Add support for ping pass-through

By default, ping pass-through is not enabled. However, you can use
ping passthrough to pass through just mods (great for modded servers)
or everything.

api/src/main/java/com/velocitypowered/api/proxy/server/ServerPing.java

@@ -182,6 +182,19 @@ public final class ServerPing {
       return this;
     }
 
+    /**
+     * Uses the modified {@code mods} list in the response.
+     * @param mods the mods list to use
+     * @return this build, for chaining
+     */
+    public Builder mods(ModInfo mods) {
+      Preconditions.checkNotNull(mods, "mods");
+      this.modType = mods.getType();
+      this.mods.clear();
+      this.mods.addAll(mods.getMods());
+      return this;
+    }
+
     public Builder clearMods() {
       this.mods.clear();
       return this;

proxy/build.gradle

@@ -67,6 +67,8 @@ dependencies {
     
     compile 'org.asynchttpclient:async-http-client:2.10.1'
     
+    compile 'com.spotify:completable-futures:0.3.2'
+
     testCompile "org.junit.jupiter:junit-jupiter-api:${junitVersion}"
     testCompile "org.junit.jupiter:junit-jupiter-engine:${junitVersion}"
 }

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

@@ -77,6 +77,7 @@ import org.asynchttpclient.AsyncHttpClient;
 import org.checkerframework.checker.nullness.qual.EnsuresNonNull;
 import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
 import org.checkerframework.checker.nullness.qual.NonNull;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import org.checkerframework.checker.nullness.qual.RequiresNonNull;
 
 public class VelocityServer implements ProxyServer {
@@ -263,11 +264,7 @@ public class VelocityServer implements ProxyServer {
     logger.info("Loaded {} plugins", pluginManager.getPlugins().size());
   }
 
-  public Bootstrap createBootstrap() {
+  public Bootstrap createBootstrap(@Nullable EventLoopGroup group) {
-    return this.cm.createWorker();
-  }
-
-  public Bootstrap createBootstrap(EventLoopGroup group) {
     return this.cm.createWorker(group);
   }
 

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

@@ -0,0 +1,7 @@
+package com.velocitypowered.proxy.config;
+
+public enum PingPassthroughMode {
+  DISABLED,
+  MODS,
+  ALL
+}

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

@@ -72,8 +72,13 @@ public class VelocityConfiguration extends AnnotatedConfig implements ProxyConfi
   @ConfigKey("forwarding-secret")
   private byte[] forwardingSecret = generateRandomString(12).getBytes(StandardCharsets.UTF_8);
 
-  @Comment({"Announce whether or not your server supports Forge. If you run a modded server, we",
+  @Comment({
-      "suggest turning this on."})
+      "Announce whether or not your server supports Forge. If you run a modded server, we",
+      "suggest turning this on.",
+      "",
+      "If your network runs one modpack consistently, consider using ping-passthrough = \"mods\"",
+      "instead for a nicer display in the server list."
+  })
   @ConfigKey("announce-forge")
   private boolean announceForge = false;
 
@@ -82,6 +87,20 @@ public class VelocityConfiguration extends AnnotatedConfig implements ProxyConfi
   @ConfigKey("kick-existing-players")
   private boolean onlineModeKickExistingPlayers = false;
 
+  @Comment({
+      "Should Velocity pass server list ping requests to a backend server?",
+      "Available options:",
+      "- \"disabled\": No pass-through will be done. The velocity.toml and server-icon.png",
+      "              will determine the initial server list ping response.",
+      "- \"mods\":     Passes only the mod list from your backend server into the response.",
+      "              This is the recommended replacement for announce-forge = true. If no backend",
+      "              servers can be contacted, Velocity will not display any mod information.",
+      "- \"all\":      Passes everything from the backend server into the response. The Velocity",
+      "              configuration is used if no servers could be contacted."
+  })
+  @ConfigKey("ping-passthrough")
+  private PingPassthroughMode pingPassthrough;
+
   @Table("[servers]")
   private final Servers servers;
 
