[core] Stats: do not count discarded packets as dropped (#2932).

Valid for a broadcast group member dropping existing packets from the RCV buffer because they were read from another member.

Co-authored-by: Sektor van Skijlen <ethouris@gmail.com>

srtcore/buffer_rcv.cpp

@@ -206,7 +206,7 @@ int CRcvBuffer::insert(CUnit* unit)
     return 0;
 }
 
-int CRcvBuffer::dropUpTo(int32_t seqno)
+std::pair<int, int> CRcvBuffer::dropUpTo(int32_t seqno)
 {
     IF_RCVBUF_DEBUG(ScopedLog scoped_log);
     IF_RCVBUF_DEBUG(scoped_log.ss << "CRcvBuffer::dropUpTo: seqno " << seqno << " m_iStartSeqNo " << m_iStartSeqNo);
@@ -215,16 +215,23 @@ int CRcvBuffer::dropUpTo(int32_t seqno)
     if (len <= 0)
     {
         IF_RCVBUF_DEBUG(scoped_log.ss << ". Nothing to drop.");
-        return 0;
+        return std::make_pair(0, 0);
     }
 
     m_iMaxPosOff -= len;
     if (m_iMaxPosOff < 0)
         m_iMaxPosOff = 0;
 
-    const int iDropCnt = len;
+    int iNumDropped = 0; // Number of dropped packets that were missing.
+    int iNumDiscarded = 0; // The number of dropped packets that existed in the buffer.
     while (len > 0)
     {
+        // Note! Dropping a EntryState_Read must not be counted as a drop because it was read.
+        // Note! Dropping a EntryState_Drop must not be counted as a drop because it was already dropped and counted earlier.
+        if (m_entries[m_iStartPos].status == EntryState_Avail)
+			++iNumDiscarded;
+        else if (m_entries[m_iStartPos].status == EntryState_Empty)
+			++iNumDropped;
         dropUnitInPos(m_iStartPos);
         m_entries[m_iStartPos].status = EntryState_Empty;
         SRT_ASSERT(m_entries[m_iStartPos].pUnit == NULL && m_entries[m_iStartPos].status == EntryState_Empty);
@@ -246,7 +253,7 @@ int CRcvBuffer::dropUpTo(int32_t seqno)
     }
     if (!m_tsbpd.isEnabled() && m_bMessageAPI)
         updateFirstReadableOutOfOrder();
-    return iDropCnt;
+    return std::make_pair(iNumDropped, iNumDiscarded);
 }
 
 int CRcvBuffer::dropAll()
@@ -255,7 +262,8 @@ int CRcvBuffer::dropAll()
         return 0;
 
     const int end_seqno = CSeqNo::incseq(m_iStartSeqNo, m_iMaxPosOff);
-    return dropUpTo(end_seqno);
+    const std::pair<int, int> numDropped = dropUpTo(end_seqno);
+    return numDropped.first + numDropped.second;
 }
 
 int CRcvBuffer::dropMessage(int32_t seqnolo, int32_t seqnohi, int32_t msgno, DropActionIfExists actionOnExisting)

srtcore/buffer_rcv.h

@@ -66,8 +66,8 @@ class CRcvBuffer
 
     /// Drop packets in the receiver buffer from the current position up to the seqno (excluding seqno).
     /// @param [in] seqno drop units up to this sequence number
-    /// @return  number of dropped packets.
-    int dropUpTo(int32_t seqno);
+    /// @return number of dropped (missing) and discarded (available) packets as a pair(dropped, discarded).
+    std::pair<int, int> dropUpTo(int32_t seqno);
 
     /// @brief Drop all the packets in the receiver buffer.
     /// The starting position and seqno are shifted right after the last packet in the buffer.

srtcore/core.cpp

@@ -5576,24 +5576,30 @@ void * srt::CUDT::tsbpd(void* param)
     return NULL;
 }
 
-int srt::CUDT::rcvDropTooLateUpTo(int seqno)
+int srt::CUDT::rcvDropTooLateUpTo(int seqno, DropReason reason)
 {
     // Make sure that it would not drop over m_iRcvCurrSeqNo, which may break senders.
     if (CSeqNo::seqcmp(seqno, CSeqNo::incseq(m_iRcvCurrSeqNo)) > 0)
         seqno = CSeqNo::incseq(m_iRcvCurrSeqNo);
 
     dropFromLossLists(SRT_SEQNO_NONE, CSeqNo::decseq(seqno));
 
