Fix EntrypointPatchFML125 (#732)

* Unbreak EntrypointPatchFML125, again

It's almost like only 3 people use this or something

* Handle null values, cleaner logic

* Add explanation for workaround

.gitignore

@@ -22,4 +22,7 @@
 !/proguard.conf
 
 !/minecraft
-!/junit
+!/junit
+
+# MacOS junk files
+.DS_Store

minecraft/src/main/java/net/fabricmc/loader/impl/game/minecraft/patch/EntrypointPatchFML125.java

@@ -16,6 +16,8 @@
 
 package net.fabricmc.loader.impl.game.minecraft.patch;
 
+import java.io.IOException;
+import java.io.InputStream;
 import java.util.function.Consumer;
 import java.util.function.Function;
 
@@ -27,6 +29,7 @@
 import net.fabricmc.loader.impl.game.patch.GamePatch;
 import net.fabricmc.loader.impl.launch.FabricLauncher;
 import net.fabricmc.loader.impl.launch.knot.Knot;
+import net.fabricmc.loader.impl.util.LoaderUtil;
 import net.fabricmc.loader.impl.util.log.Log;
 import net.fabricmc.loader.impl.util.log.LogCategory;
 
@@ -46,7 +49,20 @@ public void process(FabricLauncher launcher, Function<String, ClassReader> class
 
 			Log.debug(LogCategory.GAME_PATCH, "Detected 1.2.5 FML - Knotifying ModClassLoader...");
 
-			ClassNode patchedClassLoader = readClass(classSource.apply(FROM));
+			// ModClassLoader_125_FML isn't in the game's class path, so it's loaded from the launcher's class path instead
+			ClassNode patchedClassLoader = new ClassNode();
+
+			try (InputStream stream = launcher.getResourceAsStream(LoaderUtil.getClassFileName(FROM))) {
+				if (stream != null) {
+					ClassReader patchedClassLoaderReader = new ClassReader(stream);
+					patchedClassLoaderReader.accept(patchedClassLoader, 0);
+				} else {
+					throw new IOException("Could not find class " + FROM + " in the launcher classpath while transforming ModClassLoader");
+				}
+			} catch (IOException e) {
+				throw new RuntimeException("An error occurred while reading class " + FROM + " while transforming ModClassLoader", e);
+			}
+
 			ClassNode remappedClassLoader = new ClassNode();
 
 			patchedClassLoader.accept(new ClassRemapper(remappedClassLoader, new Remapper() {