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

src/ledger/file_test.go

@@ -1,6 +1,7 @@
 package ledger
 
 import (
+	"bytes"
 	"encoding/base64"
 	"fmt"
 	"os"
@@ -401,22 +402,39 @@ func TestFilesTempSetLastNLines(t *testing.T) {
 		c := d
 		t.Run(name, func(t *testing.T) {
 			p := path.Join(t.TempDir(), base64.URLEncoding.EncodeToString([]byte(t.Name())))
-			os.WriteFile(p, []byte(c.given), os.ModePerm)
+			realp := p + ".real"
-			files := Files([]string{p})
+
-			if err := files.TempSetLastNLines(c.n, c.input); err != nil {
+			os.WriteFile(realp, []byte(c.given), os.ModePerm)
-				s := err.Error()
+			if err := os.Symlink(realp, p); err != nil {
-				if _, err := os.Stat(s); err == nil {
-					got, _ := os.ReadFile(s)
-					if string(got) != c.want {
-						t.Errorf("want\n%s, got\n%s", c.want, got)
-					}
-				}
 				t.Fatal(err)
 			}
+			if stat, err := os.Lstat(p); err != nil {
+				t.Error(err)
+			} else if stat.Mode().IsRegular() {
+				t.Error("p is already a regular file")
+			}
+
+			files := Files([]string{p})
+			if err := files.TempSetLastNLines(c.n, c.input); err != nil {
+				t.Fatal(err)
+			}
+
 			got, _ := os.ReadFile(p)
 			if string(got) != c.want {
 				t.Errorf("want\n%s, got\n%s", c.want, got)
 			}
+
+			realb, _ := os.ReadFile(realp)
+			b, _ := os.ReadFile(realp)
+			if !bytes.Equal(b, realb) {
+				t.Errorf("%s no longer links to %s", p, realp)
+			}
+
+			if stat, err := os.Lstat(p); err != nil {
+				t.Error(err)
+			} else if stat.Mode().IsRegular() {
+				t.Error("p is now a regular file")
+			}
 		})
 	}
 }