Add --fabric.addMods / -Dfabric.addMods for adding extra path separator separated mods, list file if prefixed with @

src/main/java/net/fabricmc/loader/impl/FabricLoaderImpl.java

@@ -43,6 +43,7 @@
 import net.fabricmc.loader.api.MappingResolver;
 import net.fabricmc.loader.api.SemanticVersion;
 import net.fabricmc.loader.api.entrypoint.EntrypointContainer;
+import net.fabricmc.loader.impl.discovery.ArgumentModCandidateFinder;
 import net.fabricmc.loader.impl.discovery.ClasspathModCandidateFinder;
 import net.fabricmc.loader.impl.discovery.DirectoryModCandidateFinder;
 import net.fabricmc.loader.impl.discovery.ModCandidate;
@@ -179,9 +180,11 @@ public void load() {
 	}
 
 	private void setup() throws ModResolutionException {
+		boolean remapRegularMods = isDevelopmentEnvironment();
 		ModDiscoverer discoverer = new ModDiscoverer();
 		discoverer.addCandidateFinder(new ClasspathModCandidateFinder());
-		discoverer.addCandidateFinder(new DirectoryModCandidateFinder(gameDir.resolve("mods"), isDevelopmentEnvironment()));
+		discoverer.addCandidateFinder(new DirectoryModCandidateFinder(gameDir.resolve("mods"), remapRegularMods));
+		discoverer.addCandidateFinder(new ArgumentModCandidateFinder(remapRegularMods));
 
 		List<ModCandidate> mods = ModResolver.resolve(discoverer.discoverMods(this));
 
@@ -201,7 +204,7 @@ private void setup() throws ModResolutionException {
 
 		// runtime mod remapping
 
-		if (isDevelopmentEnvironment()) {
+		if (remapRegularMods) {
 			if (System.getProperty(SystemProperties.REMAP_CLASSPATH_FILE) == null) {
 				Log.warn(LogCategory.MOD_REMAP, "Runtime mod remapping disabled due to no fabric.remapClasspathFile being specified. You may need to update loom.");
 			} else {

src/main/java/net/fabricmc/loader/impl/discovery/ArgumentModCandidateFinder.java

@@ -0,0 +1,147 @@
+/*
+ * Copyright 2016 FabricMC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package net.fabricmc.loader.impl.discovery;
+
+import java.io.BufferedReader;
+import java.io.File;
+import java.io.IOException;
+import java.nio.file.FileVisitResult;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.nio.file.SimpleFileVisitor;
+import java.nio.file.attribute.BasicFileAttributes;
+import java.util.ArrayList;
+import java.util.List;
+
+import net.fabricmc.loader.impl.FabricLoaderImpl;
+import net.fabricmc.loader.impl.util.Arguments;
+import net.fabricmc.loader.impl.util.SystemProperties;
+import net.fabricmc.loader.impl.util.log.Log;
+import net.fabricmc.loader.impl.util.log.LogCategory;
+
+public class ArgumentModCandidateFinder implements ModCandidateFinder {
+	private final boolean requiresRemap;
+
+	public ArgumentModCandidateFinder(boolean requiresRemap) {
+		this.requiresRemap = requiresRemap;
+	}
+
+	@Override
+	public void findCandidates(ModCandidateConsumer out) {
+		String list = System.getProperty(SystemProperties.ADD_MODS);
+		if (list != null) addMods(list, "system property", out);
+
+		list = FabricLoaderImpl.INSTANCE.getGameProvider().getArguments().remove(Arguments.ADD_MODS);
+		if (list != null) addMods(list, "argument", out);
+	}
+
+	private void addMods(String list, String source, ModCandidateConsumer out) {
+		for (String pathStr : list.split(File.pathSeparator)) {
+			if (pathStr.isEmpty()) continue;
+
+			if (pathStr.startsWith("@")) {
+				Path path = Paths.get(pathStr.substring(1));
+
+				if (!Files.isRegularFile(path)) {
+					Log.warn(LogCategory.DISCOVERY, "Missing/invalid %s provided mod list file %s", source, path);
+					continue;
+				}
+
+				try (BufferedReader reader = Files.newBufferedReader(path)) {
+					String fileSource = String.format("%s file %s", source, path);
+					String line;
+
+					while ((line = reader.readLine()) != null) {
+						line = line.trim();
+						if (line.isEmpty()) continue;
+
+						addMod(line, fileSource, out);
+					}
+				} catch (IOException e) {
+					throw new RuntimeException(String.format("Error reading %s provided mod list file %s", source, path), e);
+				}
+			} else {
+				addMod(pathStr, source, out);
+			}
+		}
+	}
+
+	private void addMod(String pathStr, String source, ModCandidateConsumer out) {
+		Path path = Paths.get(pathStr).toAbsolutePath().normalize();
+
+		if (!Files.exists(path)) { // missing
+			Log.warn(LogCategory.DISCOVERY, "Missing %s provided mod path %s", source, path);
+		} else if (Files.isDirectory(path)) { // directory for extracted mod (in-dev usually) or jars (like mods, but recursive)
+			if (isHidden(path)) {
+				Log.warn(LogCategory.DISCOVERY, "Ignoring hidden %s provided mod path %s", source, path);
+				return;
+			}
+
+			if (Files.exists(path.resolve("fabric.mod.json"))) { // extracted mod
+				out.accept(path, requiresRemap);
+			} else { // dir containing jars
+				try {
+					List<String> skipped = new ArrayList<>();
+
+					Files.walkFileTree(path, new SimpleFileVisitor<Path>() {
+						@Override
+						public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) throws IOException {
+							if (DirectoryModCandidateFinder.isValidFile(file)) {
+								out.accept(file, requiresRemap);
+							} else {
+								skipped.add(path.relativize(file).toString());
+							}
+
+							return FileVisitResult.CONTINUE;
+						}
+
+						@Override
+						public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs) throws IOException {
+							if (isHidden(dir)) {
+								return FileVisitResult.SKIP_SUBTREE;
+							} else {
+								return FileVisitResult.CONTINUE;
+							}
+						}
+					});
+
+					if (!skipped.isEmpty()) {
+						Log.warn(LogCategory.DISCOVERY, "Incompatible files in %s provided mod directory %s (non-jar or hidden): %s", source, path, String.join(", ", skipped));
+					}
+				} catch (IOException e) {
+					Log.warn(LogCategory.DISCOVERY, "Error processing %s provided mod path %s: %s", source, path, e);
+				}
+			}
+		} else { // single file
+			if (!DirectoryModCandidateFinder.isValidFile(path)) {
+				Log.warn(LogCategory.DISCOVERY, "Incompatible file in %s provided mod path %s (non-jar or hidden)", source, path);
+			} else {
+				out.accept(path, requiresRemap);
+			}
+		}
+	}
+
+	private static boolean isHidden(Path path) {
+		try {
+			return path.getFileName().toString().startsWith(".") || Files.isHidden(path);
+		} catch (IOException e) {
+			Log.warn(LogCategory.DISCOVERY, "Error determining whether %s is hidden: %s", path, e);
+			return true;
+		}
+	}
+}

