Remove JarProcessor#isInvalid and impl getId correctly.

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

@@ -30,7 +30,6 @@ import java.io.UncheckedIOException;
 import java.nio.file.Files;
 import java.nio.file.NoSuchFileException;
 import java.nio.file.Path;
-import java.util.Arrays;
 
 import com.google.common.hash.Hashing;
 import org.gradle.api.Project;
@@ -39,6 +38,7 @@ import net.fabricmc.accesswidener.AccessWidener;
 import net.fabricmc.accesswidener.AccessWidenerReader;
 import net.fabricmc.loom.LoomGradleExtension;
 import net.fabricmc.loom.configuration.processors.JarProcessor;
+import net.fabricmc.loom.util.Checksum;
 import net.fabricmc.loom.util.ZipUtils;
 
 public class AccessWidenerJarProcessor implements JarProcessor {
@@ -57,7 +57,7 @@ public class AccessWidenerJarProcessor implements JarProcessor {
 
 	@Override
 	public String getId() {
-		return "loom:access_widener";
+		return "loom:access_widener:" + Checksum.toHex(inputHash);
 	}
 
 	@Override
@@ -91,21 +91,4 @@ public class AccessWidenerJarProcessor implements JarProcessor {
 			throw new UncheckedIOException("Failed to write aw jar hash", e);
 		}
 	}
-
-	@Override
-	public boolean isInvalid(File file) {
-		byte[] hash;
-
-		try {
-			hash = ZipUtils.unpackNullable(file.toPath(), HASH_FILENAME);
-		} catch (IOException e) {
-			return true;
-		}
-
-		if (hash == null) {
-			return true;
-		}
-
-		return !Arrays.equals(inputHash, hash); // TODO how do we know if the current jar as the correct access applied? save the hash of the input?
-	}
 }

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

@@ -185,12 +185,6 @@ public class TransitiveAccessWidenerJarProcessor implements JarProcessor {
 		}
 	}
 
-	@Override
-	public boolean isInvalid(File file) {
-		// The hash is handled by getId()
-		return false;
-	}
-
 	private static class TransitiveDetectorVisitor implements AccessWidenerVisitor {
 		private boolean transitive = false;
 

src/main/java/net/fabricmc/loom/configuration/ifaceinject/InterfaceInjectionProcessor.java

@@ -29,7 +29,6 @@ import java.io.IOException;
 import java.nio.charset.StandardCharsets;
 import java.nio.file.Path;
 import java.util.ArrayList;
-import java.util.Arrays;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.LinkedHashSet;
@@ -38,6 +37,7 @@ import java.util.Map;
 import java.util.Set;
 import java.util.stream.Collectors;
 
+import com.google.common.base.Preconditions;
 import com.google.common.hash.Hasher;
 import com.google.common.hash.Hashing;
 import com.google.gson.Gson;
@@ -54,6 +54,7 @@ import net.fabricmc.loom.LoomGradleExtension;
 import net.fabricmc.loom.api.mappings.layered.MappingsNamespace;
 import net.fabricmc.loom.configuration.RemappedConfigurationEntry;
 import net.fabricmc.loom.configuration.processors.JarProcessor;
+import net.fabricmc.loom.util.Checksum;
 import net.fabricmc.loom.util.Constants;
 import net.fabricmc.loom.util.Pair;
 import net.fabricmc.loom.util.TinyRemapperHelper;
@@ -83,6 +84,12 @@ public class InterfaceInjectionProcessor implements JarProcessor {
 		return injectedInterfaces.isEmpty();
 	}
 
+	@Override
+	public String getId() {
+		Preconditions.checkArgument(!isEmpty());
+		return "loom:interface_injection:" + Checksum.toHex(inputHash);
+	}
+
 	@Override
 	public void setup() {
 	}
@@ -148,23 +155,6 @@ public class InterfaceInjectionProcessor implements JarProcessor {
 		};
 	}
 
-	@Override
-	public boolean isInvalid(File file) {
-		byte[] hash;
-
-		try {
-			hash = ZipUtils.unpackNullable(file.toPath(), HASH_FILENAME);
-		} catch (IOException e) {
-			return true;
-		}
-
-		if (hash == null) {
-			return true;
-		}
-
-		return !Arrays.equals(inputHash, hash);
-	}
-
 	private List<InjectedInterface> getInjectedInterfaces() {
 		List<InjectedInterface> result = new ArrayList<>();
 

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

@@ -33,11 +33,9 @@ public interface JarProcessor {
 	 * <p>If the jar processor implementation class supports creating multiple jar processors with different effects,
 	 * the needed configuration should also be included in this ID. Example: {@code path.to.MyJarProcessor#someOption}.
 	 *
-	 * @return the ID of this jar processor
+	 * @return the unique ID of this jar processor
 	 */
-	default String getId() {
+	String getId();
-		return getClass().getName();
-	}
 
 	void setup();
 
@@ -45,9 +43,4 @@ public interface JarProcessor {
 	 * Currently this is a destructive process that replaces the existing jar.
 	 */
 	void process(File file);
-
-	/**
-	 * Return true to make all jar processors run again, return false to use the existing results of jar processing.
-	 */
-	boolean isInvalid(File file);
 }

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

@@ -83,7 +83,7 @@ public class JarProcessorManager {
 			throw new UncheckedIOException("Could not check jar manifest of " + file, e);
 		}
 
-		return jarProcessors.stream().anyMatch(jarProcessor -> jarProcessor.isInvalid(file));
+		return false;
 	}
 
 	private String getJarProcessorHash() {

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

@@ -30,6 +30,7 @@ import java.io.UncheckedIOException;
 
 import com.google.common.hash.HashCode;
 import com.google.common.hash.Hashing;
+import com.google.common.io.BaseEncoding;
 import com.google.common.io.Files;
 import org.gradle.api.logging.Logger;
 import org.gradle.api.logging.Logging;
@@ -70,4 +71,8 @@ public class Checksum {
 			throw new UncheckedIOException("Failed to get file hash of " + file, e);
 		}
 	}
+
+	public static String toHex(byte[] bytes) {
+		return BaseEncoding.base16().lowerCase().encode(bytes);
+	}
 }