Fix IPC on Forge 1.17.1 - 1.20.1 (#4187)

* Reapply "Implement a more robust IPC system between the launcher and client (#4159)"

This reverts commit e25d726da4.

* Put game JAR and theseus JAR ahead of other JARs in classpath

* Fix 1.17-1.20 Forge by forcefully removing Multi-Release manifest entry

* Fix formatting

---------

Co-authored-by: Prospector <6166773+Prospector@users.noreply.github.com>

packages/app-lib/java/build.gradle.kts

@@ -1,3 +1,14 @@
+import com.github.jengelman.gradle.plugins.shadow.tasks.ShadowCopyAction
+import com.github.jengelman.gradle.plugins.shadow.transformers.CacheableTransformer
+import com.github.jengelman.gradle.plugins.shadow.transformers.ResourceTransformer
+import com.github.jengelman.gradle.plugins.shadow.transformers.TransformerContext
+import org.apache.tools.zip.ZipEntry
+import org.apache.tools.zip.ZipOutputStream
+import java.io.IOException
+import java.util.jar.JarFile
+import java.util.jar.Attributes as JarAttributes
+import java.util.jar.Manifest as JarManifest
+
 plugins {
     java
     id("com.diffplug.spotless") version "7.0.4"
@@ -11,6 +22,7 @@ repositories {
 dependencies {
     implementation("org.ow2.asm:asm:9.8")
     implementation("org.ow2.asm:asm-tree:9.8")
+    implementation("com.google.code.gson:gson:2.13.1")
 
     testImplementation(libs.junit.jupiter)
     testRuntimeOnly("org.junit.platform:junit-platform-launcher")
@@ -46,6 +58,50 @@ tasks.shadowJar {
 
     enableRelocation = true
     relocationPrefix = "com.modrinth.theseus.shadow"
+
+    // Adapted from ManifestResourceTransformer to do one thing: remove Multi-Release.
+    // Multi-Release gets added by shadow because gson has Multi-Release set to true, however
+    // shadow strips the actual versions directory, as gson only has a module-info.class in there.
+    // However, older versions of SecureJarHandler crash if Multi-Release is set to true but the
+    // versions directory is missing.
+    transform(@CacheableTransformer object : ResourceTransformer {
+        private var manifestDiscovered = false
+        private var manifest: JarManifest? = null
+
+        override fun canTransformResource(element: FileTreeElement): Boolean {
+            return JarFile.MANIFEST_NAME.equals(element.path, ignoreCase = true)
+        }
+
+        override fun transform(context: TransformerContext) {
+            if (!manifestDiscovered) {
+                try {
+                    manifest = JarManifest(context.inputStream)
+                    manifestDiscovered = true
+                } catch (e: IOException) {
+                    logger.warn("Failed to read MANIFEST.MF", e)
+                }
+            }
+        }
+
+        override fun hasTransformedResource(): Boolean = true
+
+        override fun modifyOutputStream(
+            os: ZipOutputStream,
+            preserveFileTimestamps: Boolean
+        ) {
+            // If we didn't find a manifest, then let's create one.
+            if (manifest == null) {
+                manifest = JarManifest()
+            }
+
+            manifest!!.mainAttributes.remove(JarAttributes.Name.MULTI_RELEASE)
+
+            os.putNextEntry(ZipEntry(JarFile.MANIFEST_NAME).apply {
+                time = ShadowCopyAction.CONSTANT_TIME_FOR_ZIP_ENTRIES
+            })
+            manifest!!.write(os)
+        }
+    })
 }
 
 tasks.named<Test>("test") {