@@ -114,8 +133,8 @@ public class VelocityConfiguration extends AnnotatedConfig implements ProxyConfi
 
   private VelocityConfiguration(String bind, String motd, int showMaxPlayers, boolean onlineMode,
       boolean announceForge, PlayerInfoForwarding playerInfoForwardingMode, byte[] forwardingSecret,
-      boolean onlineModeKickExistingPlayers, Servers servers, ForcedHosts forcedHosts,
+      boolean onlineModeKickExistingPlayers, PingPassthroughMode pingPassthrough, Servers servers,
-      Advanced advanced, Query query, Metrics metrics) {
+      ForcedHosts forcedHosts, Advanced advanced, Query query, Metrics metrics) {
     this.bind = bind;
     this.motd = motd;
     this.showMaxPlayers = showMaxPlayers;
@@ -124,6 +143,7 @@ public class VelocityConfiguration extends AnnotatedConfig implements ProxyConfi
     this.playerInfoForwardingMode = playerInfoForwardingMode;
     this.forwardingSecret = forwardingSecret;
     this.onlineModeKickExistingPlayers = onlineModeKickExistingPlayers;
+    this.pingPassthrough = pingPassthrough;
     this.servers = servers;
     this.forcedHosts = forcedHosts;
     this.advanced = advanced;
@@ -380,6 +400,10 @@ public class VelocityConfiguration extends AnnotatedConfig implements ProxyConfi
     return metrics;
   }
 
+  public PingPassthroughMode getPingPassthrough() {
+    return pingPassthrough;
+  }
+
   @Override
   public String toString() {
     return MoreObjects.toStringHelper(this)
@@ -427,6 +451,8 @@ public class VelocityConfiguration extends AnnotatedConfig implements ProxyConfi
 
     String forwardingModeName = toml.getString("player-info-forwarding-mode", "MODERN")
         .toUpperCase(Locale.US);
+    String passThroughName = toml.getString("ping-passthrough", "DISABLED")
+        .toUpperCase(Locale.US);
 
     return new VelocityConfiguration(
         toml.getString("bind", "0.0.0.0:25577"),
@@ -437,6 +463,7 @@ public class VelocityConfiguration extends AnnotatedConfig implements ProxyConfi
         PlayerInfoForwarding.valueOf(forwardingModeName),
         forwardingSecret,
         toml.getBoolean("kick-existing-players", false),
+        PingPassthroughMode.valueOf(passThroughName),
         servers,
         forcedHosts,
         advanced,

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

@@ -1,12 +1,15 @@
 package com.velocitypowered.proxy.connection.client;
 
 import com.google.common.collect.ImmutableList;
+import com.spotify.futures.CompletableFutures;
 import com.velocitypowered.api.event.proxy.ProxyPingEvent;
 import com.velocitypowered.api.network.ProtocolVersion;
 import com.velocitypowered.api.proxy.InboundConnection;
+import com.velocitypowered.api.proxy.server.RegisteredServer;
 import com.velocitypowered.api.proxy.server.ServerPing;
 import com.velocitypowered.api.util.ModInfo;
 import com.velocitypowered.proxy.VelocityServer;
+import com.velocitypowered.proxy.config.PingPassthroughMode;
 import com.velocitypowered.proxy.config.VelocityConfiguration;
 import com.velocitypowered.proxy.connection.MinecraftConnection;
 import com.velocitypowered.proxy.connection.MinecraftSessionHandler;
@@ -15,7 +18,13 @@ import com.velocitypowered.proxy.protocol.packet.LegacyPing;
 import com.velocitypowered.proxy.protocol.packet.StatusPing;
 import com.velocitypowered.proxy.protocol.packet.StatusRequest;
 import com.velocitypowered.proxy.protocol.packet.StatusResponse;
+import com.velocitypowered.proxy.server.VelocityRegisteredServer;
 import io.netty.buffer.ByteBuf;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Optional;
+import java.util.concurrent.CompletableFuture;
+import java.util.stream.Stream;
 
 public class StatusSessionHandler implements MinecraftSessionHandler {
 
@@ -30,12 +39,10 @@ public class StatusSessionHandler implements MinecraftSessionHandler {
     this.inboundWrapper = inboundWrapper;
   }
 
