wifi: Clear TXOP holder when NAV is reset or counts down to zero

src/wifi/model/he/he-frame-exchange-manager.cc

@@ -1465,7 +1465,7 @@ HeFrameExchangeManager::SendQosNullFramesInTbPpdu(const CtrlTriggerHeader& trigg
     if (trigger.GetCsRequired() && hdr.GetAddr2() != m_txopHolder && m_navEnd > Simulator::Now())
     {
         NS_LOG_DEBUG("Carrier Sensing required and channel busy (TA="
-                     << hdr.GetAddr2() << ", TxopHolder=" << m_txopHolder
+                     << hdr.GetAddr2() << ", TxopHolder=" << (m_txopHolder ? *m_txopHolder : "")
                      << ", NAV end=" << m_navEnd.As(Time::S) << "), do nothing");
         return;
     }

src/wifi/model/qos-frame-exchange-manager.cc

@@ -664,6 +664,13 @@ QosFrameExchangeManager::PreProcessFrame(Ptr<const WifiPsdu> psdu, const WifiTxV
         }
     }
 
+    // before updating the NAV, check if the NAV counted down to zero. In such a
+    // case, clear the saved TXOP holder address.
+    if (m_navEnd <= Simulator::Now())
+    {
+        m_txopHolder.reset();
+    }
+
     FrameExchangeManager::PreProcessFrame(psdu, txVector);
 }
 
@@ -698,6 +705,14 @@ QosFrameExchangeManager::SetTxopHolder(Ptr<const WifiPsdu> psdu, const WifiTxVec
     }
 }
 
+void
+QosFrameExchangeManager::NavResetTimeout()
+{
+    NS_LOG_FUNCTION(this);
+    m_txopHolder.reset();
+    FrameExchangeManager::NavResetTimeout();
+}
+
 void
 QosFrameExchangeManager::ReceiveMpdu(Ptr<const WifiMpdu> mpdu,
                                      RxSignalInfo rxSignalInfo,

src/wifi/model/qos-frame-exchange-manager.h

@@ -22,6 +22,8 @@
 
 #include "frame-exchange-manager.h"
 
+#include <optional>
+
 namespace ns3
 {
 
@@ -111,6 +113,7 @@ class QosFrameExchangeManager : public FrameExchangeManager
                      bool inAmpdu) override;
     void PreProcessFrame(Ptr<const WifiPsdu> psdu, const WifiTxVector& txVector) override;
     void PostProcessFrame(Ptr<const WifiPsdu> psdu, const WifiTxVector& txVector) override;
+    void NavResetTimeout() override;
     Time GetFrameDurationId(const WifiMacHeader& header,
                             uint32_t size,
                             const WifiTxParameters& txParams,
@@ -174,10 +177,10 @@ class QosFrameExchangeManager : public FrameExchangeManager
      */
     virtual void SetTxopHolder(Ptr<const WifiPsdu> psdu, const WifiTxVector& txVector);
 
-    Ptr<QosTxop> m_edca;       //!< the EDCAF that gained channel access
-    Mac48Address m_txopHolder; //!< MAC address of the TXOP holder
-    bool m_setQosQueueSize;    /**< whether to set the Queue Size subfield of the
-                                    QoS Control field of QoS data frames */
+    Ptr<QosTxop> m_edca;                      //!< the EDCAF that gained channel access
+    std::optional<Mac48Address> m_txopHolder; //!< MAC address of the TXOP holder
+    bool m_setQosQueueSize;                   /**< whether to set the Queue Size subfield of the
+                                                   QoS Control field of QoS data frames */
 
   private:
     /**