Provide better errors when game-layer mod files try to provide class …

…processors

loader/src/main/java/net/neoforged/fml/loading/FMLLoader.java

@@ -42,6 +42,7 @@
 import net.neoforged.fml.IBindingsProvider;
 import net.neoforged.fml.ModList;
 import net.neoforged.fml.ModLoader;
+import net.neoforged.fml.ModLoadingException;
 import net.neoforged.fml.ModLoadingIssue;
 import net.neoforged.fml.classloading.JarModuleFinder;
 import net.neoforged.fml.classloading.ResourceMaskingClassLoader;
@@ -141,7 +142,15 @@ record DiscoveryResult(
             List<ModFile> pluginContent,
             List<ModFile> gameContent,
             List<ModFile> gameLibraryContent,
-            List<ModLoadingIssue> discoveryIssues) {}
+            List<ModLoadingIssue> discoveryIssues) {
+        public List<ModFile> allGameContent() {
+            var content = new ArrayList<ModFile>(
+                    gameContent.size() + gameLibraryContent.size());
+            content.addAll(gameContent);
+            content.addAll(gameLibraryContent);
+            return content;
+        }
+    }
 
     private FMLLoader(ClassLoader currentClassLoader, String[] programArgs, Dist dist, boolean production, Path gameDir) {
         this.currentClassLoader = currentClassLoader;
@@ -323,10 +332,7 @@ public static FMLLoader create(@Nullable Instrumentation instrumentation, Startu
             // BUILD GAME LAYER
             // NOTE: This is where Mixin contributes its synthetic SecureJar to ensure it's generated classes are handled by the TCL
             var gameContent = new ArrayList<SecureJar>();
-            for (var modFile : discoveryResult.gameLibraryContent) {
-                gameContent.add(modFile.getSecureJar());
-            }
-            for (var modFile : discoveryResult.gameContent) {
+            for (var modFile : discoveryResult.allGameContent()) {
                 gameContent.add(modFile.getSecureJar());
             }
 
@@ -364,6 +370,21 @@ private static ClassProcessorSet createClassProcessorSet(StartupArgs startupArgs
             LaunchContextAdapter launchContext,
             DiscoveryResult discoveryResult,
             MixinFacade mixinFacade) {
+        // Validate that the modder didn't try to provide transformation services from game content
+        var issues = new ArrayList<ModLoadingIssue>();
+        for (var modFile : discoveryResult.allGameContent()) {
+            var descriptor = modFile.getSecureJar().moduleDataProvider().descriptor();
+            for (var provides : descriptor.provides()) {
+                if (provides.service().equals(ClassProcessorProvider.class.getName())
+                        || provides.service().equals(ClassProcessor.class.getName())) {
+                    issues.add(ModLoadingIssue.error("fml.modloadingissue.classprocessor_in_game_content").withAffectedModFile(modFile));
+                }
+            }
+        }
+        if (!issues.isEmpty()) {
+            throw new ModLoadingException(issues);
+        }
+
         // Add our own launch plugins explicitly.
         var builtInProcessors = new ArrayList<ClassProcessor>();
         builtInProcessors.add(createAccessTransformerService(discoveryResult));

loader/src/main/resources/lang/en_us.json

@@ -26,6 +26,7 @@
   "fml.modloadingissue.dependencyloading.conflictingdependencies": "Some mods have requested conflicting versions of: §6{0}§r. Requested by: §e{1}§r.",
   "fml.modloadingissue.dependencyloading.mismatchedcontaineddependencies": "Some mods have agreed upon an acceptable version range for : §6{0}§r, but no jar was provided which matched the range. Requested by: §e{1}§r.",
   "fml.modloadingissue.coremod_error": "An error occurred while loading core-mod {0} from {1}",
+  "fml.modloadingissue.classprocessor_in_game_content": "Mod file {101} is trying to provide a ClassProcessor service. Mods and game libraries cannot provide such services, only libraries can.",
   "fml.modloadingissue.corrupted_installation": "Your NeoForge installation is corrupted. Please try to reinstall NeoForge.",
   "fml.modloadingissue.missing_minecraft_jar": "The patched Minecraft jar is missing. Please try to reinstall NeoForge.",
   "fml.modloadingissue.corrupted_minecraft_jar": "The patched Minecraft jar is corrupted. Please try to reinstall NeoForge.",