-  private ServerPing createInitialPing() {
+  private ServerPing constructLocalPing(ProtocolVersion version) {
     VelocityConfiguration configuration = server.getConfiguration();
-    ProtocolVersion shownVersion = ProtocolVersion.isSupported(connection.getProtocolVersion())
-        ? connection.getProtocolVersion() : ProtocolVersion.MAXIMUM_VERSION;
     return new ServerPing(
-        new ServerPing.Version(shownVersion.getProtocol(),
+        new ServerPing.Version(version.getProtocol(),
             "Velocity " + ProtocolVersion.SUPPORTED_VERSION_STRING),
         new ServerPing.Players(server.getPlayerCount(), configuration.getShowMaxPlayers(),
             ImmutableList.of()),
@@ -45,12 +52,69 @@ public class StatusSessionHandler implements MinecraftSessionHandler {
     );
   }
 
+  private CompletableFuture<ServerPing> createInitialPing() {
+    VelocityConfiguration configuration = server.getConfiguration();
+    ProtocolVersion shownVersion = ProtocolVersion.isSupported(connection.getProtocolVersion())
+        ? connection.getProtocolVersion() : ProtocolVersion.MAXIMUM_VERSION;
+
+    PingPassthroughMode passthrough = configuration.getPingPassthrough();
+    if (passthrough == PingPassthroughMode.DISABLED) {
+      return CompletableFuture.completedFuture(constructLocalPing(shownVersion));
+    } else {
+      return attemptPingPassthrough(configuration.getPingPassthrough(),
+          configuration.getAttemptConnectionOrder(), shownVersion);
+    }
+  }
+
+  private CompletableFuture<ServerPing> attemptPingPassthrough(PingPassthroughMode mode,
+      List<String> servers, ProtocolVersion pingingVersion) {
+    ServerPing fallback = constructLocalPing(pingingVersion);
+    List<CompletableFuture<ServerPing>> pings = new ArrayList<>();
+    for (String s : servers) {
+      Optional<RegisteredServer> rs = server.getServer(s);
+      if (!rs.isPresent()) {
+        continue;
+      }
+      VelocityRegisteredServer vrs = (VelocityRegisteredServer) rs.get();
+      pings.add(vrs.ping(connection.eventLoop(), pingingVersion));
+    }
+    if (pings.isEmpty()) {
+      return CompletableFuture.completedFuture(fallback);
+    }
+
+    CompletableFuture<List<ServerPing>> pingResponses = CompletableFutures.successfulAsList(pings,
+        (ex) -> fallback);
+    switch (mode) {
+      case ALL:
+        return pingResponses.thenApply(responses -> {
+          // Find the first non-fallback
+          return responses.stream()
+              .filter(ping -> ping != fallback)
+              .findFirst()
+              .orElse(fallback);
+        });
+      case MODS:
+        return pingResponses.thenApply(responses -> {
+          // Find the first non-fallback that contains a non-empty mod list
+          Optional<ModInfo> modInfo = responses.stream()
+              .filter(ping -> ping != fallback)
+              .map(ServerPing::getModinfo)
+              .flatMap(o -> o.map(Stream::of).orElseGet(Stream::empty))
+              .findFirst();
+          return modInfo.map(mi -> fallback.asBuilder().mods(mi).build()).orElse(fallback);
+        });
+      default:
+        // Not possible, but covered for completeness.
+        return CompletableFuture.completedFuture(fallback);
+    }
+  }
+
   @Override
   public boolean handle(LegacyPing packet) {
-    ServerPing initialPing = createInitialPing();
+    createInitialPing()
-    ProxyPingEvent event = new ProxyPingEvent(inboundWrapper, initialPing);
+        .thenCompose(ping -> server.getEventManager().fire(new ProxyPingEvent(inboundWrapper,
-    server.getEventManager().fire(event)
+            ping)))
-        .thenRunAsync(() -> {
+        .thenAcceptAsync(event -> {
           connection.closeWith(LegacyDisconnect.fromServerPing(event.getPing(),
               packet.getVersion()));
         }, connection.eventLoop());
@@ -65,11 +129,11 @@ public class StatusSessionHandler implements MinecraftSessionHandler {
 
