[core] Fixed SRT_EPOLL_IN epoll event loss problem (#1843)

srtcore/core.cpp

@@ -7554,13 +7554,27 @@ int CUDT::sendCtrlAck(CPacket& ctrlpkt, int size)
         }
         else
         {
-            if (m_config.bSynRecving)
             {
-                // signal a waiting "recv" call if there is any data available
-                CSync::lock_signal(m_RecvDataCond, m_RecvLock);
+                UniqueLock rdlock (m_RecvLock);
+                CSync      rdcond (m_RecvDataCond, rdlock);
+                if (m_config.bSynRecving)
+                {
+                    // signal a waiting "recv" call if there is any data available
+                    rdcond.signal_locked(rdlock);
+                }
+                // acknowledge any waiting epolls to read
+                // fix SRT_EPOLL_IN event loss but rcvbuffer still have data：
+                // 1. user call receive/receivemessage(about line number:6482)
+                // 2. after read/receive, if rcvbuffer is empty, will set SRT_EPOLL_IN event to false
+                // 3. but if we do not do some lock work here, will cause some sync problems between threads:
+                //      (1) user thread: call receive/receivemessage
+                //      (2) user thread: read data
+                //      (3) user thread: no data in rcvbuffer, set SRT_EPOLL_IN event to false
+                //      (4) receive thread: receive data and set SRT_EPOLL_IN to true
+                //      (5) user thread: set SRT_EPOLL_IN to false
+                // 4. so , m_RecvLock must be used here to protect epoll event
+                s_UDTUnited.m_EPoll.update_events(m_SocketID, m_sPollID, SRT_EPOLL_IN, true);
             }
-            // acknowledge any waiting epolls to read
-            s_UDTUnited.m_EPoll.update_events(m_SocketID, m_sPollID, SRT_EPOLL_IN, true);
 #if ENABLE_EXPERIMENTAL_BONDING
             if (m_parent->m_GroupOf)
             {