnectEvent implements
    * @param originalServer the server the player was trying to connect to
    */
   public ServerPreConnectEvent(Player player, RegisteredServer originalServer) {
+    this(player, originalServer,
+        player.getCurrentServer().map(ServerConnection::getServer).orElse(null));
+  }
+
+  /**
+   * Creates the ServerPreConnectEvent.
+   * @param player the player who is connecting to a server
+   * @param originalServer the server the player was trying to connect to
+   * @param previousServer the server the player ís connected to
+   */
+  public ServerPreConnectEvent(Player player, RegisteredServer originalServer,
+      @Nullable RegisteredServer previousServer) {
     this.player = Preconditions.checkNotNull(player, "player");
     this.originalServer = Preconditions.checkNotNull(originalServer, "originalServer");
+    this.previousServer = previousServer;
     this.result = ServerResult.allowed(originalServer);
   }
 
@@ -61,13 +76,24 @@ public final class ServerPreConnectEvent implements
   /**
    * Returns the server that the player originally tried to connect to. To get the server the
    * player will connect to, see the {@link ServerResult} of this event. To get the server the
-   * player is currently on when this event is fired, use {@link Player#getCurrentServer()}.
+   * player is currently on when this event is fired, use {@link #getPreviousServer()}.
    * @return the server that the player originally tried to connect to
    */
   public RegisteredServer getOriginalServer() {
     return originalServer;
   }
 
+  /**
+   * Returns the server that the player is currently connected to. Prefer this method over using
+   * {@link Player#getCurrentServer()} as the current server might get reset after server kicks to
+   * prevent connection issues. This is {@code null} if they were not connected to another server
+   * beforehand (for instance, if the player has just joined the proxy).
+   * @return the server the player is currently connected to.
+   */
+  public @Nullable RegisteredServer getPreviousServer() {
+    return previousServer;
+  }
+
   @Override
   public String toString() {
     return "ServerPreConnectEvent{"

api/src/main/java/com/velocitypowered/api/proxy/ServerConnection.java

@@ -11,6 +11,7 @@ import com.velocitypowered.api.proxy.messages.ChannelMessageSink;
 import com.velocitypowered.api.proxy.messages.ChannelMessageSource;
 import com.velocitypowered.api.proxy.server.RegisteredServer;
 import com.velocitypowered.api.proxy.server.ServerInfo;
+import java.util.Optional;
 
 /**
  * Represents a connection to a backend server from the proxy for a client.
@@ -24,6 +25,14 @@ public interface ServerConnection extends ChannelMessageSource, ChannelMessageSi
    */
   RegisteredServer getServer();
 
+  /**
+   * Returns the server that the player associated with this connection was connected to before
+   * switching to this connection.
+   *
+   * @return the server the player was connected to.
+   */
+  Optional<RegisteredServer> getPreviousServer();
+
   /**
    * Returns the server info for this connection.
    *