Make parallel task dispatches respect dependency ordering

loader/src/main/java/net/neoforged/fml/ModLoader.java

@@ -11,6 +11,7 @@
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
+import java.util.IdentityHashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
@@ -190,22 +191,58 @@ public static void waitForTask(String name, Runnable periodicTask, CompletableFu
         }
     }
 
+    /**
+     * Exception that is fired when a mod loading future cannot be executed because a dependent future failed.
+     * It is only used for control flow and easy filtering out, but never logged or propagated further.
+     */
+    private static class DependentFutureFailedException extends RuntimeException {}
+
     /**
      * Dispatches a task across all mod containers in parallel, with progress displayed on the loading screen.
      */
     public static void dispatchParallelTask(String name, Executor parallelExecutor, Runnable periodicTask, Consumer<ModContainer> task) {
         var progress = StartupNotificationManager.addProgressBar(name, modList.size());
         try {
             periodicTask.run();
+            Map<IModInfo, CompletableFuture<Void>> modFutures = new IdentityHashMap<>();
             var futureList = modList.getSortedMods().stream()
                     .map(modContainer -> {
-                        return CompletableFuture.runAsync(() -> {
-                            ModLoadingContext.get().setActiveContainer(modContainer);
-                            task.accept(modContainer);
-                        }, parallelExecutor).whenComplete((result, exception) -> {
-                            progress.increment();
-                            ModLoadingContext.get().setActiveContainer(null);
-                        });
+                        // Build future for all dependencies first
+                        var deps = LoadingModList.get().getDependencies(modContainer.getModInfo());
+                        @SuppressWarnings("unchecked")
+                        CompletableFuture<Void>[] depFutures = new CompletableFuture[deps.size()];
+                        for (int i = 0; i < deps.size(); ++i) {
+                            depFutures[i] = modFutures.get(deps.get(i));
+                            if (depFutures[i] == null) {
+                                throw new IllegalStateException("Dependency future for mod %s which is a dependency of %s not found!".formatted(
+                                        deps.get(i).getModId(), modContainer.getModId()));
+                            }
+                        }
+
+                        var combinedFuture = depFutures.length == 0 ? CompletableFuture.completedFuture(null) : CompletableFuture.allOf(depFutures);
+
+                        // Build the future for this container
+                        var future = combinedFuture.<Void>handleAsync((void_, exception) -> {
+                            if (exception != null) {
+                                // If there was any exception, short circuit.
+                                // The exception will already be handled by `waitForFuture` since it comes from another mod.
+                                LOGGER.error("Skipping {} future for mod {} because a dependency threw an exception.", name, modContainer.getModId());
+                                progress.increment();
+                                // Throw a marker exception to make sure that dependencies of *this* task don't get executed.
+                                throw new DependentFutureFailedException();
+                            }
+
+                            try {
+                                ModLoadingContext.get().setActiveContainer(modContainer);
+                                task.accept(modContainer);
+                            } finally {
+                                progress.increment();
+                                ModLoadingContext.get().setActiveContainer(null);
+                            }
+                            return null;
+                        }, parallelExecutor);
+                        modFutures.put(modContainer.getModInfo(), future);
+                        return future;
                     })
                     .toList();
             var singleFuture = ModList.gather(futureList)
@@ -226,7 +263,9 @@ private static void waitForFuture(String name, Runnable periodicTask, Completabl
                 // Merge all potential modloading issues
                 var errorCount = 0;
                 for (var error : e.getCause().getSuppressed()) {
-                    if (error instanceof ModLoadingException modLoadingException) {
+                    if (error instanceof DependentFutureFailedException) {
+                        continue;
+                    } else if (error instanceof ModLoadingException modLoadingException) {
                         loadingIssues.addAll(modLoadingException.getIssues());
                     } else {
                         loadingIssues.add(ModLoadingIssue.error("fml.modloading.uncaughterror", name).withCause(e));