Function write_completion 

pub fn write_completion(
    workdir: &Path,
    outcome: &DelegationOutcome,
) -> Result<(), String>

Write outcome to COMPLETION_FILE, replacing whatever was there.

Written to a sibling temporary file and renamed into place, never truncated in place. The parent reads this file the moment the completion’s task begins, and report_completion rewrites it immediately after posting that completion — so a plain truncating write leaves a window in which the parent reads an empty or half-written file and records the delegation’s outcome as unknown. The rename makes every reader see either the whole previous content or the whole new content. The temporary name is unique because the child and the watcher behind it can both reach this function.