receiveHead

    pub fn receiveHead(reader: *Reader) HeadError![]const u8 {
        reader.trailers = &.{};
        const in = reader.in;
        const max_head_len = reader.max_head_len;
        var hp: HeadParser = .{};
        var head_len: usize = 0;
        while (true) {
            if (head_len >= max_head_len) return error.HttpHeadersOversize;
            const remaining = in.buffered()[head_len..];
            if (remaining.len == 0) {
                in.fillMore() catch |err| switch (err) {
                    error.EndOfStream => switch (head_len) {
                        0 => return error.HttpConnectionClosing,
                        else => return error.HttpRequestTruncated,
                    },
                    error.ReadFailed => |e| return e,
                };
                continue;
            }
            head_len += hp.feed(remaining);
            if (hp.state == .finished) {
                reader.state = .received_head;
                const head_buffer = in.buffered()[0..head_len];
                in.toss(head_len);
                return head_buffer;
            }
        }
    }