First shot at fixing the configuration cache / remote build cache conflict.

Author: nedtwigg

lib/src/main/java/com/diffplug/spotless/ConfigurationCacheHack.java

@@ -0,0 +1,87 @@
+/*
+ * Copyright 2024 DiffPlug
+ *
+ * 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 com.diffplug.spotless;
+
+import java.io.IOException;
+import java.util.AbstractList;
+import java.util.ArrayList;
+import java.util.Collection;
+
+/**
+ * Gradle requires three things:
+ * - Gradle defines cache equality based on your serialized representation
+ * - Combined with remote build cache, you cannot have any absolute paths in your serialized representation
+ * - Combined with configuration cache, you must be able to roundtrip yourself through serialization
+ *
+ * These requirements are at odds with each other, as described in these issues
+ * - Gradle issue to define custom equality https://github.com/gradle/gradle/issues/29816
+ * - Spotless plea for developer cache instead of configuration cache https://github.com/diffplug/spotless/issues/987
+ * - Spotless cache miss bug fixed by this class https://github.com/diffplug/spotless/issues/2168
+ *
+ * The point of this class is to create containers which can optimize the serialized representation for either
+ * - roundtrip integrity
+ * - equality
+ *
+ * It is a horrific hack, but it works, and it's the only way I can figure
+ * to make Spotless work with all of Gradle's cache systems.
+ */
+public class ConfigurationCacheHack {
+	static boolean SERIALIZE_FOR_ROUNDTRIP = false;
+
+	public enum OptimizeFor {
+		ROUNDTRIP, EQUALITY,
+	}
+
+	public static class StepList extends AbstractList<FormatterStep> {
+		private final boolean optimizeForEquality;
+		private transient ArrayList<FormatterStep> backingList = new ArrayList<>();
+
+		public StepList(OptimizeFor optimizeFor) {
+			this.optimizeForEquality = optimizeFor == OptimizeFor.EQUALITY;
+		}
+
+		@Override
+		public void clear() {
+			backingList.clear();
+		}
+
+		@Override
+		public boolean addAll(Collection<? extends FormatterStep> c) {
+			return backingList.addAll(c);
+		}
+
+		private void writeObject(java.io.ObjectOutputStream out) throws IOException {
+			out.defaultWriteObject();
+			SERIALIZE_FOR_ROUNDTRIP = !optimizeForEquality;
+			out.writeObject(backingList);
+		}
+
+		private void readObject(java.io.ObjectInputStream in) throws IOException, ClassNotFoundException {
+			in.defaultReadObject();
+			backingList = (ArrayList<FormatterStep>) in.readObject();
+		}
+
+		@Override
+		public FormatterStep get(int index) {
+			return backingList.get(index);
+		}
+
+		@Override
+		public int size() {
+			return backingList.size();
+		}
+	}
+}

lib/src/main/java/com/diffplug/spotless/Formatter.java

@@ -35,6 +35,8 @@
 
 import javax.annotation.Nullable;
 
+import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
+
 /** Formatter which performs the full formatting. */
 public final class Formatter implements Serializable, AutoCloseable {
 	private static final long serialVersionUID = 1L;
@@ -57,11 +59,13 @@ private Formatter(String name, LineEnding.Policy lineEndingsPolicy, Charset enco
 	}
 
 	// override serialize output
+	@SuppressFBWarnings("ST_WRITE_TO_STATIC_FROM_INSTANCE_METHOD")
 	private void writeObject(ObjectOutputStream out) throws IOException {
 		out.writeObject(name);
 		out.writeObject(lineEndingsPolicy);
 		out.writeObject(encoding.name());
 		out.writeObject(rootDir.toString());
+		ConfigurationCacheHack.SERIALIZE_FOR_ROUNDTRIP = true;
 		out.writeObject(steps);
 		out.writeObject(exceptionPolicy);
 	}

lib/src/main/java/com/diffplug/spotless/FormatterStepSerializationRoundtrip.java

@@ -26,6 +26,7 @@ class FormatterStepSerializationRoundtrip<RoundtripState extends Serializable, E
 	private final String name;
 	private final transient ThrowingEx.Supplier<RoundtripState> initializer;
 	private @Nullable RoundtripState roundtripStateInternal;
+	private @Nullable EqualityState equalityStateInternal;
 	private final SerializedFunction<RoundtripState, EqualityState> equalityStateExtractor;
 	private final SerializedFunction<EqualityState, FormatterFunc> equalityStateToFormatter;
 
