chunkedSendFile

    pub fn chunkedSendFile(w: *Writer, file_reader: *File.Reader, limit: std.Io.Limit) Writer.FileError!usize {
        const bw: *BodyWriter = @alignCast(@fieldParentPtr("writer", w));
        assert(!bw.isEliding());
        const data_len = Writer.countSendFileLowerBound(w.end, file_reader, limit) orelse {
            // If the file size is unknown, we cannot lower to a `sendFile` since we would
            // have to flush the chunk header before knowing the chunk length.
            return error.Unimplemented;
        };
        if (data_len == 0) return error.EndOfStream;
        const out = bw.http_protocol_output;
        l: switch (bw.state.chunk_len) {
            0 => {
                const header_buf = try out.writableArray(chunk_header_template.len);
                @memcpy(header_buf, chunk_header_template);
                writeHex(header_buf[0..chunk_len_digits], data_len);
                bw.state.chunk_len = data_len + 2;
                continue :l bw.state.chunk_len;
            },
            1 => unreachable, // Wrote more data than specified in chunk header.
            2 => {
                try out.writeAll("\r\n");
                bw.state.chunk_len = 0;
                continue :l 0;
            },
            else => {
                const chunk_limit: std.Io.Limit = .limited(bw.state.chunk_len - 2);
                const n = if (chunk_limit.subtract(w.buffered().len)) |sendfile_limit|
                    try out.sendFileHeader(w.buffered(), file_reader, sendfile_limit.min(limit))
                else
                    try out.write(chunk_limit.slice(w.buffered()));
                bw.state.chunk_len -= n;
                return w.consume(n);
            },
        }
    }