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

srtcore/core.cpp

@@ -7904,7 +7904,6 @@
     SRT_ASSERT(ctrlpkt.getMsgTimeStamp() != 0);
     int nbsent = 0;
     int local_prevack = 0;
-
 #if ENABLE_HEAVY_LOGGING
     struct SaveBack
     {
@@ -7927,21 +7926,22 @@
     // 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);
-
+    // The full ACK should be sent to indicate there is now available space in the RCV buffer
+    // 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;
 
-    if (m_iRcvLastAckAck == ack)
+    if (m_iRcvLastAckAck == ack && !bNeedFullAck)
     {
-        HLOGC(xtlog.Debug,
-              log << CONID() << "sendCtrl(UMSG_ACK): last ACK %" << ack << "(" << reason << ") == last ACKACK");
-        return nbsent;
+        HLOGC(xtlog.Debug,      
+                log << CONID() << "sendCtrl(UMSG_ACK): last ACK %" << ack << "(" << reason << ") == last ACKACK");     
+        return nbsent;    
     }
-
     // send out a lite ACK
     // to save time on buffer processing and bandwidth/AS measurement, a lite ACK only feeds back an ACK number
-    if (size == SEND_LITE_ACK)
+    if (size == SEND_LITE_ACK && !bNeedFullAck)
     {
         bufflock.unlock();
         ctrlpkt.pack(UMSG_ACK, NULL, &ack, size);
@@ -8079,7 +8079,7 @@
             CGlobEvent::triggerEvent();
         }
     }
-    else if (ack == m_iRcvLastAck)
+    else if (ack == m_iRcvLastAck && !bNeedFullAck)
     {
         // If the ACK was just sent already AND elapsed time did not exceed RTT,
         if ((steady_clock::now() - m_tsLastAckTime) <
@@ -8090,7 +8090,7 @@
             return nbsent;
         }
     }
-    else
+    else if (!bNeedFullAck)
     {
         // Not possible (m_iRcvCurrSeqNo+1 <% m_iRcvLastAck ?)
         LOGC(xtlog.Error, log << CONID() << "sendCtrl(UMSG_ACK): IPE: curr %" << ack << " <% last %" << m_iRcvLastAck);
@@ -8101,8 +8101,8 @@
     // [[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 || bNeedFullAck)
     {
         // NOTE: The BSTATS feature turns on extra fields above size 6
         // also known as ACKD_TOTAL_SIZE_VER100.
         int32_t data[ACKD_TOTAL_SIZE];
@@ -8117,10 +8117,7 @@
         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;
-
+        m_bBufferWasFull = data[ACKD_BUFFERLEFT] == 0;
         if (steady_clock::now() - m_tsLastAckTime > m_tdACKInterval)
         {
             int rcvRate;
@@ -8295,7 +8292,6 @@
             m_tsLastRspAckTime = currtime;
             m_iReXmitCount         = 1; // Reset re-transmit count since last ACK
         }
-
         return;
     }
 
@@ -8336,14 +8332,25 @@
             return;
         }
 
-        if (CSeqNo::seqcmp(ackdata_seqno, m_iSndLastAck) >= 0)
+    if (CSeqNo::seqcmp(ackdata_seqno, m_iSndLastAck) >= 0)
+    {
+        const int cwnd1   = std::min(int(m_iFlowWindowSize), int(m_dCongestionWindow));
+        const bool bWasStuck = cwnd1<= getFlightSpan();
+        // Update Flow Window Size, must update before and together with m_iSndLastAck
+        m_iFlowWindowSize = ackdata[ACKD_BUFFERLEFT];
+        m_iSndLastAck     = ackdata_seqno;
+        m_tsLastRspAckTime  = currtime;
+        m_iReXmitCount    = 1; // Reset re-transmit count since last ACK
+
+        const int cwnd    = std::min(int(m_iFlowWindowSize), int(m_dCongestionWindow));
+        if (bWasStuck && cwnd > getFlightSpan())
         {
-            // Update Flow Window Size, must update before and together with m_iSndLastAck
-            m_iFlowWindowSize = ackdata[ACKD_BUFFERLEFT];
-            m_iSndLastAck     = ackdata_seqno;
-            m_tsLastRspAckTime  = currtime;
-            m_iReXmitCount    = 1; // Reset re-transmit count since last ACK
+            m_pSndQueue->m_pSndUList->update(this, CSndUList::DONT_RESCHEDULE);
+            HLOGC(gglog.Debug,
+                    log << CONID() << "processCtrlAck: could reschedule SND. iFlowWindowSize " << m_iFlowWindowSize
+                    << " SPAN " << getFlightSpan() << " ackdataseqno %" << ackdata_seqno);
         }
+    }
 
         /*
          * We must not ignore full ack received by peer

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;