Remove system neighbor DEL operation in m_toSync if SET operation for (#2853)

*Remove system neighbor DEL operation in m_toSync if SET operation for the same neighbor succeeds. This is to avoid mistakenly removing the system neighbor.

Fix sonic-net/sonic-buildimage#15266.

Author: ysmanman

orchagent/neighorch.cpp

@@ -1420,6 +1420,17 @@ void NeighOrch::doVoqSystemNeighTask(Consumer &consumer)
                     {
                         SWSS_LOG_NOTICE("VOQ encap index updated for neighbor %s", kfvKey(t).c_str());
                         it = consumer.m_toSync.erase(it);
+
+                       /* Remove remaining DEL operation in m_toSync for the same neighbor.
+                        * Since DEL operation is supposed to be executed before SET for the same neighbor
+                        * A remaining DEL after the SET operation means the DEL operation failed previously and should not be executed anymore
+                        */
+                       auto rit = make_reverse_iterator(it);
+                       while (rit != consumer.m_toSync.rend() && rit->first == key && kfvOp(rit->second) == DEL_COMMAND)
+                       {
+                           consumer.m_toSync.erase(next(rit).base());
+                           SWSS_LOG_NOTICE("Removed pending system neighbor DEL operation for %s after SET operation", key.c_str());
+                       }
                     }
                     continue;
                 }
@@ -1481,6 +1492,7 @@ void NeighOrch::doVoqSystemNeighTask(Consumer &consumer)
                 else
                 {
                     it++;
+                    continue;
                 }
             }
             else
@@ -1489,6 +1501,17 @@ void NeighOrch::doVoqSystemNeighTask(Consumer &consumer)
                 SWSS_LOG_INFO("System neighbor %s already exists", kfvKey(t).c_str());
                 it = consumer.m_toSync.erase(it);
             }
+
+            /* Remove remaining DEL operation in m_toSync for the same neighbor.
+             * Since DEL operation is supposed to be executed before SET for the same neighbor
+             * A remaining DEL after the SET operation means the DEL operation failed previously and should not be executed anymore
+             */
+            auto rit = make_reverse_iterator(it);
+            while (rit != consumer.m_toSync.rend() && rit->first == key && kfvOp(rit->second) == DEL_COMMAND)
+            {
+                consumer.m_toSync.erase(next(rit).base());
+                SWSS_LOG_NOTICE("Removed pending system neighbor DEL operation for %s after SET operation", key.c_str());
+            }
         }
         else if (op == DEL_COMMAND)
         {