Fix and test jar processor caching.

src/main/java/net/fabricmc/loom/configuration/accesswidener/AccessWidenerEntry.java

@@ -42,5 +42,7 @@ public interface AccessWidenerEntry {
 	@Nullable
 	String mappingId();
 
+	String getSortKey();
+
 	void read(AccessWidenerVisitor visitor, LazyCloseable<TinyRemapper> remapper) throws IOException;
 }

src/main/java/net/fabricmc/loom/configuration/accesswidener/AccessWidenerJarProcessor.java

@@ -24,10 +24,13 @@
 
 package net.fabricmc.loom.configuration.accesswidener;
 
+import java.io.FileNotFoundException;
 import java.io.IOException;
+import java.io.UncheckedIOException;
+import java.nio.file.Files;
 import java.nio.file.Path;
 import java.util.ArrayList;
-import java.util.Collections;
+import java.util.Comparator;
 import java.util.List;
 
 import javax.inject.Inject;
@@ -62,8 +65,14 @@ public class AccessWidenerJarProcessor implements MinecraftJarProcessor<AccessWi
 		List<AccessWidenerEntry> accessWideners = new ArrayList<>();
 
 		if (localAccessWidenerProperty.isPresent()) {
+			Path path = localAccessWidenerProperty.get().getAsFile().toPath();
+
+			if (Files.notExists(path)) {
+				throw new UncheckedIOException(new FileNotFoundException("Could not find access widener file at {%s}".formatted(path)));
+			}
+
 			// Add the access widener specified in the extension
-			accessWideners.add(new LocalAccessWidenerEntry(localAccessWidenerProperty.get().getAsFile().toPath()));
+			accessWideners.add(LocalAccessWidenerEntry.create(path));
 		}
 
 		/* Uncomment to read all access wideners from local mods.
@@ -84,7 +93,7 @@ public class AccessWidenerJarProcessor implements MinecraftJarProcessor<AccessWi
 			return null;
 		}
 
-		return new Spec(Collections.unmodifiableList(accessWideners));
+		return new Spec(accessWideners.stream().sorted(Comparator.comparing(AccessWidenerEntry::getSortKey)).toList());
 	}
 
 	@Override

src/main/java/net/fabricmc/loom/configuration/accesswidener/LocalAccessWidenerEntry.java

@@ -25,6 +25,7 @@
 package net.fabricmc.loom.configuration.accesswidener;
 
 import java.io.IOException;
+import java.io.UncheckedIOException;
 import java.nio.file.Files;
 import java.nio.file.Path;
 
@@ -32,11 +33,20 @@ import org.jetbrains.annotations.Nullable;
 
 import net.fabricmc.accesswidener.AccessWidenerReader;
 import net.fabricmc.accesswidener.AccessWidenerVisitor;
+import net.fabricmc.loom.util.Checksum;
 import net.fabricmc.loom.util.LazyCloseable;
 import net.fabricmc.loom.util.fmj.ModEnvironment;
 import net.fabricmc.tinyremapper.TinyRemapper;
 
-public record LocalAccessWidenerEntry(Path path) implements AccessWidenerEntry {
+public record LocalAccessWidenerEntry(Path path, String hash) implements AccessWidenerEntry {
+	public static LocalAccessWidenerEntry create(Path path) {
+		try {
+			return new LocalAccessWidenerEntry(path, Checksum.sha1Hex(path));
+		} catch (IOException e) {
+			throw new UncheckedIOException("Failed to create LocalAccessWidenerEntry", e);
+		}
+	}
+
 	@Override
 	public void read(AccessWidenerVisitor visitor, LazyCloseable<TinyRemapper> remapper) throws IOException {
 		var reader = new AccessWidenerReader(visitor);
@@ -52,4 +62,14 @@ public record LocalAccessWidenerEntry(Path path) implements AccessWidenerEntry {
 	public @Nullable String mappingId() {
 		return null;
 	}
+
+	@Override
+	public String getSortKey() {
+		return "local";
+	}
+
+	@Override
+	public int hashCode() {
+		return hash.hashCode();
+	}
 }

src/main/java/net/fabricmc/loom/configuration/accesswidener/ModAccessWidenerEntry.java

@@ -61,6 +61,11 @@ public record ModAccessWidenerEntry(FabricModJson mod, String path, ModEnvironme
 		return transitiveOnly ? mod.getId() : null;
 	}
 
+	@Override
+	public String getSortKey() {
+		return mod.getId() + ":" + path;
+	}
+
 	@Override
 	public void read(AccessWidenerVisitor visitor, LazyCloseable<TinyRemapper> remapper) throws IOException {
 		if (transitiveOnly) {

src/main/java/net/fabricmc/loom/configuration/processors/MinecraftJarProcessorManager.java

@@ -34,10 +34,13 @@ import java.util.Collections;
 import java.util.Comparator;
 import java.util.List;
 import java.util.Objects;
+import java.util.StringJoiner;
 import java.util.stream.Collectors;
 
 import org.gradle.api.Project;
 import org.jetbrains.annotations.Nullable;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 import net.fabricmc.loom.LoomGradleExtension;
 import net.fabricmc.loom.api.processor.MappingProcessorContext;
@@ -49,6 +52,7 @@ import net.fabricmc.mappingio.tree.MemoryMappingTree;
 
 public final class MinecraftJarProcessorManager {
 	private static final String CACHE_VALUE_FILE_PATH = "META-INF/Loom-Jar-Processor-Cache";
+	private static final Logger LOGGER = LoggerFactory.getLogger(MinecraftJarProcessorManager.class);
 
 	private final List<ProcessorEntry<?>> jarProcessors;
 