loader/src/test/java/net/neoforged/fml/classloading/JarMetadataTest.java

@@ -232,6 +232,6 @@ private static ModuleDescriptor getJdkModuleDescriptor(Path testJar) {
 
     @FunctionalInterface
     interface ModFileCustomizer {
-        void customize(ModFileBuilder builder) throws IOException;
+        void customize(ModFileBuilder<?> builder) throws IOException;
     }
 }

loader/src/test/java/net/neoforged/fml/loading/SimpleProcessorsTest.java

@@ -9,8 +9,12 @@
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
+import java.io.IOException;
 import java.util.Set;
 import net.neoforged.fml.ModLoadingException;
+import net.neoforged.fml.testlib.ModFileBuilder;
+import net.neoforged.fml.testlib.SimulatedInstallation;
+import net.neoforged.fml.testlib.args.ClientInstallationTypesSource;
 import net.neoforged.jarjar.metadata.ContainedJarIdentifier;
 import net.neoforged.neoforgespi.locating.IModFile;
 import net.neoforged.neoforgespi.transformation.ClassProcessor;
@@ -19,6 +23,7 @@
 import net.neoforged.neoforgespi.transformation.SimpleClassProcessor;
 import net.neoforged.neoforgespi.transformation.SimpleTransformationContext;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
 import org.objectweb.asm.tree.ClassNode;
 
 /**
@@ -119,4 +124,45 @@ public void createProcessors(Context context, Collector collector) {
 
         assertEquals("fml:test", loader.getClassTransformerAuditLog().getAuditString("testmod.TestClass"));
     }
+
+    /**
+     * Class processors shouldn't be able to be loaded from mod jars, and the modder should get a proper error
+     * for this case.
+     */
+    @ParameterizedTest
+    @ClientInstallationTypesSource
+    public void testProcessorProvidedByModJar(SimulatedInstallation.Type type) throws Exception {
+        testProcessorProvidedByGameContent(type, ModFileBuilder::withTestmodModsToml);
+    }
+
+    /**
+     * Class processors shouldn't be able to be loaded from game libraries, and the modder should get a proper error
+     * for this case.
+     */
+    @ParameterizedTest
+    @ClientInstallationTypesSource
+    public void testProcessorProvidedByGameLibrary(SimulatedInstallation.Type type) throws Exception {
+        testProcessorProvidedByGameContent(type, builder -> builder.withModTypeManifest("GAMELIBRARY"));
+    }
+
+    private void testProcessorProvidedByGameContent(SimulatedInstallation.Type type, ModFileBuilder.ModJarCustomizer customizer) throws IOException {
+        installation.setup(type);
+        installation.buildInstallationAppropriateModProject("testmod", "testmod.jar", builder -> {
+            customizer.customize(builder);
+            builder.addClass("testmod.TestProvider", """
+                    public class TestProvider implements net.neoforged.neoforgespi.transformation.ClassProcessorProvider {
+                        @Override
+                        public void createProcessors(Context context, Collector collector) {
+                            throw new RuntimeException();
+                        }
+                    }""")
+                    .addService(ClassProcessorProvider.class, "testmod.TestProvider");
+        });
+
+        var e = assertThrows(ModLoadingException.class, () -> launchAndLoad("neoforgeclient"));
+        var issues = getTranslatedIssues(e.getIssues());
+        assertThat(issues).hasSize(1);
+        assertThat(issues.getFirst()).matches(
+                "ERROR: Mod file .* is trying to provide a ClassProcessor service. Mods and game libraries cannot provide such services, only libraries can.");
+    }
 }