@@ -43,10 +44,13 @@ public String getName() {
 
 	@Override
 	protected EqualityState stateSupplier() throws Exception {
-		if (roundtripStateInternal == null) {
-			roundtripStateInternal = initializer.get();
+		if (equalityStateInternal == null) {
+			if (roundtripStateInternal == null) {
+				roundtripStateInternal = initializer.get();
+			}
+			equalityStateInternal = equalityStateExtractor.apply(roundtripStateInternal);
 		}
-		return equalityStateExtractor.apply(roundtripStateInternal);
+		return equalityStateInternal;
 	}
 
 	@Override
@@ -56,8 +60,14 @@ protected FormatterFunc stateToFormatter(EqualityState equalityState) throws Exc
 
 	// override serialize output
 	private void writeObject(java.io.ObjectOutputStream out) throws IOException {
-		if (roundtripStateInternal == null) {
-			roundtripStateInternal = ThrowingEx.get(initializer::get);
+		if (ConfigurationCacheHack.SERIALIZE_FOR_ROUNDTRIP) {
+			if (roundtripStateInternal == null) {
+				roundtripStateInternal = ThrowingEx.get(initializer::get);
+			}
+			equalityStateInternal = null;
+		} else {
+			equalityStateInternal = ThrowingEx.get(this::stateSupplier);
+			roundtripStateInternal = null;
 		}
 		out.defaultWriteObject();
 	}

plugin-gradle/src/main/java/com/diffplug/gradle/spotless/SpotlessTask.java

@@ -17,7 +17,6 @@
 
 import java.io.File;
 import java.nio.charset.Charset;
-import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
 import java.util.Locale;
@@ -37,6 +36,7 @@
 import org.gradle.api.tasks.PathSensitivity;
 import org.gradle.work.Incremental;
 
+import com.diffplug.spotless.ConfigurationCacheHack;
 import com.diffplug.spotless.FormatExceptionPolicy;
 import com.diffplug.spotless.FormatExceptionPolicyStrict;
 import com.diffplug.spotless.Formatter;
@@ -150,17 +150,25 @@ public File getOutputDirectory() {
 		return outputDirectory;
 	}
 
-	protected final List<FormatterStep> steps = new ArrayList<>();
+	private final List<FormatterStep> stepsInternalRoundtrip = new ConfigurationCacheHack.StepList(ConfigurationCacheHack.OptimizeFor.ROUNDTRIP);
+	private final List<FormatterStep> stepsInternalEquality = new ConfigurationCacheHack.StepList(ConfigurationCacheHack.OptimizeFor.EQUALITY);
+
+	@Internal
+	public List<FormatterStep> getStepsInternalRoundtrip() {
+		return Collections.unmodifiableList(stepsInternalRoundtrip);
+	}
 
 	@Input
-	public List<FormatterStep> getSteps() {
-		return Collections.unmodifiableList(steps);
+	public List<FormatterStep> getStepsInternalEquality() {
+		return Collections.unmodifiableList(stepsInternalEquality);
 	}
 
 	public void setSteps(List<FormatterStep> steps) {
 		PluginGradlePreconditions.requireElementsNonNull(steps);
-		this.steps.clear();
-		this.steps.addAll(steps);
+		this.stepsInternalRoundtrip.clear();
+		this.stepsInternalEquality.clear();
+		this.stepsInternalRoundtrip.addAll(steps);
+		this.stepsInternalEquality.addAll(steps);
 	}
 
 	/** Returns the name of this format. */
@@ -179,7 +187,7 @@ Formatter buildFormatter() {
 				.lineEndingsPolicy(getLineEndingsPolicy().get())
 				.encoding(Charset.forName(encoding))
 				.rootDir(getProjectDir().get().getAsFile().toPath())
-				.steps(steps)
+				.steps(stepsInternalRoundtrip)
 				.exceptionPolicy(exceptionPolicy)
 				.build();
 	}