Add support for extracting xattrs on unix (#74)

When extended file attributes are set on a file in the archive, try to
preserve them on extraction but do not fail when the user does not have
permission to set them or if they are not supported. This should prevent
existing usages from breaking while also enabling users to preserve
extended file attributes.

Author: maxmoehl

extractor/tgz_extractor.go

@@ -105,5 +105,9 @@ func extractTarArchiveFile(header *tar.Header, dest string, input io.Reader) err
 	defer fileCopy.Close()
 
 	_, err = io.Copy(fileCopy, input)
-	return err
+	if err != nil {
+		return err
+	}
+
+	return setXattrsFromTar(filePath, header)
 }

extractor/xattr_unix.go

@@ -0,0 +1,29 @@
+//go:build unix
+
+package extractor
+
+import (
+	"archive/tar"
+	"errors"
+	"strings"
+	"syscall"
+
+	"golang.org/x/sys/unix"
+)
+
+func setXattrsFromTar(path string, hdr *tar.Header) (err error) {
+	const paxSchilyXattr = "SCHILY.xattr."
+
+	for key, value := range hdr.PAXRecords {
+		if !strings.HasPrefix(key, paxSchilyXattr) {
+			continue
+		}
+
+		err = unix.Lsetxattr(path, key[len(paxSchilyXattr):], []byte(value), 0)
+		if err != nil && !errors.Is(err, syscall.ENOTSUP) && !errors.Is(err, syscall.EPERM) {
+			return err
+		}
+	}
+
+	return nil
+}

extractor/xattr_windows.go

@@ -0,0 +1,9 @@
+//go:build windows
+
+package extractor
+
+import "archive/tar"
+
+func setXattrsFromTar(_ string, _ *tar.Header) error {
+	return nil
+}