Memory usage improvements for transform cache (#336)

* Add system property to enable useTempFile in ZipFS

This avoids storing data in memory but slows down cache building,
so we gate it behind a system property that's off by default

* Delete previous transform cache file if recreating

* Avoid storing all class bytes in a map at once for internals hider transformation

src/main/java/org/quiltmc/loader/impl/transformer/TransformCache.java

@@ -148,6 +148,8 @@ private static QuiltZipPath checkTransformCache(Path transformCacheFolder, Map<S
 			QuiltZipPath inner = fs.getRoot();
 			if (!FasterFiles.isRegularFile(inner.resolve(FILE_TRANSFORM_COMPLETE))) {
 				Log.info(LogCategory.CACHE, "Not reusing previous transform cache since it's incomplete!");
+				// delete the previous transform cache to prevent FileAlreadyExistsException later
+				try { Files.deleteIfExists(cacheFile); } catch(IOException ignored) {}
 				return null;
 			}
 			Path optionFile = inner.resolve("options.txt");
@@ -312,17 +314,21 @@ private static void populateTransformCache(Path root, List<ModLoadOption> modLis
 		}
 
 		InternalsHiderTransform internalsHider = new InternalsHiderTransform(InternalsHiderTransform.Target.MOD);
-		Map<Path, ClassData> classes = new HashMap<>();
+		Map<Path, ModLoadOption> classes = new HashMap<>();
+
+		// the double read is necessary to avoid storing all classes in memory at once, and thus having memory complexity
+		// proportional to mod count
 
 		forEachClassFile(root, modList, (mod, file) -> {
 			byte[] classBytes = Files.readAllBytes(file);
-			classes.put(file, new ClassData(mod, classBytes));
+			classes.put(file, mod);
 			internalsHider.scanClass(mod, file, classBytes);
 			return null;
 		});
 
-		for (Map.Entry<Path, ClassData> entry : classes.entrySet()) {
-			byte[] newBytes = internalsHider.run(entry.getValue().mod, entry.getValue().classBytes);
+		for (Map.Entry<Path, ModLoadOption> entry : classes.entrySet()) {
+			byte[] classBytes = Files.readAllBytes(entry.getKey());
+			byte[] newBytes = internalsHider.run(entry.getValue(), classBytes);
 			if (newBytes != null) {
 				Files.write(entry.getKey(), newBytes);
 			}

src/main/java/org/quiltmc/loader/impl/util/FileSystemUtil.java

@@ -25,7 +25,7 @@
 import java.nio.file.FileSystemNotFoundException;
 import java.nio.file.FileSystems;
 import java.nio.file.Path;
-import java.util.Collections;
+import java.util.HashMap;
 import java.util.Map;
 import java.util.zip.ZipError;
 
@@ -56,8 +56,17 @@ public void close() throws IOException {
 
 	private FileSystemUtil() { }
 
-	private static final Map<String, String> jfsArgsCreate = Collections.singletonMap("create", "true");
-	private static final Map<String, String> jfsArgsEmpty = Collections.emptyMap();
+	private static final Map<String, Object> jfsArgsCreate = new HashMap<>();
+	private static final Map<String, Object> jfsArgsEmpty = new HashMap<>();
+
+	static {
+		jfsArgsCreate.put("create", "true");
+		if(Boolean.getBoolean(SystemProperties.USE_ZIPFS_TEMP_FILE)) {
+			// must be Boolean.TRUE for Java 8
+			jfsArgsCreate.put("useTempFile", Boolean.TRUE);
+			jfsArgsEmpty.put("useTempFile", Boolean.TRUE);
+		}
+	}
 
 	public static FileSystemDelegate getJarFileSystem(Path path, boolean create) throws IOException {
 		return getJarFileSystem(path.toUri(), create);

src/main/java/org/quiltmc/loader/impl/util/SystemProperties.java

@@ -74,6 +74,8 @@ public final class SystemProperties {
 	public static final String DISABLE_OPTIMIZED_COMPRESSED_TRANSFORM_CACHE = "loader.transform_cache.disable_optimised_compression";
 	public static final String DISABLE_PRELOAD_TRANSFORM_CACHE = "loader.transform_cache.disable_preload";
 	public static final String LOG_CACHE_KEY_CHANGES = "loader.transform_cache.log_changed_keys";
+	// enable useTempFile in ZipFileSystem, reduces memory usage when writing transform cache at the cost of speed
+	public static final String USE_ZIPFS_TEMP_FILE = "loader.zipfs.use_temp_file";
 	public static final String DISABLE_BEACON = "loader.disable_beacon";
 
 	private SystemProperties() {