What the caller believes is on disk, as UTF-8 text, or null if the file must not exist yet.
Checked again just before the new content takes its place; if it no longer holds, the write is
abandoned with an error and the file is untouched. Omit to skip the check.
What the caller believes is on disk, as UTF-8 text, or
nullif the file must not exist yet. Checked again just before the new content takes its place; if it no longer holds, the write is abandoned with an error and the file is untouched. Omit to skip the check.