Function tail_file 

pub fn tail_file(
    filename: &str,
    config: &TailConfig,
    out: &mut impl Write,
    tool_name: &str,
) -> Result<bool>

Process a single file/stdin for tail.

On Linux, the sendfile fast paths bypass out and write directly to stdout (fd 1). Callers MUST ensure out wraps stdout when these paths are active. The out.flush() call drains any buffered data before sendfile takes over.