Allow for declaring an oldForkCommit to bring in objects from the active fork for 3 way apply.

paperweight-core/src/main/kotlin/io/papermc/paperweight/core/PaperweightCore.kt

@@ -28,6 +28,7 @@ import io.papermc.paperweight.core.extension.PaperweightCoreExtension
 import io.papermc.paperweight.core.taskcontainers.CoreTasks
 import io.papermc.paperweight.core.taskcontainers.DevBundleTasks
 import io.papermc.paperweight.core.taskcontainers.PaperclipTasks
+import io.papermc.paperweight.core.tasks.patching.ApplyFilePatches
 import io.papermc.paperweight.core.tasks.patchroulette.PatchRouletteTasks
 import io.papermc.paperweight.core.util.coreExt
 import io.papermc.paperweight.tasks.*
@@ -197,6 +198,26 @@ abstract class PaperweightCore : Plugin<Project> {
                     coreExt.paper.rejectsDir,
                     layout.projectDirectory.dir("src/minecraft/java"),
                 )
+            } else if (coreExt.activeFork.isPresent && coreExt.updatingMinecraft.oldForkCommit.isPresent) {
+                // old commit fetching for forks through a gradle property
+                target.tasks.named<ApplyFilePatches>("applyMinecraftSourcePatches").configure {
+                    additionalRemote = coreExt.activeFork.map { fork ->
+                        layout.cache
+                            .resolve(
+                                "$PAPER_PATH/old${fork.name.capitalized()}/${coreExt.updatingMinecraft.oldForkCommit.get()}/${fork.name}-server/src/minecraft/java"
+                            )
+                            .absolutePathString()
+                    }
+                    emitRejects = false
+                }
+
+                PatchRouletteTasks(
+                    target,
+                    coreExt.activeFork.get().name.lowercase(), // TODO: make this lazy
+                    coreExt.minecraftVersion,
+                    coreExt.activeFork.flatMap { it.rejectsDir },
+                    layout.projectDirectory.dir("src/minecraft/java"),
+                )
             }
         }
     }

paperweight-core/src/main/kotlin/io/papermc/paperweight/core/extension/UpdatingMinecraftExtension.kt

@@ -26,4 +26,5 @@ import org.gradle.api.provider.Property
 
 interface UpdatingMinecraftExtension {
     val oldPaperCommit: Property<String>
+    val oldForkCommit: Property<String>
 }

paperweight-core/src/main/kotlin/io/papermc/paperweight/core/taskcontainers/MinecraftPatchingTasks.kt

@@ -30,6 +30,7 @@ import io.papermc.paperweight.core.tasks.patching.ApplyFilePatches
 import io.papermc.paperweight.core.tasks.patching.ApplyFilePatchesFuzzy
 import io.papermc.paperweight.core.tasks.patching.FixupFilePatches
 import io.papermc.paperweight.core.tasks.patching.RebuildFilePatches
+import io.papermc.paperweight.core.util.coreExt
 import io.papermc.paperweight.tasks.*
 import io.papermc.paperweight.util.*
 import io.papermc.paperweight.util.constants.*
@@ -167,6 +168,11 @@ class MinecraftPatchingTasks(
             outputDir.set(layout.cache.resolve(paperTaskOutput()))
             identifier.set(configName)
 
+            if (namePart == "Minecraft") {
+                oldCommit.convention(project.coreExt.updatingMinecraft.oldForkCommit)
+                oldOutputDir.set(layout.cache.resolve("$PAPER_PATH/old${configName.capitalized()}"))
+            }
+
             libraryImports.set(importLibFiles.flatMap { it.outputDir })
             atFile.set(mergeCollectedAts.flatMap { it.outputFile })
             ats.jst.from(project.configurations.named(JST_CONFIG))

paperweight-core/src/main/kotlin/io/papermc/paperweight/core/tasks/SetupForkMinecraftSources.kt

@@ -22,12 +22,17 @@
 
 package io.papermc.paperweight.core.tasks
 
+import io.papermc.paperweight.PaperweightException
 import io.papermc.paperweight.core.util.ApplySourceATs
+import io.papermc.paperweight.core.util.coreExt
 import io.papermc.paperweight.tasks.*
 import io.papermc.paperweight.util.*
 import io.papermc.paperweight.util.constants.paperTaskOutput
+import java.util.concurrent.TimeUnit
 import kotlin.io.path.*
 import org.eclipse.jgit.api.Git
+import org.eclipse.jgit.api.ResetCommand
+import org.eclipse.jgit.transport.URIish
 import org.gradle.api.file.DirectoryProperty
 import org.gradle.api.file.RegularFileProperty
 import org.gradle.api.provider.Property
@@ -49,6 +54,10 @@ abstract class SetupForkMinecraftSources : JavaLauncherTask() {
     @get:OutputDirectory
     abstract val outputDir: DirectoryProperty
 
+    @get:OutputDirectory
+    @get:Optional
+    abstract val oldOutputDir: DirectoryProperty
+
     @get:Internal
     abstract val atWorkingDir: DirectoryProperty
 
