Fix generated event handlers for events on interfaces.

Author: LexManos

eventbus-test/src/test/java/net/minecraftforge/eventbus/test/general/InterfaceEventHandler.java

@@ -0,0 +1,53 @@
+/*
+ * Copyright (c) Forge Development LLC
+ * SPDX-License-Identifier: LGPL-2.1-only
+ */
+
+package net.minecraftforge.eventbus.test.general;
+
+import java.util.function.Consumer;
+import java.util.function.Supplier;
+
+import net.minecraftforge.eventbus.api.BusBuilder;
+import net.minecraftforge.eventbus.api.Event;
+import net.minecraftforge.eventbus.api.IEventBus;
+import net.minecraftforge.eventbus.api.SubscribeEvent;
+import net.minecraftforge.eventbus.test.ITestHandler;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+public class InterfaceEventHandler implements ITestHandler {
+    private static boolean hit = false;
+
+    public InterfaceEventHandler(boolean hasTransformer) {
+    }
+
+    @Override
+    public void test(Consumer<Class<?>> validator, Supplier<BusBuilder> builder) {
+        var bus = builder.get().build();
+        assertDoesNotThrow(() -> bus.register(STATIC.class));
+        testCall(bus, true, "STATIC");
+        assertDoesNotThrow(() -> bus.register(new INSTANCE() {}));
+        testCall(bus, true, "STATIC");
+    }
+
+    private void testCall(IEventBus bus, boolean expected, String name) {
+        hit = false;
+        bus.post(new Event());
+        assertEquals(expected, hit, name + " did not behave correctly");
+    }
+
+    public interface STATIC {
+        @SubscribeEvent
+        static void handler(Event e) {
+            hit = true;
+        }
+    }
+    
+    public interface INSTANCE {
+        @SubscribeEvent
+        default void handler(Event e) {
+            hit = true;
+        }
+    }
+}

src/main/java/net/minecraftforge/eventbus/ClassLoaderFactory.java

@@ -52,6 +52,7 @@ protected static void transformNode(String name, Method callback, ClassNode targ
         MethodVisitor mv;
 
         boolean isStatic = Modifier.isStatic(callback.getModifiers());
+        boolean isInterface = Modifier.isInterface(callback.getDeclaringClass().getModifiers());
         String desc = name.replace('.',  '/');
         String instType = Type.getInternalName(callback.getDeclaringClass());
         String eventType = Type.getInternalName(callback.getParameterTypes()[0]);
@@ -95,7 +96,14 @@ protected static void transformNode(String name, Method callback, ClassNode targ
             }
             mv.visitVarInsn(ALOAD, 1);
             mv.visitTypeInsn(CHECKCAST, eventType);
-            mv.visitMethodInsn(isStatic ? INVOKESTATIC : INVOKEVIRTUAL, instType, callback.getName(), Type.getMethodDescriptor(callback), false);
+
+            var opcode = INVOKEVIRTUAL;
+            if (isStatic)
+                opcode = INVOKESTATIC;
+            else if (isInterface)
+                opcode = INVOKEINTERFACE;
+
+            mv.visitMethodInsn(opcode, instType, callback.getName(), Type.getMethodDescriptor(callback), isInterface);
             mv.visitInsn(RETURN);
             mv.visitMaxs(2, 2);
             mv.visitEnd();