From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Mariell Hoversholm <proximyst@proximyst.com>
Date: Mon, 27 Apr 2020 12:31:59 +0200
Subject: [PATCH] Prioritise own classes where possible

This adds the server property `Paper.DisableClassPrioritization` to disable
prioritization of own classes for plugins' classloaders.

This value is by default not present, and this will therefore break any
plugins which abuse behaviour related to not using their own classes
while still loading their own. This is often an issue with failing to
relocate or shade properly, such as when shading plugin APIs like Vault.

A plugin's classloader will first look in the same jar as it is loading
in for a requested class, then load it. It does not re-use other
plugins' classes if it has the chance to avoid doing so.

If a class is not found in the same jar as it is loading for and it does
find it elsewhere, it will still choose the class elsewhere. This is
intended behaviour, as it will only prioritise classes it has in its own
jar, no other plugins' classes will be prioritised in any other order
than the one they were registered in.

The patch in general terms just loads the class in the plugin's jar
before it starts looking elsewhere for it.

diff --git a/src/main/java/org/bukkit/plugin/java/JavaPluginLoader.java b/src/main/java/org/bukkit/plugin/java/JavaPluginLoader.java
index cf809eda2a3feb6abccf7286068280f430452135..4b54af83ef8fd18696d2d21ed52b61f13bff7988 100644
--- a/src/main/java/org/bukkit/plugin/java/JavaPluginLoader.java
+++ b/src/main/java/org/bukkit/plugin/java/JavaPluginLoader.java
@@ -51,6 +51,7 @@ import org.yaml.snakeyaml.error.YAMLException;
  */
 public final class JavaPluginLoader implements PluginLoader {
     final Server server;
+    private static final boolean DISABLE_CLASS_PRIORITIZATION = Boolean.getBoolean("Paper.DisableClassPrioritization"); // Paper
     private final Pattern[] fileFilters = new Pattern[]{Pattern.compile("\\.jar$")};
     private final Map<String, java.util.concurrent.locks.ReentrantReadWriteLock> classLoadLock = new java.util.HashMap<String, java.util.concurrent.locks.ReentrantReadWriteLock>(); // Paper
     private final Map<String, Integer> classLoadLockCount = new java.util.HashMap<String, Integer>(); // Paper
@@ -203,6 +204,11 @@ public final class JavaPluginLoader implements PluginLoader {
 
     @Nullable
     Class<?> getClassByName(final String name, boolean resolve, PluginDescriptionFile description) {
+        // Paper start - prioritize self
+        return getClassByName(name, resolve, description, null);
+    }
+    Class<?> getClassByName(final String name, boolean resolve, PluginDescriptionFile description, PluginClassLoader requester) {
+        // Paper end
         // Paper start - make MT safe
         java.util.concurrent.locks.ReentrantReadWriteLock lock;
         synchronized (classLoadLock) {
@@ -210,6 +216,13 @@ public final class JavaPluginLoader implements PluginLoader {
             classLoadLockCount.compute(name, (x, prev) -> prev != null ? prev + 1 : 1);
         }
         lock.writeLock().lock();try {
+            // Paper start - prioritize self
+            if (!DISABLE_CLASS_PRIORITIZATION && requester != null) {
+                try {
+                return requester.loadClass0(name, false, false, ((SimplePluginManager) server.getPluginManager()).isTransitiveDepend(description, requester.getDescription()));
+                } catch (ClassNotFoundException cnfe) {}
+            }
+            // Paper end
         // Paper end
         for (PluginClassLoader loader : loaders) {
             try {
diff --git a/src/main/java/org/bukkit/plugin/java/PluginClassLoader.java b/src/main/java/org/bukkit/plugin/java/PluginClassLoader.java
index 0db641f5d5e1293b236ad0d2e3a156802ffed839..064c758b19bc8c9a4e94769dd205a1bdcc972a89 100644
--- a/src/main/java/org/bukkit/plugin/java/PluginClassLoader.java
+++ b/src/main/java/org/bukkit/plugin/java/PluginClassLoader.java
@@ -33,7 +33,7 @@ public final class PluginClassLoader extends URLClassLoader { // Spigot
     public JavaPlugin getPlugin() { return plugin; } // Spigot
     private final JavaPluginLoader loader;
     private final Map<String, Class<?>> classes = new ConcurrentHashMap<String, Class<?>>();
-    private final PluginDescriptionFile description;
+    private final PluginDescriptionFile description; PluginDescriptionFile getDescription() { return description; } // Paper
     private final File dataFolder;
     private final File file;
     private final JarFile jar;
@@ -123,7 +123,7 @@ public final class PluginClassLoader extends URLClassLoader { // Spigot
 
         if (checkGlobal) {
             // This ignores the libraries of other plugins, unless they are transitive dependencies.
-            Class<?> result = loader.getClassByName(name, resolve, description);
+            Class<?> result = loader.getClassByName(name, resolve, description, this);  // Paper - prioritize self
 
             if (result != null) {
                 // If the class was loaded from a library instead of a PluginClassLoader, we can assume that its associated plugin is a transitive dependency and can therefore skip this check.