@@ -66,9 +75,17 @@ abstract class SetupForkMinecraftSources : JavaLauncherTask() {
     @get:Input
     abstract val identifier: Property<String>
 
+    @get:Internal
+    abstract val forkName: Property<String>
+
+    @get:Input
+    @get:Optional
+    abstract val oldCommit: Property<String>
+
     override fun init() {
         super.init()
         atWorkingDir.set(layout.cache.resolve(paperTaskOutput(name = "${name}_atWorkingDir")))
+        forkName.convention(project.coreExt.activeFork.map { it.name.capitalized() })
     }
 
     @TaskAction
@@ -78,6 +95,10 @@ abstract class SetupForkMinecraftSources : JavaLauncherTask() {
 
         val git = Git.open(outputDir.path.toFile())
 
+        if (oldCommit.isPresent) {
+            setupOld()
+        }
+
         if (atFile.isPresent && atFile.path.readText().isNotBlank()) {
             println("Applying access transformers...")
             ats.run(
@@ -104,4 +125,82 @@ abstract class SetupForkMinecraftSources : JavaLauncherTask() {
 
         git.close()
     }
+
+    private fun setupOld() {
+        val name = forkName.get()
+        logger.lifecycle("Setting up $name commit ${oldCommit.get()} to use as base for 3-way apply...")
+
+        val rootProjectDir = layout.projectDirectory.dir("../").path
+        val oldDir = oldOutputDir.get().path.resolve(oldCommit.get())
+        val oldLog = oldOutputDir.get().path.resolve("${oldCommit.get()}.log")
+
+        val oldGit: Git
+        if (oldDir.exists()) {
+            oldGit = Git.open(oldDir.toFile())
+        } else {
+            oldDir.createParentDirectories()
+            oldGit = Git.init()
+                .setDirectory(oldDir.toFile())
+                .setInitialBranch("main")
+                .call()
+            oldGit.remoteRemove().setRemoteName("origin").call()
+            oldGit.remoteAdd().setName("origin").setUri(URIish(rootProjectDir.absolutePathString())).call()
+        }
+
+        val upstream = Git.open(rootProjectDir.toFile())
+        val upstreamConfig = upstream.repository.config
+        val upstreamReachableSHA1 = upstreamConfig.getString("uploadpack", null, "allowreachablesha1inwant")
+        val upstreamConfigContainsUploadPack = upstreamConfig.sections.contains("uploadpack")
+        try {
+            // Temporarily allow fetching reachable sha1 refs from the "upstream" repository.
+            upstreamConfig.setBoolean("uploadpack", null, "allowreachablesha1inwant", true)
+            upstreamConfig.save()
+            oldGit.fetch().setDepth(1).setRemote("origin").setRefSpecs(oldCommit.get()).call()
+            oldGit.reset().setMode(ResetCommand.ResetType.HARD).setRef(oldCommit.get()).call()
+        } finally {
+            if (upstreamReachableSHA1 == null) {
+                if (upstreamConfigContainsUploadPack) {
+                    upstreamConfig.unset("uploadpack", null, "allowreachablesha1inwant")
+                } else {
+                    upstreamConfig.unsetSection("uploadpack", null)
+                }
+            } else {
+                upstreamConfig.setString("uploadpack", null, "allowreachablesha1inwant", upstreamReachableSHA1)
+            }
+            upstreamConfig.save()
+            upstream.close()
+        }
+
+        oldGit.close()
+
+        val isWindows = System.getProperty("os.name").lowercase().contains("win")
+        oldLog.outputStream().use { logOut ->
+            val args = arrayOf(
+                "applyAllPatches",
+                "--console",
+                "plain",
+                "--stacktrace",
+                "-Dpaperweight.debug=true"
+            )
+            val command = if (isWindows) {
+                listOf("cmd.exe", "/C", "gradlew.bat " + args.joinToString(" "))
+            } else {
+                listOf("./gradlew", *args)
+            }
+            val processBuilder = ProcessBuilder(command)
+            processBuilder.directory(oldDir)
+            val process = processBuilder.start()
+
+            val outFuture = redirect(process.inputStream, logOut)
+            val errFuture = redirect(process.errorStream, logOut)
+
+            val exit = process.waitFor()
+            outFuture.get(500L, TimeUnit.MILLISECONDS)
+            errFuture.get(500L, TimeUnit.MILLISECONDS)
+
+            if (exit != 0) {
+                throw PaperweightException("Failed to apply old $name, see log at $oldLog")
+            }
+        }
+    }
 }

paperweight-lib/src/main/kotlin/io/papermc/paperweight/util/constants/constants.kt

@@ -65,7 +65,7 @@ const val PLUGIN_REMAPPER_REPO_NAME = "paperweightPluginRemapperRepository"
 const val MACHE_REPO_NAME = "paperweightMacheRepository"
 
 const val CACHE_PATH = "caches"
-private const val PAPER_PATH = "paperweight"
+const val PAPER_PATH = "paperweight"
 
 const val LOCK_DIR = "$PAPER_PATH/lock"
 const val USERDEV_SETUP_LOCK = "$LOCK_DIR/userdev/setup.lock"