@@ -73,14 +77,17 @@ public final class MinecraftJarProcessorManager {
 		List<ProcessorEntry<?>> entries = new ArrayList<>();
 
 		for (MinecraftJarProcessor<?> processor : processors) {
+			LOGGER.debug("Building processor spec for {}", processor.getName());
 			MinecraftJarProcessor.Spec spec = processor.buildSpec(context);
 
 			if (spec != null) {
+				LOGGER.debug("Adding processor entry for {}", processor.getName());
 				entries.add(new ProcessorEntry<>(processor, spec));
 			}
 		}
 
 		if (entries.isEmpty()) {
+			LOGGER.debug("No processor entries");
 			return null;
 		}
 
@@ -94,10 +101,23 @@ public final class MinecraftJarProcessorManager {
 				.collect(Collectors.joining("::"));
 	}
 
+	private String getDebugString() {
+		final StringJoiner sj = new StringJoiner("\n");
+
+		for (ProcessorEntry<?> jarProcessor : jarProcessors) {
+			sj.add(jarProcessor.name() + ":");
+			sj.add("\tHash: " + jarProcessor.hashCode());
+			sj.add("\tStr: " + jarProcessor.toString());
+		}
+
+		return sj.toString();
+	}
+
 	public boolean requiresProcessingJar(Path jar) {
 		Objects.requireNonNull(jar);
 
 		if (Files.notExists(jar)) {
+			LOGGER.debug("{} does not exist, generating", jar);
 			return true;
 		}
 
@@ -110,11 +130,23 @@ public final class MinecraftJarProcessorManager {
 		}
 
 		if (existingCache == null) {
+			LOGGER.info("{} does not contain a processor cache value, regenerating", jar);
 			return true;
 		}
 
 		final String existingCacheValue = new String(existingCache, StandardCharsets.UTF_8);
-		return !existingCacheValue.equals(getCacheValue());
+		final String expectedCacheValue = getCacheValue();
+		final boolean matches = existingCacheValue.equals(expectedCacheValue);
+
+		if (!matches) {
+			LOGGER.info("{} has an invalid cache, got {} expected {}", jar, existingCacheValue, expectedCacheValue);
+
+			if (LOGGER.isInfoEnabled()) {
+				LOGGER.info("Expected state: {}", getDebugString());
+			}
+		}
+
+		return !matches;
 	}
 
 	public void processJar(Path jar, ProcessorContext context) throws IOException {

src/main/java/net/fabricmc/loom/util/Checksum.java

@@ -62,7 +62,7 @@ public class Checksum {
 			HashCode hash = Files.asByteSource(file).hash(Hashing.sha256());
 			return hash.asBytes();
 		} catch (IOException e) {
-			throw new RuntimeException("Failed to get file hash");
+			throw new UncheckedIOException("Failed to get file hash", e);
 		}
 	}
 

src/main/java/net/fabricmc/loom/util/fmj/FabricModJson.java

@@ -33,8 +33,9 @@ import java.util.Objects;
 import com.google.gson.JsonElement;
 import com.google.gson.JsonObject;
 import org.jetbrains.annotations.Nullable;
+import org.jetbrains.annotations.VisibleForTesting;
 
-public abstract sealed class FabricModJson permits FabricModJsonV0, FabricModJsonV1, FabricModJsonV2 {
+public abstract sealed class FabricModJson permits FabricModJsonV0, FabricModJsonV1, FabricModJsonV2, FabricModJson.Mockable {
 	protected final JsonObject jsonObject;
 	private final FabricModJsonSource source;
 
@@ -59,4 +60,22 @@ public abstract sealed class FabricModJson permits FabricModJsonV0, FabricModJso
 	public final FabricModJsonSource getSource() {
 		return source;
 	}
+
+	@Override
+	public final String toString() {
+		return getClass().getName() + "[id=%s, version=%s, classTweakers=%s]".formatted(getId(), getVersion(), getClassTweakers());
+	}
+
+	@Override
+	public final int hashCode() {
+		return Objects.hash(getId(), getVersion());
+	}
+
+	@VisibleForTesting
+	public abstract non-sealed class Mockable extends FabricModJson {
+		private Mockable() {
+			super(null, null);
+			throw new AssertionError();
+		}
+	}
 }

src/main/java/net/fabricmc/loom/util/fmj/ModEnvironment.java

@@ -24,17 +24,21 @@
 
 package net.fabricmc.loom.util.fmj;
 
-public enum ModEnvironment {
-	UNIVERSAL(true, true),
-	CLIENT(true, false),
-	SERVER(false, true);
+import java.util.Objects;
+
+public final class ModEnvironment {
+	public static final ModEnvironment UNIVERSAL = new ModEnvironment(true, true, "universal");
+	public static final ModEnvironment CLIENT = new ModEnvironment(true, false, "client");
+	public static final ModEnvironment SERVER = new ModEnvironment(false, true, "server");
 
 	private final boolean client;
 	private final boolean server;
+	private final String name;
 
-	ModEnvironment(boolean client, boolean server) {
+	private ModEnvironment(boolean client, boolean server, String name) {
 		this.client = client;
 		this.server = server;
+		this.name = name;
 	}
 
 	public boolean isClient() {
@@ -44,4 +48,17 @@ public enum ModEnvironment {
 	public boolean isServer() {
 		return server;
 	}
+
+	@Override
+	public boolean equals(Object o) {
+		if (this == o) return true;
+		if (o == null || getClass() != o.getClass()) return false;
+		ModEnvironment that = (ModEnvironment) o;
+		return name.equals(that.name);
+	}
+
+	@Override
+	public int hashCode() {
+		return Objects.hash(name);
+	}
 }