fix: use os.CreateTemp for atomic rewrites

file_handling.go

@@ -98,18 +98,33 @@ func (f *File) Write(content string) error {
 		return err
 	}
 
-	tempName := filepath.Join(f.Dir(), RandomString(20))
-	if err := os.WriteFile(tempName, []byte(content), mode); err != nil {
+	tempFile, err := os.CreateTemp(f.Dir(), ".find-replace-*")
+	if err != nil {
 		return fmt.Errorf("create tempfile in %v: %w", f.Dir(), err)
 	}
-	// Make sure the temp file is removed if the rename below fails. On
-	// success, the rename has already moved the file to f.Path so this is
-	// a no-op (we deliberately ignore the not-exist error).
-	defer os.Remove(tempName)
+	tempName := tempFile.Name()
+	removeTemp := true
+	defer func() {
+		if removeTemp {
+			_ = os.Remove(tempName)
+		}
+	}()
+	if err := tempFile.Chmod(mode); err != nil {
+		_ = tempFile.Close()
+		return fmt.Errorf("chmod temp file %v: %w", tempName, err)
+	}
+	if _, err := tempFile.WriteString(content); err != nil {
+		_ = tempFile.Close()
+		return fmt.Errorf("write temp file %v: %w", tempName, err)
+	}
+	if err := tempFile.Close(); err != nil {
+		return fmt.Errorf("close temp file %v: %w", tempName, err)
+	}
 
 	log.Printf("Rewriting %v", f.Path)
 	if err := os.Rename(tempName, f.Path); err != nil {
 		return fmt.Errorf("atomically move temp file %v to %v: %w", tempName, f.Path, err)
 	}
+	removeTemp = false
 	return nil
 }

file_handling_test.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"os"
 	"path/filepath"
 	"testing"
 )
@@ -77,3 +78,28 @@ func TestNewFile(t *testing.T) {
 		})
 	}
 }
+
+func TestWriteUsesCreateTempPrefix(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "target.txt")
+	if err := os.WriteFile(path, []byte("old"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	f, err := NewFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := f.Write("new"); err != nil {
+		t.Fatal(err)
+	}
+	entries, err := os.ReadDir(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, e := range entries {
+		if e.Name() != "target.txt" {
+			t.Fatalf("unexpected leftover file %q", e.Name())
+		}
+	}
+}
+