Add symlink follow into unmount util.

Signed-off-by: Lantao Liu <lantaol@google.com>

pkg/os/testing/fake_os.go

@@ -40,18 +40,19 @@ type CalledDetail struct {
 // of the real call.
 type FakeOS struct {
 	sync.Mutex
-	MkdirAllFn            func(string, os.FileMode) error
-	RemoveAllFn           func(string) error
-	OpenFifoFn            func(context.Context, string, int, os.FileMode) (io.ReadWriteCloser, error)
-	StatFn                func(string) (os.FileInfo, error)
-	ResolveSymbolicLinkFn func(string) (string, error)
-	CopyFileFn            func(string, string, os.FileMode) error
-	WriteFileFn           func(string, []byte, os.FileMode) error
-	MountFn               func(source string, target string, fstype string, flags uintptr, data string) error
-	UnmountFn             func(target string, flags int) error
-	LookupMountFn         func(path string) (containerdmount.Info, error)
-	calls                 []CalledDetail
-	errors                map[string]error
+	MkdirAllFn             func(string, os.FileMode) error
+	RemoveAllFn            func(string) error
+	OpenFifoFn             func(context.Context, string, int, os.FileMode) (io.ReadWriteCloser, error)
+	StatFn                 func(string) (os.FileInfo, error)
+	ResolveSymbolicLinkFn  func(string) (string, error)
+	FollowSymlinkInScopeFn func(string, string) (string, error)
+	CopyFileFn             func(string, string, os.FileMode) error
+	WriteFileFn            func(string, []byte, os.FileMode) error
+	MountFn                func(source string, target string, fstype string, flags uintptr, data string) error
+	UnmountFn              func(target string, flags int) error
+	LookupMountFn          func(path string) (containerdmount.Info, error)
+	calls                  []CalledDetail
+	errors                 map[string]error
 }
 
 var _ osInterface.OS = &FakeOS{}
@@ -176,6 +177,19 @@ func (f *FakeOS) ResolveSymbolicLink(path string) (string, error) {
 	return path, nil
 }
 
+// FollowSymlinkInScope is a fake call that invokes FollowSymlinkInScope or returns its input
+func (f *FakeOS) FollowSymlinkInScope(path, scope string) (string, error) {
+	f.appendCalls("FollowSymlinkInScope", path, scope)
+	if err := f.getError("FollowSymlinkInScope"); err != nil {
+		return "", err
+	}
+
+	if f.FollowSymlinkInScopeFn != nil {
+		return f.FollowSymlinkInScopeFn(path, scope)
+	}
+	return path, nil
+}
+
 // CopyFile is a fake call that invokes CopyFileFn or just return nil.
 func (f *FakeOS) CopyFile(src, dest string, perm os.FileMode) error {
 	f.appendCalls("CopyFile", src, dest, perm)