i think files.TempLastNLines are symlink friendly...

src/ledger/file.go

@@ -36,26 +36,51 @@ func (files Files) TempGetLastNLines(n int) ([]string, error) {
 
 func (files Files) TempSetLastNLines(n int, lines []string) error {
 	p := files.paths()[0]
-	w, err := ioutil.TempFile(os.TempDir(), path.Base(p))
+
+	newFile, err := func() (string, error) {
+		w, err := ioutil.TempFile(os.TempDir(), path.Base(p))
+		if err != nil {
+			return "", err
+		}
+		defer w.Close()
+
+		r, err := os.Open(p)
+		if err != nil {
+			return "", err
+		}
+		defer r.Close()
+
+		if _, err := peekLastNLines(w, bufio.NewReader(r), n); err != nil {
+			return "", err
+		}
+		for i := range lines {
+			if _, err := fmt.Fprintln(w, lines[i]); err != nil {
+				return "", err
+			}
+		}
+		if err := w.Close(); err != nil {
+			return "", err
+		}
+		return w.Name(), nil
+	}()
 	if err != nil {
 		return err
 	}
-	defer w.Close()
 
-	r, err := os.Open(p)
+	r, err := os.Open(newFile)
 	if err != nil {
 		return err
 	}
 	defer r.Close()
 
-	if _, err := peekLastNLines(w, bufio.NewReader(r), n); err != nil {
+	w, err := os.Create(p)
+	if err != nil {
 		return err
 	}
-	for i := range lines {
-		fmt.Fprintln(w, lines[i])
-	}
+	defer w.Close()
 
-	return os.Rename(w.Name(), p)
+	_, err = io.Copy(w, r)
+	return err
 }
 
 func peekLastNLines(w io.Writer, r *bufio.Reader, n int) ([]string, error) {