Allow existing players to be kicked from server if in online-mode

Fixes #226

proxy/src/main/java/com/velocitypowered/proxy/VelocityServer.java

@@ -70,6 +70,7 @@ import java.util.function.IntFunction;
 import java.util.stream.Collectors;
 import net.kyori.text.Component;
 import net.kyori.text.TextComponent;
+import net.kyori.text.TranslatableComponent;
 import net.kyori.text.serializer.gson.GsonComponentSerializer;
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
@@ -454,6 +455,9 @@ public class VelocityServer implements ProxyServer {
    * @return {@code true} if we can register the connection, {@code false} if not
    */
   public boolean canRegisterConnection(ConnectedPlayer connection) {
+    if (configuration.isOnlineMode() && configuration.isOnlineModeKickExistingPlayers()) {
+      return true;
+    }
     String lowerName = connection.getUsername().toLowerCase(Locale.US);
     return !(connectionsByName.containsKey(lowerName)
         || connectionsByUuid.containsKey(connection.getUniqueId()));
@@ -466,12 +470,24 @@ public class VelocityServer implements ProxyServer {
    */
   public boolean registerConnection(ConnectedPlayer connection) {
     String lowerName = connection.getUsername().toLowerCase(Locale.US);
-    if (connectionsByName.putIfAbsent(lowerName, connection) != null) {
-      return false;
-    }
-    if (connectionsByUuid.putIfAbsent(connection.getUniqueId(), connection) != null) {
-      connectionsByName.remove(lowerName, connection);
-      return false;
+
+    if (!this.configuration.isOnlineModeKickExistingPlayers()) {
+      if (connectionsByName.putIfAbsent(lowerName, connection) != null) {
+        return false;
+      }
+      if (connectionsByUuid.putIfAbsent(connection.getUniqueId(), connection) != null) {
+        connectionsByName.remove(lowerName, connection);
+        return false;
+      }
+    } else {
+      ConnectedPlayer existing = connectionsByUuid.get(connection.getUniqueId());
+      if (existing != null) {
+        existing.disconnect(TranslatableComponent.of("multiplayer.disconnect.duplicate_login"));
+      }
+
+      // We can now replace the entries as needed.
+      connectionsByName.put(lowerName, connection);
+      connectionsByUuid.put(connection.getUniqueId(), connection);
     }
     return true;
   }

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

@@ -77,6 +77,11 @@ public class VelocityConfiguration extends AnnotatedConfig implements ProxyConfi
   @ConfigKey("announce-forge")
   private boolean announceForge = false;
 
+  @Comment({"If enabled (default is false) and the proxy is in online mode, Velocity will kick",
+      "any existing player who is online if a duplicate connection attempt is made."})
+  @ConfigKey("kick-existing-players")
+  private boolean onlineModeKickExistingPlayers = false;
+
   @Table("[servers]")
   private final Servers servers;
 
@@ -109,7 +114,8 @@ public class VelocityConfiguration extends AnnotatedConfig implements ProxyConfi
 
   private VelocityConfiguration(String bind, String motd, int showMaxPlayers, boolean onlineMode,
       boolean announceForge, PlayerInfoForwarding playerInfoForwardingMode, byte[] forwardingSecret,
-      Servers servers, ForcedHosts forcedHosts, Advanced advanced, Query query, Metrics metrics) {
+      boolean onlineModeKickExistingPlayers, Servers servers, ForcedHosts forcedHosts,
+      Advanced advanced, Query query, Metrics metrics) {
     this.bind = bind;
     this.motd = motd;
     this.showMaxPlayers = showMaxPlayers;
@@ -117,6 +123,7 @@ public class VelocityConfiguration extends AnnotatedConfig implements ProxyConfi
     this.announceForge = announceForge;
     this.playerInfoForwardingMode = playerInfoForwardingMode;
     this.forwardingSecret = forwardingSecret;
+    this.onlineModeKickExistingPlayers = onlineModeKickExistingPlayers;
     this.servers = servers;
     this.forcedHosts = forcedHosts;
     this.advanced = advanced;
@@ -425,6 +432,7 @@ public class VelocityConfiguration extends AnnotatedConfig implements ProxyConfi
         toml.getBoolean("announce-forge", false),
         PlayerInfoForwarding.valueOf(forwardingModeName),
         forwardingSecret,
+        toml.getBoolean("kick-existing-players", false),
         servers,
         forcedHosts,
         advanced,
@@ -443,6 +451,10 @@ public class VelocityConfiguration extends AnnotatedConfig implements ProxyConfi
     return builder.toString();
   }
 
+  public boolean isOnlineModeKickExistingPlayers() {
+    return onlineModeKickExistingPlayers;
+  }
+
   private static class Servers {
 
     @IsMap