fix DownloadUtil not downloading file if ETag present, but file is not

src/main/java/net/fabricmc/loom/providers/MinecraftProvider.java

@@ -42,6 +42,7 @@ import java.net.URL;
 import java.nio.charset.StandardCharsets;
 import java.util.Optional;
 import java.util.function.Consumer;
+import java.util.zip.ZipError;
 
 public class MinecraftProvider extends DependencyProvider {
 
@@ -89,7 +90,15 @@ public class MinecraftProvider extends DependencyProvider {
 		libraryProvider.provide(this, project);
 
 		if (!MINECRAFT_MERGED_JAR.exists()) {
-			mergeJars(project.getLogger());
+			try {
+				mergeJars(project.getLogger());
+			} catch (ZipError e) {
+				DownloadUtil.delete(MINECRAFT_CLIENT_JAR);
+				DownloadUtil.delete(MINECRAFT_SERVER_JAR);
+
+				project.getLogger().error("Could not merge JARs! Deleting source JARs - please re-run the command and move on.", e);
+				throw new RuntimeException();
+			}
 		}
 	}
 

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

@@ -86,7 +86,7 @@ public class DownloadUtil {
 		}
 
 		long modifyTime = connection.getHeaderFieldDate("Last-Modified", -1);
-		if (code == HttpURLConnection.HTTP_NOT_MODIFIED || modifyTime > 0 && to.exists() && to.lastModified() >= modifyTime) {
+		if (to.exists() && (code == HttpURLConnection.HTTP_NOT_MODIFIED || modifyTime > 0 && to.lastModified() >= modifyTime)) {
 			if (!quiet) logger.info("'{}' Not Modified, skipping.", to);
 			return; //What we've got is already fine
 		}
@@ -180,4 +180,20 @@ public class DownloadUtil {
 			return String.format("%.2f GB", bytes / (1024.0 * 1024.0 * 1024.0));
 		}
 	}
+
+	/**
+	 * Delete the file along with the corresponding ETag, if it exists.
+	 *
+	 * @param file The file to delete.
+	 */
+	public static void delete(File file) {
+		if (file.exists()) {
+			file.delete();
+		}
+
+		File etagFile = getETagFile(file);
+		if (etagFile.exists()) {
+			etagFile.delete();
+		}
+	}
 }