[core] Fix hang up on not enough space in the RCV buffer.

srtcore/core.cpp

@@ -7904,7 +7904,7 @@ int srt::CUDT::sendCtrlAck(CPacket& ctrlpkt, int size)
     SRT_ASSERT(ctrlpkt.getMsgTimeStamp() != 0);
     int nbsent = 0;
     int local_prevack = 0;
-
+    bool sendAckAgain = false; //Send ack again after a buffer full was detected
 #if ENABLE_HEAVY_LOGGING
     struct SaveBack
     {
@@ -7927,12 +7927,16 @@ int srt::CUDT::sendCtrlAck(CPacket& ctrlpkt, int size)
     // The TSBPD thread may change the first lost sequence record (TLPKTDROP).
     // To avoid it the m_RcvBufferLock has to be acquired.
     UniqueLock bufflock(m_RcvBufferLock);
-
+    if(m_bBufferWasFull && getAvailRcvBufferSizeNoLock() > 0)
+    {
+        sendAckAgain = true;
+        m_bBufferWasFull = false;
+    }
     int32_t ack;    // First unacknowledged packet sequence number (acknowledge up to ack).
     if (!getFirstNoncontSequence((ack), (reason)))
         return nbsent;
 
-    if (m_iRcvLastAckAck == ack)
+    if (m_iRcvLastAckAck == ack && !sendAckAgain)
     {
         HLOGC(xtlog.Debug,
               log << CONID() << "sendCtrl(UMSG_ACK): last ACK %" << ack << "(" << reason << ") == last ACKACK");
@@ -8101,7 +8105,7 @@ int srt::CUDT::sendCtrlAck(CPacket& ctrlpkt, int size)
     // [[using locked(m_RcvBufferLock)]];
 
     // Send out the ACK only if has not been received by the sender before
-    if (CSeqNo::seqcmp(m_iRcvLastAck, m_iRcvLastAckAck) > 0)
+    if (CSeqNo::seqcmp(m_iRcvLastAck, m_iRcvLastAckAck) > 0 || sendAckAgain)
     {
         // NOTE: The BSTATS feature turns on extra fields above size 6
         // also known as ACKD_TOTAL_SIZE_VER100.
@@ -8117,10 +8121,10 @@ int srt::CUDT::sendCtrlAck(CPacket& ctrlpkt, int size)
         data[ACKD_RTT] = m_iSRTT;
         data[ACKD_RTTVAR] = m_iRTTVar;
         data[ACKD_BUFFERLEFT] = (int) getAvailRcvBufferSizeNoLock();
-        // a minimum flow window of 2 is used, even if buffer is full, to break potential deadlock
-        if (data[ACKD_BUFFERLEFT] < 2)
-            data[ACKD_BUFFERLEFT] = 2;
-
+        if (data[ACKD_BUFFERLEFT] == 0)
+        {
+            m_bBufferWasFull = true;
+        }
         if (steady_clock::now() - m_tsLastAckTime > m_tdACKInterval)
         {
             int rcvRate;

srtcore/core.h

@@ -934,7 +934,7 @@ class CUDT
     int32_t m_iAckSeqNo;                         // Last ACK sequence number
     sync::atomic<int32_t> m_iRcvCurrSeqNo;       // (RCV) Largest received sequence number. RcvQTh, TSBPDTh.
     int32_t m_iRcvCurrPhySeqNo;                  // Same as m_iRcvCurrSeqNo, but physical only (disregarding a filter)
-
+    bool m_bBufferWasFull;                        // Indicate that RX buffer was full last time a ack was sent
     int32_t m_iPeerISN;                          // Initial Sequence Number of the peer side
 
     uint32_t m_uPeerSrtVersion;