Fix tab completions not being forwarded to backend if proxy command exists but is inaccessible (#1329)

api/src/main/java/com/velocitypowered/api/command/CommandManager.java

@@ -10,6 +10,7 @@ package com.velocitypowered.api.command;
 import com.velocitypowered.api.event.command.CommandExecuteEvent;
 import java.util.Collection;
 import java.util.concurrent.CompletableFuture;
+import java.util.function.Predicate;
 import org.checkerframework.checker.nullness.qual.Nullable;
 
 /**
@@ -126,4 +127,15 @@ public interface CommandManager {
    * @return true if the alias is registered; false otherwise
    */
   boolean hasCommand(String alias);
+
+  /**
+   * Returns whether the given alias is registered on this manager
+   * and can be used by the given {@link CommandSource}.
+   * See {@link com.mojang.brigadier.builder.ArgumentBuilder#requires(Predicate)}
+   *
+   * @param alias the command alias to check
+   * @param source the command source
+   * @return true if the alias is registered and usable; false otherwise
+   */
+  boolean hasCommand(String alias, CommandSource source);
 }

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

@@ -347,8 +347,19 @@ public class VelocityCommandManager implements CommandManager {
 
   @Override
   public boolean hasCommand(final String alias) {
+    return getCommand(alias) != null;
+  }
+
+  @Override
+  public boolean hasCommand(String alias, CommandSource source) {
+    Preconditions.checkNotNull(source, "source");
+    CommandNode<CommandSource> command = getCommand(alias);
+    return command != null && command.canUse(source);
+  }
+
+  CommandNode<CommandSource> getCommand(final String alias) {
     Preconditions.checkNotNull(alias, "alias");
-    return dispatcher.getRoot().getChild(alias.toLowerCase(Locale.ENGLISH)) != null;
+    return dispatcher.getRoot().getChild(alias.toLowerCase(Locale.ENGLISH));
   }
 
   @VisibleForTesting // this constitutes unsafe publication

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

@@ -630,7 +630,7 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
     }
 
     String commandLabel = command.substring(0, commandEndPosition);
-    if (!server.getCommandManager().hasCommand(commandLabel)) {
+    if (!server.getCommandManager().hasCommand(commandLabel, player)) {
       if (player.getProtocolVersion().lessThan(ProtocolVersion.MINECRAFT_1_13)) {
         // Outstanding tab completes are recorded for use with 1.12 clients and below to provide
         // additional tab completion support.