Fixes Tar.Extract when extracting hardlinked files

This chance relativizes the Linkname on tar headers when extracting
hardlinked files.

This change also adds a couple of tests around the Tar.Unarchive and
Tar.Extract functions.

tar.go

@@ -512,6 +512,11 @@ func (t *Tar) Extract(source, target, destination string) error {
 			}
 			th.Name = end
 
+			// relativize any hardlink names
+			if th.Typeflag == tar.TypeLink {
+				th.Linkname = filepath.Join(filepath.Base(filepath.Dir(th.Linkname)), filepath.Base(th.Linkname))
+			}
+
 			err = t.untarFile(f, destination, th)
 			if err != nil {
 				return fmt.Errorf("extracting file %s: %v", th.Name, err)

tar_test.go

@@ -0,0 +1,67 @@
+package archiver_test
+
+import (
+	"io/ioutil"
+	"os"
+	"path"
+	"testing"
+
+	"github.com/mholt/archiver"
+)
+
+func requireRegularFile(t *testing.T, path string) os.FileInfo {
+	fileInfo, err := os.Stat(path)
+	if err != nil {
+		t.Fatalf("fileInfo on '%s': %v", path, err)
+	}
+
+	if !fileInfo.Mode().IsRegular() {
+		t.Fatalf("'%s' expected to be a regular file", path)
+	}
+
+	return fileInfo
+}
+
+func assertSameFile(t *testing.T, f1, f2 os.FileInfo) {
+	if !os.SameFile(f1, f2) {
+		t.Errorf("expected '%s' and '%s' to be the same file", f1.Name(), f2.Name())
+	}
+}
+
+func TestDefaultTar_Unarchive_HardlinkSuccess(t *testing.T) {
+	source := "testdata/gnu-hardlinks.tar"
+
+	destination, err := ioutil.TempDir("", "archiver_tar_test")
+	if err != nil {
+		t.Fatalf("creating temp dir: %v", err)
+	}
+	defer os.RemoveAll(destination)
+
+	err = archiver.DefaultTar.Unarchive(source, destination)
+	if err != nil {
+		t.Fatalf("unarchiving '%s' to '%s': %v", source, destination, err)
+	}
+
+	fileaInfo := requireRegularFile(t, path.Join(destination, "dir-1", "dir-2", "file-a"))
+	filebInfo := requireRegularFile(t, path.Join(destination, "dir-1", "dir-2", "file-b"))
+	assertSameFile(t, fileaInfo, filebInfo)
+}
+
+func TestDefaultTar_Extract_HardlinkSuccess(t *testing.T) {
+	source := "testdata/gnu-hardlinks.tar"
+
+	destination, err := ioutil.TempDir("", "archiver_tar_test")
+	if err != nil {
+		t.Fatalf("creating temp dir: %v", err)
+	}
+	defer os.RemoveAll(destination)
+
+	err = archiver.DefaultTar.Extract(source, path.Join("dir-1", "dir-2"), destination)
+	if err != nil {
+		t.Fatalf("unarchiving '%s' to '%s': %v", source, destination, err)
+	}
+
+	fileaInfo := requireRegularFile(t, path.Join(destination, "dir-2", "file-a"))
+	filebInfo := requireRegularFile(t, path.Join(destination, "dir-2", "file-b"))
+	assertSameFile(t, fileaInfo, filebInfo)
+}