Thread Safe Vanilla Command permission checking

Datapacks check this on load and are built concurrently. This was breaking them badly due
to race conditions.

Plus, .canUse we want to be safe for async anyways.

Fixes #3722

Spigot-Server-Patches/0543-Thread-Safe-Vanilla-Command-permission-checking.patch

@@ -0,0 +1,52 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Aikar <aikar@aikar.co>
+Date: Sat, 11 Jul 2020 03:54:28 -0400
+Subject: [PATCH] Thread Safe Vanilla Command permission checking
+
+Datapacks check this on load and are built concurrently. This was breaking them badly due
+to race conditions.
+
+Plus, .canUse we want to be safe for async anyways.
+
+diff --git a/src/main/java/com/mojang/brigadier/tree/CommandNode.java b/src/main/java/com/mojang/brigadier/tree/CommandNode.java
+index 5e3fc338b0fe69a74465567e69721bc28c64dd17..ca0d664487a65a4768f0e65af6a320fd1875de8f 100644
+--- a/src/main/java/com/mojang/brigadier/tree/CommandNode.java
++++ b/src/main/java/com/mojang/brigadier/tree/CommandNode.java
+@@ -75,10 +75,10 @@ public abstract class CommandNode<S> implements Comparable<CommandNode<S>> {
+         // CraftBukkit start
+         if (source instanceof CommandListenerWrapper) {
+             try {
+-                ((CommandListenerWrapper) source).currentCommand = this;
++                ((CommandListenerWrapper) source).currentCommand.set(this); // Paper
+                 return requirement.test(source);
+             } finally {
+-                ((CommandListenerWrapper) source).currentCommand = null;
++                ((CommandListenerWrapper) source).currentCommand.set(null); // Paper
+             }
+         }
+         // CraftBukkit end
+diff --git a/src/main/java/net/minecraft/server/CommandListenerWrapper.java b/src/main/java/net/minecraft/server/CommandListenerWrapper.java
+index fa0f247e51c20ea0e3cbaccbc8dbb2a153d6121f..a4995ddc82100bc460cd2021dcfa8389769d5da0 100644
+--- a/src/main/java/net/minecraft/server/CommandListenerWrapper.java
++++ b/src/main/java/net/minecraft/server/CommandListenerWrapper.java
+@@ -33,7 +33,7 @@ public class CommandListenerWrapper implements ICompletionProvider, com.destroys
+     private final ResultConsumer<CommandListenerWrapper> l;
+     private final ArgumentAnchor.Anchor m;
+     private final Vec2F n;
+-    public CommandNode currentCommand; // CraftBukkit
++    public ThreadLocal<CommandNode> currentCommand = new ThreadLocal<>(); // CraftBukkit // Paper
+ 
+     public CommandListenerWrapper(ICommandListener icommandlistener, Vec3D vec3d, Vec2F vec2f, WorldServer worldserver, int i, String s, IChatBaseComponent ichatbasecomponent, MinecraftServer minecraftserver, @Nullable Entity entity) {
+         this(icommandlistener, vec3d, vec2f, worldserver, i, s, ichatbasecomponent, minecraftserver, entity, false, (commandcontext, flag, j) -> {
+@@ -143,8 +143,11 @@ public class CommandListenerWrapper implements ICompletionProvider, com.destroys
+     @Override
+     public boolean hasPermission(int i) {
+         // CraftBukkit start
++        // Paper start - fix concurrency issue
++        CommandNode currentCommand = this.currentCommand.get();
+         if (currentCommand != null) {
+             return hasPermission(i, org.bukkit.craftbukkit.command.VanillaCommandWrapper.getPermission(currentCommand));
++            // Paper end
+         }
+         // CraftBukkit end
+