feat: Add basic API for server links (#1353)

* feat: add basic server links API

* refactor: more precondition checking on links

* refactor: remove whitespace

* refactor: adjust method order, JDs in ServerLink

* refactor: remove "throws" from constructors

* refactor: add @NotNull annotations

* refactor: requested changes

* refactor: just use `List#copyOf`

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

@@ -21,6 +21,7 @@ import com.velocitypowered.api.proxy.player.TabList;
 import com.velocitypowered.api.proxy.server.RegisteredServer;
 import com.velocitypowered.api.util.GameProfile;
 import com.velocitypowered.api.util.ModInfo;
+import com.velocitypowered.api.util.ServerLink;
 import java.net.InetSocketAddress;
 import java.util.Collection;
 import java.util.List;
@@ -461,4 +462,16 @@ public interface Player extends
    * @sinceMinecraft 1.20.5
    */
   void requestCookie(Key key);
+
+  /**
+   * Send the player a list of custom links to display in their client's pause menu.
+   *
+   * <p>Note that later packets sent by the backend server may override links sent by the proxy.
+   *
+   * @param links an ordered list of {@link ServerLink}s to send to the player
+   * @throws IllegalArgumentException if the player is from a version lower than 1.21
+   * @since 3.3.0
+   * @sinceMinecraft 1.21
+   */
+  void setServerLinks(@NotNull List<ServerLink> links);
 }

api/src/main/java/com/velocitypowered/api/util/ServerLink.java

@@ -0,0 +1,101 @@
+/*
+ * Copyright (C) 2021-2024 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.util;
+
+import com.google.common.base.Preconditions;
+import java.net.URI;
+import java.util.Optional;
+import net.kyori.adventure.text.Component;
+import org.jetbrains.annotations.Nullable;
+
+/**
+ * Represents a custom URL servers can show in player pause menus.
+ * Links can be of a built-in type or use a custom component text label.
+ */
+public final class ServerLink {
+
+  private @Nullable Type type;
+  private @Nullable Component label;
+  private final URI url;
+
+  private ServerLink(Component label, String url) {
+    this.label = Preconditions.checkNotNull(label, "label");
+    this.url = URI.create(url);
+  }
+
+  private ServerLink(Type type, String url) {
+    this.type = Preconditions.checkNotNull(type, "type");
+    this.url = URI.create(url);
+  }
+
+  /**
+   * Construct a server link with a custom component label.
+   *
+   * @param label a custom component label to display
+   * @param link  the URL to open when clicked
+   */
+  public static ServerLink serverLink(Component label, String link) {
+    return new ServerLink(label, link);
+  }
+
+  /**
+   * Construct a server link with a built-in type.
+   *
+   * @param type the {@link Type built-in type} of link
+   * @param link the URL to open when clicked
+   */
+  public static ServerLink serverLink(Type type, String link) {
+    return new ServerLink(type, link);
+  }
+
+  /**
+   * Get the type of the server link.
+   *
+   * @return the type of the server link
+   */
+  public Optional<Type> getBuiltInType() {
+    return Optional.ofNullable(type);
+  }
+
+  /**
+   * Get the custom component label of the server link.
+   *
+   * @return the custom component label of the server link
+   */
+  public Optional<Component> getCustomLabel() {
+    return Optional.ofNullable(label);
+  }
+
+  /**
+   * Get the link {@link URI}.
+   *
+   * @return the link {@link URI}
+   */
+  public URI getUrl() {
+    return url;
+  }
+
+  /**
+   * Built-in types of server links.
+   *
+   * @apiNote {@link Type#BUG_REPORT} links are shown on the connection error screen
+   */
+  public enum Type {
+    BUG_REPORT,
+    COMMUNITY_GUIDELINES,
+    SUPPORT,
+    STATUS,
+    FEEDBACK,
+    COMMUNITY,
+    WEBSITE,
+    FORUMS,
+    NEWS,
+    ANNOUNCEMENTS
+  }
+
+}

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

@@ -55,6 +55,7 @@ import com.velocitypowered.api.proxy.player.ResourcePackInfo;
 import com.velocitypowered.api.proxy.server.RegisteredServer;
 import com.velocitypowered.api.util.GameProfile;
 import com.velocitypowered.api.util.ModInfo;
+import com.velocitypowered.api.util.ServerLink;
 import com.velocitypowered.proxy.VelocityServer;
 import com.velocitypowered.proxy.adventure.VelocityBossBarImplementation;
 import com.velocitypowered.proxy.connection.MinecraftConnection;
@@ -83,6 +84,7 @@ import com.velocitypowered.proxy.protocol.packet.chat.ComponentHolder;
 import com.velocitypowered.proxy.protocol.packet.chat.PlayerChatCompletionPacket;
 import com.velocitypowered.proxy.protocol.packet.chat.builder.ChatBuilderFactory;
 import com.velocitypowered.proxy.protocol.packet.chat.legacy.LegacyChatPacket;
+import com.velocitypowered.proxy.protocol.packet.config.ClientboundServerLinksPacket;
 import com.velocitypowered.proxy.protocol.packet.config.StartUpdatePacket;
 import com.velocitypowered.proxy.protocol.packet.title.GenericTitlePacket;
 import com.velocitypowered.proxy.protocol.util.ByteBufDataOutput;
@@ -1059,6 +1061,22 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player,
         }, connection.eventLoop());
   }
 
+  @Override
+  public void setServerLinks(final @NotNull List<ServerLink> links) {
+    Preconditions.checkNotNull(links, "links");
+    Preconditions.checkArgument(
+        this.getProtocolVersion().noLessThan(ProtocolVersion.MINECRAFT_1_21),
+        "Player version must be at least 1.21 to be able to set server links");
+
+    if (connection.getState() != StateRegistry.PLAY
+        && connection.getState() != StateRegistry.CONFIG) {
+      throw new IllegalStateException("Can only send server links in CONFIGURATION or PLAY protocol");
+    }
+
+    connection.write(new ClientboundServerLinksPacket(List.copyOf(links).stream()
+        .map(l -> new ClientboundServerLinksPacket.ServerLink(l, getProtocolVersion())).toList()));
+  }
+
   @Override
   public void addCustomChatCompletions(@NotNull Collection<String> completions) {
     Preconditions.checkNotNull(completions, "completions");

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/config/ClientboundServerLinksPacket.java

@@ -18,6 +18,7 @@
 package com.velocitypowered.proxy.protocol.packet.config;
 
 import com.velocitypowered.api.network.ProtocolVersion;
+import com.velocitypowered.api.util.ServerLink;
 import com.velocitypowered.proxy.connection.MinecraftSessionHandler;
 import com.velocitypowered.proxy.protocol.MinecraftPacket;
 import com.velocitypowered.proxy.protocol.ProtocolUtils;
@@ -66,6 +67,13 @@ public class ClientboundServerLinksPacket implements MinecraftPacket {
     }
 
     public record ServerLink(int id, ComponentHolder displayName, String url) {
+
+        public ServerLink(com.velocitypowered.api.util.ServerLink link, ProtocolVersion protocolVersion) {
+            this(link.getBuiltInType().map(Enum::ordinal).orElse(-1),
+                link.getCustomLabel().map(c -> new ComponentHolder(protocolVersion, c)).orElse(null),
+                link.getUrl().toString());
+        }
+
         private static ServerLink read(ByteBuf buf, ProtocolVersion version) {
             if (buf.readBoolean()) {
                 return new ServerLink(ProtocolUtils.readVarInt(buf), null, ProtocolUtils.readString(buf));