[core] Fixed group rcv buffering of ignored packet

[maxsharabayko]

One of the member links can provide a packet ahead of the current position after another link has provided a valid packet.
This AHEAD packet is saved in group's buffer. However, the payload of the packet was incorrect, as it was read to a buffer different from the one being saved.

Fixes #1799

Ignoring Packet

Already have the one to deliver.

if (output_size)
{
    // We have already the data, so this must fall on the floor
    char lostbuf[SRT_LIVE_MAX_PLSIZE];
    stat = ps->core().receiveMessage((lostbuf), SRT_LIVE_MAX_PLSIZE, (mctrl), CUDTUnited::ERH_RETURN);

Buffering Packet

Here buf is different from lostbuf where the packet was read to.

if (seqdiff > 1)
{
    HLOGC(grlog.Debug,
            log << "@" << id << " %" << mctrl.pktseq << " #" << mctrl.msgno << " AHEAD base=%"
                << m_RcvBaseSeqNo);
    p->packet.assign(buf, buf + stat);
    p->mctrl = mctrl;
    break; // Don't read from that socket anymore.
}

TODO

• Fix recvDiscard stats (see TODO comment in the code)

[ethouris]

Not sure what this thing changes. Ignoring the packet is based on a premise - might be that not correctly stated - that the packet with given sequence number is already received. Note that this reception function relies on packets retrieved from single links at their play time, meaning, whatever is not delivered by these links, has been dropped according to overall rules of TLPKTDROP. Here if the output_size has been set, it means that the data for the next packet to deliver have been already read, the user's buffer filled with the payload, and whatever is being done here is to make the link shifted to read the "next position", that is, it should read everything up to the sequence number of the currently delivered packet.

Reading packets that came AHEAD of the current latest read sequence is done in another place, and these are being read to a spare buffer - if I remember correctly.

[maxsharabayko]

The issue is the following.

1. Group reads the next packet. output_size = non-zero
2. Iterating to the next read-ready member link, now reading to lostbuf and mctl.

if (output_size)
{
    // We have already the data, so this must fall on the floor
    char lostbuf[SRT_LIVE_MAX_PLSIZE];
    stat = ps->core().receiveMessage((lostbuf), SRT_LIVE_MAX_PLSIZE, (mctrl), CUDTUnited::ERH_RETURN);

4. Checking the sequence number of the packet. It it happens to be the AHEAD packet, saving buf instead of lostbuf.

if (seqdiff > 1)
{
    p->packet.assign(buf, buf + stat);
    p->mctrl = mctrl;
    break; // Don't read from that socket anymore.
}

Result: group buffered a packet with AHEAD seqno, but a different payload.

[ethouris]

Minor things to consider.