readAtLeast

    pub fn readAtLeast(d: *Decoder, stream: *std.Io.Reader, their_amt: usize) !void {
        assert(!d.disable_reads);
        const existing_amt = d.cap - d.idx;
        d.their_end = d.idx + their_amt;
        if (their_amt <= existing_amt) return;
        const request_amt = their_amt - existing_amt;
        const dest = d.buf[d.cap..];
        if (request_amt > dest.len) return error.TlsRecordOverflow;
        stream.readSlice(dest[0..request_amt]) catch |err| switch (err) {
            error.EndOfStream => return error.TlsConnectionTruncated,
            error.ReadFailed => |e| return e,
        };
        d.cap += request_amt;
    }