bugfix: allow hardlink to softlink file

With `fs.RootPath`, the target file will be the file which the softlink
points to, like:

  touch /tmp/zzz
  ln -s /tmp/zzz /tmp/xxx
  ln /tmp/xxx /tmp/yyy

The `/tmp/yyy` should be same with the `/tmp/xxx`, not `/tmp/zzz`. We
should allow hardlink to softlink file.

Signed-off-by: Wei Fu <fhfuwei@163.com>

archive/tar_test.go

@@ -196,6 +196,49 @@ func TestBreakouts(t *testing.T) {
 		return nil
 	}
 	errFileDiff := errors.New("files differ")
+
+	isSymlinkFile := func(f string) func(string) error {
+		return func(root string) error {
+			fi, err := os.Lstat(filepath.Join(root, f))
+			if err != nil {
+				return err
+			}
+
+			if got := fi.Mode() & os.ModeSymlink; got != os.ModeSymlink {
+				return errors.Errorf("%s should be symlink", fi.Name())
+			}
+			return nil
+		}
+	}
+
+	sameSymlinkFile := func(f1, f2 string) func(string) error {
+		checkF1, checkF2 := isSymlinkFile(f1), isSymlinkFile(f2)
+		return func(root string) error {
+			if err := checkF1(root); err != nil {
+				return err
+			}
+
+			if err := checkF2(root); err != nil {
+				return err
+			}
+
+			t1, err := os.Readlink(filepath.Join(root, f1))
+			if err != nil {
+				return err
+			}
+
+			t2, err := os.Readlink(filepath.Join(root, f2))
+			if err != nil {
+				return err
+			}
+
+			if t1 != t2 {
+				return errors.Wrapf(errFileDiff, "%#v and %#v", t1, t2)
+			}
+			return nil
+		}
+	}
+
 	sameFile := func(f1, f2 string) func(string) error {
 		return func(root string) error {
 			p1, err := fs.RootPath(root, f1)
@@ -406,6 +449,16 @@ func TestBreakouts(t *testing.T) {
 			),
 			validator: sameFile("localpasswd", "/etc/passwd"),
 		},
+		{
+			name: "HardlinkSymlinkBeforeCreateTarget",
+			w: TarAll(
+				tc.Dir("etc", 0770),
+				tc.Symlink("/etc/passwd", "localpasswd"),
+				tc.Link("localpasswd", "localpasswd-dup"),
+				tc.File("/etc/passwd", []byte("after"), 0644),
+			),
+			validator: sameFile("localpasswd-dup", "/etc/passwd"),
+		},
 		{
 			name: "HardlinkSymlinkRelative",
 			w: TarAll(
@@ -414,7 +467,10 @@ func TestBreakouts(t *testing.T) {
 				tc.Symlink("../../../../../etc/passwd", "passwdlink"),
 				tc.Link("/passwdlink", "localpasswd"),
 			),
-			validator: sameFile("/localpasswd", "/etc/passwd"),
+			validator: all(
+				sameSymlinkFile("/localpasswd", "/passwdlink"),
+				sameFile("/localpasswd", "/etc/passwd"),
+			),
 		},
 		{
 			name: "HardlinkSymlinkAbsolute",
@@ -424,7 +480,10 @@ func TestBreakouts(t *testing.T) {
 				tc.Symlink("/etc/passwd", "passwdlink"),
 				tc.Link("/passwdlink", "localpasswd"),
 			),
-			validator: sameFile("/localpasswd", "/etc/passwd"),
+			validator: all(
+				sameSymlinkFile("/localpasswd", "/passwdlink"),
+				sameFile("/localpasswd", "/etc/passwd"),
+			),
 		},
 		{
 			name: "SymlinkParentDirectory",