lib/osutil: Preserve perms in AtomicWriter (fixes #tbd) (#6885)

lib/osutil/atomic.go

@@ -93,7 +93,12 @@ func (w *AtomicWriter) Close() error {
 		return err
 	}
 
-	err := w.fs.Rename(w.next.Name(), w.path)
+	info, err := w.fs.Lstat(w.path)
+	if err != nil && !fs.IsNotExist(err) {
+		w.err = err
+		return err
+	}
+	err = w.fs.Rename(w.next.Name(), w.path)
 	if runtime.GOOS == "windows" && fs.IsPermission(err) {
 		// On Windows, we might not be allowed to rename over the file
 		// because it's read-only. Get us some write permissions and try
@@ -105,6 +110,12 @@ func (w *AtomicWriter) Close() error {
 		w.err = err
 		return err
 	}
+	if info != nil {
+		if err := w.fs.Chmod(w.path, info.Mode()); err != nil {
+			w.err = err
+			return err
+		}
+	}
 
 	// fsync the directory too
 	if fd, err := w.fs.Open(filepath.Dir(w.next.Name())); err == nil {