-    const int iDropCnt = m_pRcvBuffer->dropUpTo(seqno);
-    if (iDropCnt > 0)
+    const std::pair<int, int> iDropDiscardedPkts = m_pRcvBuffer->dropUpTo(seqno);
+    const int iDropCnt = iDropDiscardedPkts.first;
+    const int iDiscardedCnt = iDropDiscardedPkts.second;
+    const int iDropCntTotal = iDropCnt + iDiscardedCnt;
+
+    // In case of DROP_TOO_LATE discarded packets should also be counted because they are not read from another member socket.
+    const int iDropStatCnt = (reason == DROP_DISCARD) ? iDropCnt : iDropCntTotal;
+    if (iDropStatCnt > 0)
     {
         enterCS(m_StatsLock);
         // Estimate dropped bytes from average payload size.
         const uint64_t avgpayloadsz = m_pRcvBuffer->getRcvAvgPayloadSize();
-        m_stats.rcvr.dropped.count(stats::BytesPackets(iDropCnt * avgpayloadsz, (uint32_t) iDropCnt));
+        m_stats.rcvr.dropped.count(stats::BytesPackets(iDropStatCnt * avgpayloadsz, (uint32_t)iDropStatCnt));
         leaveCS(m_StatsLock);
     }
-    return iDropCnt;
+    return iDropCntTotal;
 }
 
 void srt::CUDT::setInitialRcvSeq(int32_t isn)
@@ -7835,7 +7841,7 @@ void srt::CUDT::dropToGroupRecvBase()
         return;
 
     ScopedLock lck(m_RcvBufferLock);
-    int cnt = rcvDropTooLateUpTo(CSeqNo::incseq(group_recv_base));
+    const int cnt = rcvDropTooLateUpTo(CSeqNo::incseq(group_recv_base), DROP_DISCARD);
     if (cnt > 0)
     {
         HLOGC(grlog.Debug,
@@ -8063,6 +8069,7 @@ int srt::CUDT::sendCtrlAck(CPacket& ctrlpkt, int size)
     string reason; // just for "a reason" of giving particular % for ACK
 
 #if ENABLE_BONDING
+    // TODO: The group drops other members upon reading, maybe no longer needed here?
     dropToGroupRecvBase();
 #endif
 

srtcore/core.h

@@ -755,11 +755,18 @@ class CUDT
     // TSBPD thread main function.
     static void* tsbpd(void* param);
 
+    enum DropReason
+    {
+        DROP_TOO_LATE, //< Drop to keep up to the live pace (TLPKTDROP).
+        DROP_DISCARD   //< Drop because another group member already provided these packets.
+    };
+
     /// Drop too late packets (receiver side). Update loss lists and ACK positions.
     /// The @a seqno packet itself is not dropped.
     /// @param seqno [in] The sequence number of the first packets following those to be dropped.
+    /// @param reason A reason for dropping (see @a DropReason).
     /// @return The number of packets dropped.
-    int rcvDropTooLateUpTo(int seqno);
+    int rcvDropTooLateUpTo(int seqno, DropReason reason = DROP_TOO_LATE);
 
     static loss_seqs_t defaultPacketArrival(void* vself, CPacket& pkt);
     static loss_seqs_t groupPacketArrival(void* vself, CPacket& pkt);

srtcore/group.cpp

@@ -2338,7 +2338,7 @@ int CUDTGroup::recv(char* buf, int len, SRT_MSGCTRL& w_mc)
             ScopedLock  lg(ps->core().m_RcvBufferLock);
             if (m_RcvBaseSeqNo != SRT_SEQNO_NONE)
             {
-                const int cnt = ps->core().rcvDropTooLateUpTo(CSeqNo::incseq(m_RcvBaseSeqNo));
+                const int cnt = ps->core().rcvDropTooLateUpTo(CSeqNo::incseq(m_RcvBaseSeqNo), CUDT::DROP_DISCARD);
                 if (cnt > 0)
                 {
                     HLOGC(grlog.Debug,