fix: improve paperclip indicator checks to support e.g. leafmc (#1847)

### Motivation
With #1839 the wrapper was converted into a java agent which resulted in
a regression regarding leafmc support.

### Modification
Instead of strictly checking on the paperclip package & class name we
now only check on an existing version list (used by paperclip) and the
existance of a method called `extractAndApplyPatches` inside a class in
the same package as the main class is located in.

### Result
Paperclip, Leaf's QuantumLeaper and possible other paperclip variants
are supported.

##### Other context
Results from this conversation
https://discord.com/channels/325362837184577536/818777626663321671/1465822591880003787

---------

Co-authored-by: Pasqual Koschmieder <git@derklaro.dev>

Author: 0utplay

wrapper-jvm/impl/src/main/java/eu/cloudnetservice/wrapper/impl/transform/bukkit/PaperclipClassLoaderTransformer.java

@@ -20,53 +20,97 @@
 import java.lang.classfile.ClassModel;
 import java.lang.classfile.ClassTransform;
 import java.lang.classfile.instruction.InvokeInstruction;
+import java.lang.reflect.AccessFlag;
 import lombok.NonNull;
 import org.jetbrains.annotations.ApiStatus;
 
 /**
- * A transformer implementation that removes the paperclip call to get the parent class loader of the paperclip class
- * which results in the bootstrap class loader which does not have the files from the class path.
+ * A transformer that removes the {@code ClassLoader.getParent()} call from the paperclip main method, as it results in
+ * the paper main class being loaded by the bootstrap class loader. This then causes, for example, plugins to not have
+ * access to the wrapper classes (which reside on the class path and therefore on the system class loader).
  *
  * @since 4.0
  */
 @ApiStatus.Internal
-public class PaperclipClassLoaderTransformer implements ClassTransformer {
+public final class PaperclipClassLoaderTransformer implements ClassTransformer {
 
   private static final String MAIN_METHOD_NAME = "main";
   private static final String CLASS_LOADER_GET_PARENT_METHOD = "getParent";
   private static final String CLASS_LOADER_INTERNAL_NAME = "java/lang/ClassLoader";
-  private static final String PAPERCLIP_MAIN_CLASS = "io/papermc/paperclip/Paperclip";
+  private static final String PAPERCLIP_INDICATOR_METHOD = "extractAndApplyPatches";
+
+  // need to check by package and not by exact class name, as the real paperclip main class is just
+  // a dummy implementation to check if the correct java version for the paper version is being used
+  private final String mainClassPackage;
 
   /**
    * Constructs a new instance of this transformer, usually done via SPI.
    */
   public PaperclipClassLoaderTransformer() {
-    // used by SPI
+    this.mainClassPackage = findPaperclipMainClassPackage();
+  }
+
+  /**
+   * Finds the package where the main class of paperclip might be located in. Throws an exception in case no paperclip
+   * is on the classpath or the main class package cannot be resolved.
+   *
+   * @return the package name of the paperclip main class.
+   * @throws RuntimeException if no paperclip main package can be resolved.
+   */
+  private static @NonNull String findPaperclipMainClassPackage() {
+    // check for the 'patches.list' file included in modern paperclip (since 1.18)
+    // previously paperclip used instrumentation to add the paper jar to the system cl, making this transform obsolete
+    var hasPatchesList = ClassLoader.getSystemClassLoader().getResource("META-INF/patches.list") != null;
+    if (hasPatchesList) {
+      // 'sun.java.command' holds the main class and arguments to supply to the main class, we use this
+      // property to resolve the main class that is being invoked by the jvm
+      var mainClassAndArgs = System.getProperty("sun.java.command", "");
+      var mainClass = mainClassAndArgs.split(" ")[0];
+      if (!mainClass.isBlank()) {
+        var internalMainClassName = mainClass.replace('.', '/');
+        var lastPackageDelimiter = internalMainClassName.lastIndexOf('/');
+        if (lastPackageDelimiter != -1) {
+          return internalMainClassName.substring(0, lastPackageDelimiter);
+        }
+      }
+    }
+
+    throw new RuntimeException("not running paperclip or main class not found");
   }
 
   /**
    * {@inheritDoc}
    */
   @Override
   public @NonNull TransformWillingness classTransformWillingness(@NonNull String internalClassName) {
-    return internalClassName.equals(PAPERCLIP_MAIN_CLASS)
-      ? TransformWillingness.ACCEPT_ONCE
-      : TransformWillingness.REJECT;
+    var lastPkgDelimIdx = internalClassName.lastIndexOf('/');
+    if (lastPkgDelimIdx == this.mainClassPackage.length() && internalClassName.startsWith(this.mainClassPackage)) {
+      return TransformWillingness.ACCEPT;
+    }
+
+    return TransformWillingness.REJECT;
   }
 
   /**
    * {@inheritDoc}
    */
   @Override
   public @NonNull ClassTransform provideClassTransform(@NonNull ClassModel original) {
+    var hasPaperclipMainClassIndicatorMethod = original.methods()
+      .stream()
+      .anyMatch(method -> method.methodName().equalsString(PAPERCLIP_INDICATOR_METHOD));
+    if (!hasPaperclipMainClassIndicatorMethod) {
+      return ClassTransform.ACCEPT_ALL;
+    }
+
     return ClassTransform.transformingMethodBodies(
-      methodModel -> methodModel.methodName().equalsString(MAIN_METHOD_NAME),
+      methodModel -> methodModel.flags().has(AccessFlag.STATIC)
+        && methodModel.methodName().equalsString(MAIN_METHOD_NAME),
       (builder, element) -> {
-        if (element instanceof InvokeInstruction invoke) {
-          if (invoke.method().name().equalsString(CLASS_LOADER_GET_PARENT_METHOD)
-            && invoke.owner().asInternalName().equals(CLASS_LOADER_INTERNAL_NAME)) {
-            return;
-          }
+        if (element instanceof InvokeInstruction invokeInst
+          && invokeInst.owner().asInternalName().equals(CLASS_LOADER_INTERNAL_NAME)
+          && invokeInst.method().name().equalsString(CLASS_LOADER_GET_PARENT_METHOD)) {
+          return;
         }
 
         builder.with(element);