src/main/java/net/fabricmc/loader/impl/discovery/DirectoryModCandidateFinder.java

@@ -25,6 +25,9 @@
 import java.nio.file.attribute.BasicFileAttributes;
 import java.util.EnumSet;
 
+import net.fabricmc.loader.impl.util.log.Log;
+import net.fabricmc.loader.impl.util.log.LogCategory;
+
 public class DirectoryModCandidateFinder implements ModCandidateFinder {
 	private final Path path;
 	private final boolean requiresRemap;
@@ -52,18 +55,7 @@ public void findCandidates(ModCandidateConsumer out) {
 			Files.walkFileTree(this.path, EnumSet.of(FileVisitOption.FOLLOW_LINKS), 1, new SimpleFileVisitor<Path>() {
 				@Override
 				public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) throws IOException {
-					/*
-					 * We only propose a file as a possible mod in the following scenarios:
-					 * General: Must be a jar file
-					 *
-					 * Some OSes Generate metadata so consider the following because of OSes:
-					 * UNIX: Exclude if file is hidden; this occurs when starting a file name with `.`
-					 * MacOS: Exclude hidden + startsWith "." since Mac OS names their metadata files in the form of `.mod.jar`
-					 */
-
-					String fileName = file.getFileName().toString();
-
-					if (fileName.endsWith(".jar") && !fileName.startsWith(".") && !Files.isHidden(file)) {
+					if (isValidFile(file)) {
 						out.accept(file, requiresRemap);
 					}
 
@@ -74,4 +66,28 @@ public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) throws IO
 			throw new RuntimeException("Exception while searching for mods in '" + path + "'!", e);
 		}
 	}
+
+	static boolean isValidFile(Path path) {
+		/*
+		 * We only propose a file as a possible mod in the following scenarios:
+		 * General: Must be a jar file
+		 *
+		 * Some OSes Generate metadata so consider the following because of OSes:
+		 * UNIX: Exclude if file is hidden; this occurs when starting a file name with `.`
+		 * MacOS: Exclude hidden + startsWith "." since Mac OS names their metadata files in the form of `.mod.jar`
+		 */
+
+		if (!Files.isRegularFile(path)) return false;
+
+		try {
+			if (Files.isHidden(path)) return false;
+		} catch (IOException e) {
+			Log.warn(LogCategory.DISCOVERY, "Error checking if file %s is hidden", path, e);
+			return false;
+		}
+
+		String fileName = path.getFileName().toString();
+
+		return fileName.endsWith(".jar") && !fileName.startsWith(".");
+	}
 }

src/main/java/net/fabricmc/loader/impl/discovery/ModDiscoverer.java

@@ -69,10 +69,15 @@ public void addCandidateFinder(ModCandidateFinder f) {
 	public Collection<ModCandidate> discoverMods(FabricLoaderImpl loader) throws ModResolutionException {
 		long startTime = System.nanoTime();
 		ForkJoinPool pool = new ForkJoinPool();
+		Set<Path> paths = new HashSet<>(); // suppresses duplicate paths
 		List<Future<ModCandidate>> futures = new ArrayList<>();
 
