Function write_bytes_with_fallback 

pub fn write_bytes_with_fallback(
    path: &Path,
    bytes: &[u8],
    permissions: Option<&Permissions>,
) -> Result<(), String>

Atomic write with the read-only-directory in-place fallback (#459). Tries the crash-atomic temp+rename first; if that fails because the directory is read-only/permission-denied but an existing file inode is writable, overwrite it in place. permissions, when given, is applied to the written file.