   @Override
   public boolean handle(StatusRequest packet) {
-    ServerPing initialPing = createInitialPing();
+    createInitialPing()
-    ProxyPingEvent event = new ProxyPingEvent(inboundWrapper, initialPing);
+        .thenCompose(ping -> server.getEventManager().fire(new ProxyPingEvent(inboundWrapper,
-    server.getEventManager().fire(event)
+            ping)))
-        .thenRunAsync(
+        .thenAcceptAsync(
-            () -> {
+            (event) -> {
               StringBuilder json = new StringBuilder();
               VelocityServer.GSON.toJson(event.getPing(), json);
               connection.write(new StatusResponse(json));

proxy/src/main/java/com/velocitypowered/proxy/network/ConnectionManager.java

@@ -145,10 +145,6 @@ public final class ConnectionManager {
         });
   }
 
-  public Bootstrap createWorker() {
-    return this.createWorker(null);
-  }
-
   /**
    * Creates a TCP {@link Bootstrap} using Velocity's event loops.
    *

proxy/src/main/java/com/velocitypowered/proxy/server/PingSessionHandler.java

@@ -18,13 +18,15 @@ public class PingSessionHandler implements MinecraftSessionHandler {
   private final CompletableFuture<ServerPing> result;
   private final RegisteredServer server;
   private final MinecraftConnection connection;
+  private final ProtocolVersion version;
   private boolean completed = false;
 
   PingSessionHandler(CompletableFuture<ServerPing> result, RegisteredServer server,
-      MinecraftConnection connection) {
+      MinecraftConnection connection, ProtocolVersion version) {
     this.result = result;
     this.server = server;
     this.connection = connection;
+    this.version = version;
   }
 
   @Override
@@ -33,7 +35,7 @@ public class PingSessionHandler implements MinecraftSessionHandler {
     handshake.setNextStatus(StateRegistry.STATUS_ID);
     handshake.setServerAddress(server.getServerInfo().getAddress().getHostString());
     handshake.setPort(server.getServerInfo().getAddress().getPort());
-    handshake.setProtocolVersion(ProtocolVersion.MINIMUM_VERSION);
+    handshake.setProtocolVersion(version);
     connection.delayedWrite(handshake);
 
     connection.setState(StateRegistry.STATUS);

proxy/src/main/java/com/velocitypowered/proxy/server/VelocityRegisteredServer.java

@@ -9,6 +9,7 @@ import static com.velocitypowered.proxy.network.Connections.READ_TIMEOUT;
 
 import com.google.common.base.Preconditions;
 import com.google.common.collect.ImmutableList;
+import com.velocitypowered.api.network.ProtocolVersion;
 import com.velocitypowered.api.proxy.Player;
 import com.velocitypowered.api.proxy.ServerConnection;
 import com.velocitypowered.api.proxy.messages.ChannelIdentifier;
@@ -27,6 +28,7 @@ import io.netty.channel.Channel;
 import io.netty.channel.ChannelFuture;
 import io.netty.channel.ChannelFutureListener;
 import io.netty.channel.ChannelInitializer;
+import io.netty.channel.EventLoop;
 import io.netty.handler.timeout.ReadTimeoutHandler;
 import java.util.Collection;
 import java.util.Set;
@@ -58,11 +60,22 @@ public class VelocityRegisteredServer implements RegisteredServer {
 
   @Override
   public CompletableFuture<ServerPing> ping() {
+    return ping(null, ProtocolVersion.UNKNOWN);
+  }
+
+  /**
+   * Pings the specified server using the specified event {@code loop}, claiming to be
+   * {@code version}.
+   * @param loop the event loop to use
+   * @param version the version to report
+   * @return the server list ping response
+   */
+  public CompletableFuture<ServerPing> ping(@Nullable EventLoop loop, ProtocolVersion version) {
     if (server == null) {
       throw new IllegalStateException("No Velocity proxy instance available");
     }
     CompletableFuture<ServerPing> pingFuture = new CompletableFuture<>();
-    server.createBootstrap()
+    server.createBootstrap(loop)
         .handler(new ChannelInitializer<Channel>() {
           @Override
           protected void initChannel(Channel ch) throws Exception {
@@ -86,8 +99,8 @@ public class VelocityRegisteredServer implements RegisteredServer {
           public void operationComplete(ChannelFuture future) throws Exception {
             if (future.isSuccess()) {
               MinecraftConnection conn = future.channel().pipeline().get(MinecraftConnection.class);
-              conn.setSessionHandler(
+              conn.setSessionHandler(new PingSessionHandler(
-                  new PingSessionHandler(pingFuture, VelocityRegisteredServer.this, conn));
+                  pingFuture, VelocityRegisteredServer.this, conn, version));
             } else {
               pingFuture.completeExceptionally(future.cause());
             }