 		ModCandidateConsumer taskSubmitter = (path, requiresRemap) -> {
-			futures.add(pool.submit(new ModScanTask(path, requiresRemap)));
+			path = path.toAbsolutePath().normalize();
+
+			if (paths.add(path)) {
+				futures.add(pool.submit(new ModScanTask(path, requiresRemap)));
+			}
 		};
 
 		for (ModCandidateFinder finder : candidateFinders) {

src/main/java/net/fabricmc/loader/impl/game/GameProvider.java

@@ -24,6 +24,7 @@
 import net.fabricmc.api.EnvType;
 import net.fabricmc.loader.api.metadata.ModMetadata;
 import net.fabricmc.loader.impl.game.patch.GameTransformer;
+import net.fabricmc.loader.impl.util.Arguments;
 
 public interface GameProvider {
 	String getGameId();
@@ -42,6 +43,7 @@ public interface GameProvider {
 	GameTransformer getEntrypointTransformer();
 	void launch(ClassLoader loader);
 
+	Arguments getArguments();
 	String[] getLaunchArguments(boolean sanitize);
 
 	default boolean canOpenErrorGui() {

src/main/java/net/fabricmc/loader/impl/game/minecraft/MinecraftGameProvider.java

@@ -36,7 +36,6 @@
 import net.fabricmc.loader.impl.game.minecraft.patch.EntrypointPatch;
 import net.fabricmc.loader.impl.game.minecraft.patch.EntrypointPatchFML125;
 import net.fabricmc.loader.impl.game.patch.GameTransformer;
-import net.fabricmc.loader.impl.launch.FabricLauncherBase;
 import net.fabricmc.loader.impl.metadata.BuiltinModMetadata;
 import net.fabricmc.loader.impl.metadata.ModDependencyImpl;
 import net.fabricmc.loader.impl.util.Arguments;
@@ -110,7 +109,7 @@ public Path getLaunchDirectory() {
 			return new File(".").toPath();
 		}
 
-		return FabricLauncherBase.getLaunchDirectory(arguments).toPath();
+		return getLaunchDirectory(arguments).toPath();
 	}
 
 	@Override
@@ -166,38 +165,75 @@ public boolean locateGame(EnvType envType, String[] args, ClassLoader loader) {
 		if (version == null) version = System.getProperty(SystemProperties.GAME_VERSION);
 		versionData = McVersionLookup.getVersion(gameJar, entrypointClasses, version);
 
-		FabricLauncherBase.processArgumentMap(arguments, envType);
+		processArgumentMap(arguments, envType);
 
 		return true;
 	}
 
+	private static void processArgumentMap(Arguments argMap, EnvType envType) {
+		switch (envType) {
+		case CLIENT:
+			if (!argMap.containsKey("accessToken")) {
+				argMap.put("accessToken", "FabricMC");
+			}
+
+			if (!argMap.containsKey("version")) {
+				argMap.put("version", "Fabric");
+			}
+
+			String versionType = "";
+
+			if (argMap.containsKey("versionType") && !argMap.get("versionType").equalsIgnoreCase("release")) {
+				versionType = argMap.get("versionType") + "/";
+			}
+
+			argMap.put("versionType", versionType + "Fabric");
+
+			if (!argMap.containsKey("gameDir")) {
+				argMap.put("gameDir", getLaunchDirectory(argMap).getAbsolutePath());
+			}
+
+			break;
+		case SERVER:
+			argMap.remove("version");
+			argMap.remove("gameDir");
+			argMap.remove("assetsDir");
+			break;
+		}
+	}
+
+	private static File getLaunchDirectory(Arguments argMap) {
+		return new File(argMap.getOrDefault("gameDir", "."));
+	}
+
 	@Override
-	public String[] getLaunchArguments(boolean sanitize) {
-		if (arguments != null) {
-			List<String> list = new ArrayList<>(Arrays.asList(arguments.toArray()));
+	public Arguments getArguments() {
+		return arguments;
+	}
 
-			if (sanitize) {
-				int remove = 0;
-				Iterator<String> iterator = list.iterator();
+	@Override
+	public String[] getLaunchArguments(boolean sanitize) {
+		if (arguments == null) return new String[0];
+		if (!sanitize) return arguments.toArray();
 
-				while (iterator.hasNext()) {
-					String next = iterator.next();
+		List<String> list = new ArrayList<>(Arrays.asList(arguments.toArray()));
+		int remove = 0;
+		Iterator<String> iterator = list.iterator();
 
-					if ("--accessToken".equals(next)) {
-						remove = 2;
-					}
+		while (iterator.hasNext()) {
+			String next = iterator.next();
 
-					if (remove > 0) {
-						iterator.remove();
-						remove--;
-					}
-				}
+			if ("--accessToken".equals(next)) {
+				remove = 2;
 			}
 
-			return list.toArray(new String[0]);
+			if (remove > 0) {
+				iterator.remove();
+				remove--;
+			}
 		}
 
-		return new String[0];
+		return list.toArray(new String[0]);
 	}
 
 	@Override