Fix regression (crash) in transport-cc feedback generation

[ibc]

• Fixes #1336

Details

• Regression introduced in PR #1088, which has many issues.
• Basically there are cases in which this->transportCcFeedbackPacket is reset so it's a complete new packet and hence its baseSet is false.
• However in TransportCongestionControlServer::FillAndSendTransportCcFeedback() in which we just call this->transportCcFeedbackPacket.SetBase() once at the top, but then we enter a loop in which the packet maybe full so it's sent (or other things) and hence we reset the packet while in the loop. And then at the top of the loop this->transportCcFeedbackPacket.AddPacket() is called so the assertion fails because its SetBase() was not called on that fresh packet.
• So this PR calls SetBase() conditionally in every iteration of the loop by checking if current feedback has the base set or not.
• Also add a missing break in a case block in that loop.
• Also set proper packet count in all cases in which we reset this->transportCcFeedbackPacket.
• Fix crash scenario in which feedback-cc packet is full but cannot be sent: https://github.com/versatica/mediasoup/pull/1339/commits/b8a411b1d2b200040f053a1a79649b96f68d2085

[ibc]

This is a draft PR because I need help. See TODO above.

[ibc]

I think I know how to solve it properly. Just a question:

Does somebody remember what is feedbackPacketCount in FeedbackRtpTransportPacket? It's obviously part of the packet fields but I don't remember its meaning. Specifically if an endpoint sends 2 sequential CC feedback packets, what should be the value of that field in those packets?

And why are we setting it when initiating transport server control?

    /* Instance methods. */

    TransportCongestionControlServer::TransportCongestionControlServer(
      RTC::TransportCongestionControlServer::Listener* listener,
      RTC::BweType bweType,
      size_t maxRtcpPacketLen)
      : listener(listener), bweType(bweType), maxRtcpPacketLen(maxRtcpPacketLen)
    {
        MS_TRACE();

        switch (this->bweType)
        {
            case RTC::BweType::TRANSPORT_CC:
            {
                // Create a feedback packet.
                this->transportCcFeedbackPacket.reset(new RTC::RTCP::FeedbackRtpTransportPacket(0u, 0u));

                // Set initial packet count.
                this->transportCcFeedbackPacket->SetFeedbackPacketCount(this->transportCcFeedbackPacketCount);

[ibc]

Ah, is the number of CC feedback packet sent, right? so it's just a counter that must be incremented every time a feedback packet is sent. Right?

[ibc]

Concerns resolved in theory. PR ready for review.

[ibc]

I've done this change to test:

uint16_t FeedbackRtpTransportPacket::maxMissingPackets{ (1 << 13) - 1 };
// uint16_t FeedbackRtpTransportPacket::maxPacketStatusCount{ (1 << 16) - 1 };

and it crashes:

(ABORT) RTC::RTCP::FeedbackRtpTransport::AddPacket() | failed assertion `!IsFull()': packet is full

Exception Type:        EXC_CRASH (SIGABRT)
Exception Codes:       0x0000000000000000, 0x0000000000000000

Thread 0 Crashed::  Dispatch queue: com.apple.main-thread
0   libsystem_kernel.dylib              0x7ff80d79b1e2 __pthread_kill + 10
1   libsystem_pthread.dylib             0x7ff80d7d2ee6 pthread_kill + 263
2   libsystem_c.dylib                   0x7ff80d6f9b45 abort + 123
3   mediasoup-worker                       0x1095e6ea0 RTC::RTCP::FeedbackRtpTransportPacket::AddPacket(unsigned short, unsigned long long, unsigned long) (.cold.2) + 64
4   mediasoup-worker                       0x1092b4c47 RTC::RTCP::FeedbackRtpTransportPacket::AddPacket(unsigned short, unsigned long long, unsigned long) + 231
5   mediasoup-worker                       0x109274e22 RTC::TransportCongestionControlServer::FillAndSendTransportCcFeedback() + 210

Why? Because when this->transportCcFeedbackPacket->IsFull() we call SendTransportCcFeedback() which is supposed to send the feedback and reset this->transportCcFeedbackPacket so in the next iteration of the loop there is a fresh feedback packet which obviously is not full, however there are cases in which SendTransportCcFeedback() does NOT send the feedback and does NOT reset it:

if (!this->transportCcFeedbackPacket->IsSerializable())
{
    return;
}

So here another regression in that PR or perhaps

[ibc]

Fixing it as follows: