fix(system): write all files atomically

system/file.go

@@ -31,33 +31,55 @@ func (f *File) Permissions() (os.FileMode, error) {
 	return os.FileMode(perm), nil
 }
 
-func WriteFile(f *File) error {
+func WriteFile(f *File, root string) (string, error) {
 	if f.Encoding != "" {
-		return fmt.Errorf("Unable to write file with encoding %s", f.Encoding)
+		return "", fmt.Errorf("Unable to write file with encoding %s", f.Encoding)
 	}
 
-	if err := os.MkdirAll(path.Dir(f.Path), os.FileMode(0755)); err != nil {
-		return err
+	fullpath := path.Join(root, f.Path)
+	dir := path.Dir(fullpath)
+
+	if err := EnsureDirectoryExists(dir); err != nil {
+		return "", err
 	}
 
 	perm, err := f.Permissions()
 	if err != nil {
-		return err
+		return "", err
 	}
 
-	if err := ioutil.WriteFile(f.Path, []byte(f.Content), perm); err != nil {
-		return err
+	var tmp *os.File
+	// Create a temporary file in the same directory to ensure it's on the same filesystem
+	if tmp, err = ioutil.TempFile(dir, "cloudinit-temp"); err != nil {
+		return "", err
+	}
+
+	if err := ioutil.WriteFile(tmp.Name(), []byte(f.Content), perm); err != nil {
+		return "", err
+	}
+
+	if err := tmp.Close(); err != nil {
+		return "", err
+	}
+
+	// Ensure the permissions are as requested (since WriteFile can be affected by sticky bit)
+	if err := os.Chmod(tmp.Name(), perm); err != nil {
+		return "", err
 	}
 
 	if f.Owner != "" {
 		// We shell out since we don't have a way to look up unix groups natively
-		cmd := exec.Command("chown", f.Owner, f.Path)
+		cmd := exec.Command("chown", f.Owner, tmp.Name())
 		if err := cmd.Run(); err != nil {
-			return err
+			return "", err
 		}
 	}
 
-	return nil
+	if err := os.Rename(tmp.Name(), fullpath); err != nil {
+		return "", err
+	}
+
+	return fullpath, nil
 }
 
 func EnsureDirectoryExists(dir string) error {