[mle] remove all queued parent responses when starting to attach (#11996)

When handling a parent request, a response is not sent if the device
is not attached, or if it is attaching. However if the device starts
the attachment process between queuing the parent response and
actually sending it, the parent response will be sent from an invalid
state.

This commit causes all queued parent response messages to be removed
when the device begins the attachment process, to avoid stale messages
from being sent on the link.

src/core/thread/mle.cpp

@@ -3175,6 +3175,16 @@ void Mle::DelayedSender::ScheduleParentResponse(const ParentResponseInfo &aInfo,
     AddSchedule(kTypeParentResponse, destination, aDelay, &aInfo, sizeof(aInfo));
 }
 
+void Mle::DelayedSender::RemoveScheduledParentResponses(void)
+{
+    Ip6::Address destination;
+
+    // The unspecified address will clear all parent responses to any destination
+    destination.Clear();
+
+    RemoveMatchingSchedules(kTypeParentResponse, destination);
+}
+
 void Mle::DelayedSender::ScheduleAdvertisement(const Ip6::Address &aDestination, uint32_t aDelay)
 {
     VerifyOrExit(!HasMatchingSchedule(kTypeAdvertisement, aDestination));
@@ -4426,6 +4436,10 @@ void Mle::Attacher::Attach(AttachMode aMode)
 
     VerifyOrExit(!IsAttaching());
 
+#if OPENTHREAD_FTD
+    Get<Mle>().RemoveScheduledParentResponses();
+#endif
+
     if (!Get<Mle>().IsDetached())
     {
         mAttachCounter = 0;

src/core/thread/mle.hpp

@@ -1046,6 +1046,12 @@ public:
      */
     void ScheduleUnicastAdvertisementTo(const Router &aRouter);
 
+    /**
+     * Remove all parent responses that have been scheduled to be sent at a later time. Used when a precondition
+     * of the parent responses has changed.
+     */
+    void RemoveScheduledParentResponses(void) { mDelayedSender.RemoveScheduledParentResponses(); }
+
 #if OPENTHREAD_CONFIG_MLE_STEERING_DATA_SET_OOB_ENABLE
     /**
      * Sets steering data out of band
@@ -1698,6 +1704,7 @@ private:
         void ScheduleChildUpdateRequestToParent(uint32_t aDelay);
 #if OPENTHREAD_FTD
         void ScheduleParentResponse(const ParentResponseInfo &aInfo, uint32_t aDelay);
+        void RemoveScheduledParentResponses(void);
         void ScheduleAdvertisement(const Ip6::Address &aDestination, uint32_t aDelay);
         void ScheduleMulticastDataResponse(uint32_t aDelay);
         void ScheduleLinkRequest(const Router &aRouter, uint32_t aDelay);