fix generateDiff task when target version doesn't exist

buildSrc/src/main/java/quilt/internal/plugin/ProcessMappingsPlugin.java

@@ -133,7 +133,7 @@ protected ProcessMappingsExtension applyImpl(@NotNull Project project) {
                     insertAutoGeneratedMappings.flatMap(AddProposedMappingsTask::getOutputMappings)
                 ));
 
-                // TODO LATER move this to build/ once generate-diff.yml uses generateDiff
+                // TODO LATER move this to build/ once generate-diff.yml uses lazilyDiffTarget
                 task.getOutput().convention(this.getProjectDir().dir("namedSrc"));
             }
         );

buildSrc/src/main/java/quilt/internal/plugin/TargetDiffPlugin.java

@@ -27,8 +27,10 @@
 import quilt.internal.task.decompile.DecompileVineflowerTask;
 import quilt.internal.task.diff.DecompileTargetVineflowerTask;
 import quilt.internal.task.diff.DiffDirectoriesTask;
+import quilt.internal.task.diff.DiffTargetTask;
 import quilt.internal.task.diff.DownloadTargetMappingJarTask;
 import quilt.internal.task.diff.ExtractTargetMappingJarTask;
+import quilt.internal.task.diff.LazilyDiffTargetTask;
 import quilt.internal.task.diff.RemapTargetMinecraftJarTask;
 import quilt.internal.task.diff.RemapTargetUnpickDefinitionsTask;
 import quilt.internal.task.diff.TargetVersionConsumingTask;
@@ -39,15 +41,17 @@
 
 import java.io.FileReader;
 import java.io.IOException;
+import java.util.Collections;
 
 import static quilt.internal.constants.Constants.UNPICK_NAME;
 import static quilt.internal.task.build.MappingsV2JarTask.JAR_MAPPINGS_PATH;
 
 /**
  * {@linkplain TaskContainer#register Registers} tasks that download the latest published Quilt Mappings for the current
  * {@link QuiltMappingsExtension#getMinecraftVersion() minecraftVersion} so the
- * {@value DiffDirectoriesTask#GENERATE_DIFF_TASK_NAME} task can {@value DiffDirectoriesTask#DIFF_COMMAND}
- * them with this project's mappings.
+ * {@value DiffTargetTask#DIFF_TARGET_TASK_NAME}
+ * (or {@value LazilyDiffTargetTask#LAZILY_DIFF_TARGET_TASK_NAME}) task can
+ * {@value DiffDirectoriesTask#DIFF_COMMAND} them with this project's mappings.
  * <p>
  * The generated {@value DiffDirectoriesTask#DIFF_COMMAND} is useful when reviewing new mappings.
  * <p>
@@ -231,15 +235,14 @@ public void apply(@NotNull Project project) {
                         .map(dest -> dest.file(JAR_MAPPINGS_PATH))
                 ));
 
-                // TODO LATER move this to build/ once generate-diff.yml uses generateDiff
+                // TODO LATER move this to build/ once generate-diff.yml uses lazilyDiffTarget
                 task.getOutput().convention(this.getProjectDir().dir("namedTargetSrc"));
             }
         );
 
-        // TODO LATER use this in generate-diff.yml
-        tasks.register(
-            DiffDirectoriesTask.GENERATE_DIFF_TASK_NAME,
-            DiffDirectoriesTask.class,
+        final var diffTarget = tasks.register(
+            DiffTargetTask.DIFF_TARGET_TASK_NAME,
+            DiffTargetTask.class,
             task -> {
                 task.getAdditionalArgs().add("-bur");
 
@@ -250,6 +253,26 @@ public void apply(@NotNull Project project) {
                 task.getDest().convention(this.getBuildDir().file("target.diff"));
             }
         );
+
+        // TODO LATER use this in generate-diff.yml
+        tasks.register(
+            LazilyDiffTargetTask.LAZILY_DIFF_TARGET_TASK_NAME,
+            LazilyDiffTargetTask.class,
+            task -> {
+                // This provider is safe to get when dependOn is evaluated because it comes from a ValueSource.
+                // Configuring diffTarget's targetVersion to a provider mapped from a task output would break this.
+                task.dependsOn(this.getProviders().provider(() -> {
+                    final Property<String> targetVersion = diffTarget.get().getTargetVersion();
+                    targetVersion.finalizeValue();
+
+                    if (targetVersion.isPresent()) {
+                        return diffTarget;
+                    } else {
+                        return Collections.emptyList();
+                    }
+                }));
+            }
+        );
     }
 
     public Provider<Directory> getTargetsBuildDir() {

buildSrc/src/main/java/quilt/internal/task/diff/DecompileTargetVineflowerTask.java

