[core] Update group read state using ack instead of first unread seqno

[xiaozhihong]

I used srt backup mode in 2 links, one 4G, another Wifi, and push live stream on it.
My option is live mode, but tspbd: off, tlpktdrop: off.

When I disconnect the Wifi(main) connection, the group socket will never be readable.
And I check the code, I found the problem.

For example, the buffer state below

After the Wifi disconnected, when recv packet from 4G, the function below will be called.

updateReadState(socket_id_of_4g, 4/*first seq unread*/)

but It always false, because 4 is less than 5(CUDTGroup::m_RcvBaseSeqNo), and the group socket can not read anymore.

This PR show we should check the group readable using ack seqno instead of first unread seqno in recv buffer.
And it works well in my case.

[gou4shi1]

you can try #2046

[xiaozhihong]

you can try #2046

I have seen it, maybe your PR is too complex

[gou4shi1]

With tspbd and tlpktdrop off, SRT behaves like message mode (except congestion control), and the current group connect is only designed for tsbpd on.
For in-order message, using ack may result in useless epolls.
For out-of-order message, using ack may miss useful epolls.
I have not yet check the code of new recv buffer, maybe the new recv buffer could help to make #2046 simpler.

[xiaozhihong]

With tspbd and tlpktdrop off, SRT behaves like message mode (except congestion control), and the current group connect is only designed for tsbpd on. For in-order message, using ack may result in useless epolls. For out-of-order message, using ack may miss useful epolls. I have not yet check the code of new recv buffer, maybe the new recv buffer could help to make #2046 simpler.

@gou4shi1 Hi, I sent an email to you , can you receive it ?