Symlink to JAR fails to load mod

[Lancelotbronner]

This could be an opportunity to install mods only once and link into different profiles, making players of multiple large modpacks not have to re-download the same mods over and over again.

I'd like to help with that but I don't have much experience with Java, if anyone could just point me in the general direction of where the mod discovery happens I'd love the help!

[Player3324]

Hardlinks and symlinks should already work, if not that's a bug. There's also a mechanism to provide the desired mods through a system properts or arg: #470 - directly or in a file listing them

[Lancelotbronner]

Alright, then I guess it's a bug? Here's the stack trace for my attempt:

net.fabricmc.loader.impl.FormattedException: net.fabricmc.loader.impl.discovery.ModResolutionException: Mod discovery failed!
	at net.fabricmc.loader.impl.FabricLoaderImpl.load(FabricLoaderImpl.java:189)
	at net.fabricmc.loader.impl.launch.knot.Knot.init(Knot.java:142)
	at net.fabricmc.loader.impl.launch.knot.Knot.launch(Knot.java:71)
	at net.fabricmc.loader.impl.launch.knot.KnotClient.main(KnotClient.java:23)
Caused by: net.fabricmc.loader.impl.discovery.ModResolutionException: Mod discovery failed!
	at net.fabricmc.loader.impl.discovery.ModDiscoverer.lambda$discoverMods$1(ModDiscoverer.java:122)
	at net.fabricmc.loader.impl.util.ExceptionUtil.gatherExceptions(ExceptionUtil.java:33)
	at net.fabricmc.loader.impl.discovery.ModDiscoverer.discoverMods(ModDiscoverer.java:122)
	at net.fabricmc.loader.impl.FabricLoaderImpl.setup(FabricLoaderImpl.java:204)
	at net.fabricmc.loader.impl.FabricLoaderImpl.load(FabricLoaderImpl.java:187)
	... 3 more
Caused by: java.lang.RuntimeException: java.lang.RuntimeException: Error analyzing /Users/lancelot/Library/Application Support/CurseForge/Content/Minecraft/Instances/test/mods/Decorative Blocks-fabric-1.18.1-2.0.4.jar: java.util.zip.ZipException: zip END header not found
	at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
	at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:77)
	at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
	at java.base/java.lang.reflect.Constructor.newInstanceWithCaller(Constructor.java:499)
	at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:480)
	at java.base/java.util.concurrent.ForkJoinTask.getThrowableException(ForkJoinTask.java:562)
	at java.base/java.util.concurrent.ForkJoinTask.reportExecutionException(ForkJoinTask.java:604)
	at java.base/java.util.concurrent.ForkJoinTask.get(ForkJoinTask.java:981)
	at net.fabricmc.loader.impl.discovery.ModDiscoverer.discoverMods(ModDiscoverer.java:119)
	... 5 more
Caused by: java.lang.RuntimeException: Error analyzing /Users/lancelot/Library/Application Support/CurseForge/Content/Minecraft/Instances/test/mods/Decorative Blocks-fabric-1.18.1-2.0.4.jar: java.util.zip.ZipException: zip END header not found
	at net.fabricmc.loader.impl.discovery.ModDiscoverer$ModScanTask.compute(ModDiscoverer.java:218)
	at net.fabricmc.loader.impl.discovery.ModDiscoverer$ModScanTask.compute(ModDiscoverer.java:175)
	at java.base/java.util.concurrent.RecursiveTask.exec(RecursiveTask.java:100)
	at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:373)
	at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1182)
	at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1655)
	at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1622)
	at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:165)
Caused by: java.util.zip.ZipException: zip END header not found
	at java.base/java.util.zip.ZipFile$Source.findEND(ZipFile.java:1469)
	at java.base/java.util.zip.ZipFile$Source.initCEN(ZipFile.java:1477)
	at java.base/java.util.zip.ZipFile$Source.<init>(ZipFile.java:1315)
	at java.base/java.util.zip.ZipFile$Source.get(ZipFile.java:1277)
	at java.base/java.util.zip.ZipFile$CleanableResource.<init>(ZipFile.java:709)
	at java.base/java.util.zip.ZipFile.<init>(ZipFile.java:243)
	at java.base/java.util.zip.ZipFile.<init>(ZipFile.java:172)
	at java.base/java.util.zip.ZipFile.<init>(ZipFile.java:186)
	at net.fabricmc.loader.impl.discovery.ModDiscoverer$ModScanTask.computeJarFile(ModDiscoverer.java:237)
	at net.fabricmc.loader.impl.discovery.ModDiscoverer$ModScanTask.compute(ModDiscoverer.java:213)
	... 7 more

Seems to come from this method:

@Override
protected ModCandidate compute() {
	if (is != null) { // nested jar
		try {
			return computeJarStream();
		} catch (ParseMetadataException e) { // already contains all context
			throw ExceptionUtil.wrap(e);
		} catch (Throwable t) {
			throw new RuntimeException(String.format("Error analyzing nested jar %s from %s: %s", localPath, parentPaths, t), t);
		}
	} else { // regular classes-dir or jar
		try {
			if (paths.size() != 1 || Files.isDirectory(paths.get(0))) {
				return computeDir();
			} else {
				return computeJarFile();
			}
		} catch (ParseMetadataException e) { // already contains all context
			throw ExceptionUtil.wrap(e);
		} catch (Throwable t) {
			throw new RuntimeException(String.format("Error analyzing %s: %s", paths, t), t);
		}
	}
}

[Player3324]

that error typically means that the file is not a jar/zip file. If you use a high level linking mechanism like windows shortcuts those won't work. Symlinks are a lower level mechanism.

[Lancelotbronner]

It was macOS aliases, which I’m guessing are equally high-level, is there a way we could support it? (I’m willing to help)

If not I guess I’ll learn about how lower-level symlink work.

[Player3324]

symlinks are created with ln -s, osx aliases are different indeed and not very approachable through java or other cross platform tools.

[Lancelotbronner]

Alright, thanks a lot for the explanation and sorry to bother!