@@ -8,7 +8,7 @@
  * @see TargetDiffPlugin TargetDiffPlugin's configureEach
  */
 public abstract class DecompileTargetVineflowerTask extends DecompileVineflowerTask implements
-        TargetVersionConsumingTask {
+        UnpickVersionsMatchConsumingTask {
     /**
      * {@linkplain TaskContainer#register Registered} by {@link TargetDiffPlugin}.
      */

buildSrc/src/main/java/quilt/internal/task/diff/DiffDirectoriesTask.java

@@ -14,12 +14,11 @@
 import org.gradle.api.tasks.PathSensitive;
 import org.gradle.api.tasks.PathSensitivity;
 import org.gradle.api.tasks.TaskAction;
-import org.gradle.api.tasks.TaskContainer;
 import org.gradle.api.tasks.options.Option;
 import quilt.internal.constants.Groups;
-import quilt.internal.plugin.TargetDiffPlugin;
 
 import java.io.File;
+import java.io.FileNotFoundException;
 import java.io.FileOutputStream;
 import java.io.IOException;
 import java.util.ArrayList;
@@ -39,11 +38,6 @@
  */
 @CacheableTask
 public abstract class DiffDirectoriesTask extends Exec {
-    /**
-     * {@linkplain TaskContainer#register Registered} by {@link TargetDiffPlugin}.
-     */
-    public static final String GENERATE_DIFF_TASK_NAME = "generateDiff";
-
     public static final String DIFF_COMMAND = "diff";
 
     private static final String DIFF_COMMAND_PHRASE = DIFF_COMMAND + " command";
@@ -115,19 +109,24 @@ public DiffDirectoriesTask() {
     @Override
     @TaskAction
     public void exec() {
+        final File dest;
         try {
-            final File dest = this.getDest().get().getAsFile();
+            dest = this.getDest().get().getAsFile();
 
             dest.getParentFile().mkdirs();
 
             dest.createNewFile();
-
-            this.setStandardOutput(new FileOutputStream(dest.getAbsolutePath()));
         } catch (IOException e) {
-            throw new GradleException("Failed to access destination file", e);
+            throw new GradleException("Failed to create destination file", e);
         }
 
-        super.exec();
+        try (var out = new FileOutputStream(dest.getAbsolutePath())) {
+            this.setStandardOutput(out);
+
+            super.exec();
+        } catch (IOException e) {
+            throw new GradleException("Failed to write to destination", e);
+        }
 
         final int exitValue = this.getExecutionResult().get().getExitValue();
         switch (exitValue) {

buildSrc/src/main/java/quilt/internal/task/diff/DiffTargetTask.java

@@ -0,0 +1,11 @@
+package quilt.internal.task.diff;
+
+import org.gradle.api.tasks.TaskContainer;
+import quilt.internal.plugin.TargetDiffPlugin;
+
+public abstract class DiffTargetTask extends DiffDirectoriesTask implements UnpickVersionsMatchConsumingTask {
+    /**
+     * {@linkplain TaskContainer#register Registered} by {@link TargetDiffPlugin}.
+     */
+    public static final String DIFF_TARGET_TASK_NAME = "diffTarget";
+}

buildSrc/src/main/java/quilt/internal/task/diff/LazilyDiffTargetTask.java

@@ -0,0 +1,25 @@
+package quilt.internal.task.diff;
+
+import org.gradle.api.DefaultTask;
+import org.gradle.api.provider.Provider;
+import org.gradle.api.tasks.TaskContainer;
+import quilt.internal.constants.Groups;
+import quilt.internal.plugin.TargetDiffPlugin;
+
+public abstract class LazilyDiffTargetTask extends DefaultTask implements UnpickVersionsMatchConsumingTask {
+    /**
+     * {@linkplain TaskContainer#register Registered} by {@link TargetDiffPlugin}.
+     * <p>
+     * A wrapper for {@value DiffTargetTask#DIFF_TARGET_TASK_NAME} that conditionally depends on
+     * {@value DiffTargetTask#DIFF_TARGET_TASK_NAME} only if its
+     * {@link TargetVersionConsumingTask#getTargetVersion() targetVersion} {@link Provider#isPresent() isPresent}.
+     * <p>
+     * This is a hack to prevent unnecessarily generating sources using local mappings when
+     * {@link TargetVersionConsumingTask#getTargetVersion() targetVersion} isn't present.
+     */
+    public static final String LAZILY_DIFF_TARGET_TASK_NAME = "lazilyDiffTarget";
+
+    public LazilyDiffTargetTask() {
+        this.setGroup(Groups.DIFF);
+    }
+}