[core] Fixed data race: TLPKTDROP while checking loss list for ACK ca…

…ndidate (#1668)

srtcore/core.cpp

@@ -8056,25 +8056,23 @@ int CUDT::sendCtrlAck(CPacket& ctrlpkt, int size)
 
     local_prevack = m_iDebugPrevLastAck;
 
-    string reason; // just for "a reason" of giving particular % for ACK
+    string reason = "first lost"; // just for "a reason" of giving particular % for ACK
 #endif
 
-    // If there is no loss, the ACK is the current largest sequence number plus 1;
-    // Otherwise it is the smallest sequence number in the receiver loss list.
-    if (m_pRcvLossList->getLossLength() == 0)
-    {
-        ack = CSeqNo::incseq(m_iRcvCurrSeqNo);
-#if ENABLE_HEAVY_LOGGING
-        reason = "expected next";
-#endif
-    }
-    else
     {
+        // If there is no loss, the ACK is the current largest sequence number plus 1;
+        // Otherwise it is the smallest sequence number in the receiver loss list.
         ScopedLock lock(m_RcvLossLock);
         ack = m_pRcvLossList->getFirstLostSeq();
-#if ENABLE_HEAVY_LOGGING
-        reason = "first lost";
-#endif
+    }
+
+    // We don't need to check the length prematurely,
+    // if length is 0, this will return SRT_SEQNO_NONE.
+    // If so happened, simply use the latest received pkt + 1.
+    if (ack == SRT_SEQNO_NONE)
+    {
+        ack = CSeqNo::incseq(m_iRcvCurrSeqNo);
+        IF_HEAVY_LOGGING(reason = "expected next");
     }
 
     if (m_iRcvLastAckAck == ack)