Fix server-sent plugin message channels, fixing WDLCompanion and similar

Closes #203

proxy/src/main/java/com/velocitypowered/proxy/command/VelocityCommand.java

@@ -260,7 +260,7 @@ public class VelocityCommand implements Command {
 
     @Override
     public boolean hasPermission(CommandSource source, String @NonNull [] args) {
-      return source.getPermissionValue("velocity.command.plugins") == Tristate.TRUE;
+      return source.getPermissionValue("velocity.command.plugins").asBoolean();
     }
   }
 }

proxy/src/main/java/com/velocitypowered/proxy/connection/backend/BackendPlaySessionHandler.java

@@ -86,10 +86,19 @@ public class BackendPlaySessionHandler implements MinecraftSessionHandler {
 
   @Override
   public boolean handle(PluginMessage packet) {
-    if (!serverConn.getPlayer().canForwardPluginMessage(packet)) {
+    if (!serverConn.getPlayer().canForwardPluginMessage(serverConn.ensureConnected()
+        .getProtocolVersion(), packet)) {
       return true;
     }
 
+    // We need to specially handle REGISTER and UNREGISTER packets. Later on, we'll write them to
+    // the client.
+    if (PluginMessageUtil.isRegister(packet)) {
+      serverConn.getPlayer().getKnownChannels().addAll(PluginMessageUtil.getChannels(packet));
+    } else if (PluginMessageUtil.isUnregister(packet)) {
+      serverConn.getPlayer().getKnownChannels().removeAll(PluginMessageUtil.getChannels(packet));
+    }
+
     if (PluginMessageUtil.isMcBrand(packet)) {
       PluginMessage rewritten = PluginMessageUtil.rewriteMinecraftBrand(packet,
           server.getVersion());

proxy/src/main/java/com/velocitypowered/proxy/connection/backend/TransitionSessionHandler.java

@@ -131,15 +131,16 @@ public class TransitionSessionHandler implements MinecraftSessionHandler {
 
   @Override
   public boolean handle(PluginMessage packet) {
-    if (!serverConn.getPlayer().canForwardPluginMessage(packet)) {
+    if (!serverConn.getPlayer().canForwardPluginMessage(serverConn.ensureConnected()
+        .getProtocolVersion(), packet)) {
       return true;
     }
 
-    // We need to specially handle REGISTER and UNREGISTER packets. Later on, we'll write them to
-    // the client.
     if (PluginMessageUtil.isRegister(packet)) {
+      System.out.println("[TSH] I CAN HAZ REGISTER: " + PluginMessageUtil.getChannels(packet));
       serverConn.getPlayer().getKnownChannels().addAll(PluginMessageUtil.getChannels(packet));
     } else if (PluginMessageUtil.isUnregister(packet)) {
+      System.out.println("[TSH] I CAN HAZ UNREGISTER: " + PluginMessageUtil.getChannels(packet));
       serverConn.getPlayer().getKnownChannels().removeAll(PluginMessageUtil.getChannels(packet));
     }
 

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

@@ -43,6 +43,7 @@ import com.velocitypowered.proxy.protocol.packet.KeepAlive;
 import com.velocitypowered.proxy.protocol.packet.PluginMessage;
 import com.velocitypowered.proxy.protocol.packet.ResourcePackRequest;
 import com.velocitypowered.proxy.protocol.packet.TitlePacket;
+import com.velocitypowered.proxy.protocol.util.PluginMessageUtil;
 import com.velocitypowered.proxy.server.VelocityRegisteredServer;
 import com.velocitypowered.proxy.tablist.VelocityTabList;
 import com.velocitypowered.proxy.util.VelocityMessages;
@@ -650,16 +651,18 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player {
 
   /**
    * Determines whether or not we can forward a plugin message onto the client.
+   * @param version the Minecraft protocol version
    * @param message the plugin message to forward to the client
    * @return {@code true} if the message can be forwarded, {@code false} otherwise
    */
-  public boolean canForwardPluginMessage(PluginMessage message) {
+  public boolean canForwardPluginMessage(ProtocolVersion version, PluginMessage message) {
-    // If we're forwarding a plugin message onto the client, that implies that we have a backend
-    // connection already.
-    MinecraftConnection mc = ensureBackendConnection();
-
     boolean minecraftOrFmlMessage;
-    if (mc.getProtocolVersion().compareTo(ProtocolVersion.MINECRAFT_1_12_2) <= 0) {
+
+    if (PluginMessageUtil.isRegister(message) || PluginMessageUtil.isUnregister(message)) {
+      return true;
+    }
+
+    if (version.compareTo(ProtocolVersion.MINECRAFT_1_12_2) <= 0) {
       String channel = message.getChannel();
       minecraftOrFmlMessage = channel.startsWith("MC|") || channel
               .startsWith(LegacyForgeConstants.FORGE_LEGACY_HANDSHAKE_CHANNEL);

proxy/src/main/java/com/velocitypowered/proxy/network/http/NettyHttpClient.java

@@ -15,7 +15,6 @@ import io.netty.handler.ssl.SslContextBuilder;
 import io.netty.handler.ssl.SslHandler;
 import java.net.InetSocketAddress;
 import java.net.URL;
-import java.util.Objects;
 import java.util.concurrent.CompletableFuture;
 import javax.net.ssl.SSLEngine;