[core] Set ACK position based on RCV buffer first nonread seqno (#2931).

srtcore/buffer_rcv.cpp

@@ -760,6 +760,12 @@ CRcvBuffer::PacketInfo CRcvBuffer::getFirstReadablePacketInfo(time_point time_no
         return unreadableInfo;
 }
 
+int32_t CRcvBuffer::getFirstNonreadSeqNo() const
+{
+    const int offset = offPos(m_iStartPos, m_iFirstNonreadPos);
+    return CSeqNo::incseq(m_iStartSeqNo, offset);
+}
+
 void CRcvBuffer::countBytes(int pkts, int bytes)
 {
     ScopedLock lock(m_BytesCountLock);

srtcore/buffer_rcv.h

@@ -187,6 +187,11 @@ class CRcvBuffer
 
     PacketInfo getFirstReadablePacketInfo(time_point time_now) const;
 
+    /// @brief Get the sequence number of the first packet that can't be read
+    /// (either because it is missing, or because it is a part of a bigger message
+    /// that is not fully available yet).
+    int32_t getFirstNonreadSeqNo() const;
+
     /// Get information on packets available to be read.
     /// @returns a pair of sequence numbers (first available; first unavailable).
     /// 

srtcore/core.cpp

@@ -8023,6 +8023,27 @@ void srt::CUDT::sendCtrl(UDTMessageType pkttype, const int32_t* lparam, void* rp
 // [[using locked(m_RcvBufferLock)]]
 bool srt::CUDT::getFirstNoncontSequence(int32_t& w_seq, string& w_log_reason)
 {
+    if (m_config.bTSBPD || !m_config.bMessageAPI)
+    {
+        // The getFirstNonreadSeqNo() function retuens the sequence number of the first packet
+        // that cannot be read. In cases when a message can consist of several data packets,
+        // an existing packet of partially available message also cannot be read.
+        // If TSBPD mode is enabled, a message must consist of a single data packet only.
+        w_seq = m_pRcvBuffer->getFirstNonreadSeqNo();
+
+        const int32_t iNextSeqNo = CSeqNo::incseq(m_iRcvCurrSeqNo);
+        SRT_ASSERT(CSeqNo::seqcmp(w_seq, iNextSeqNo) <= 0);
+        w_log_reason = iNextSeqNo != w_seq ? "first lost" : "expected next";
+
+        if (CSeqNo::seqcmp(w_seq, iNextSeqNo) > 0)
+        {
+            LOGC(xtlog.Error, log << "IPE: NONCONT-SEQUENCE: RCV buffer first non-read %" << w_seq << ", RCV latest seqno %" << m_iRcvCurrSeqNo);
+            w_seq = iNextSeqNo;
+        }
+
+        return true;
+    }
+
     {
         ScopedLock losslock (m_RcvLossLock);
         const int32_t seq = m_pRcvLossList->getFirstLostSeq();
@@ -8070,6 +8091,7 @@ int srt::CUDT::sendCtrlAck(CPacket& ctrlpkt, int size)
     // since the last full ACK. It should unblock the sender to proceed further.
     const bool bNeedFullAck = (m_bBufferWasFull && getAvailRcvBufferSizeNoLock() > 0);
     int32_t ack;    // First unacknowledged packet sequence number (acknowledge up to ack).
+
     if (!getFirstNoncontSequence((ack), (reason)))
         return nbsent;