[data-poll-sender] add GetParent() (#4419)

This commit changes the `Mle::GetParentCandidate()` to return a
reference to the parent candidate and moves the selection logic
between parent or parent candidate when sending data polls to
`DataPollSender` from the newly added `GetParent()` method.

src/core/mac/data_poll_sender.cpp

@@ -62,6 +62,13 @@ DataPollSender::DataPollSender(Instance &aInstance)
 {
 }
 
+const Neighbor &DataPollSender::GetParent(void) const
+{
+    const Neighbor &parentCandidate = Get<Mle::MleRouter>().GetParentCandidate();
+
+    return parentCandidate.IsStateValid() ? parentCandidate : Get<Mle::MleRouter>().GetParent();
+}
+
 otError DataPollSender::StartPolling(void)
 {
     otError error = OT_ERROR_NONE;
@@ -90,14 +97,12 @@ void DataPollSender::StopPolling(void)
 
 otError DataPollSender::SendDataPoll(void)
 {
-    otError   error;
-    Neighbor *parent;
+    otError error;
 
     VerifyOrExit(mEnabled, error = OT_ERROR_INVALID_STATE);
     VerifyOrExit(!Get<Mac::Mac>().GetRxOnWhenIdle(), error = OT_ERROR_INVALID_STATE);
 
-    parent = Get<Mle::MleRouter>().GetParentCandidate();
-    VerifyOrExit((parent != NULL) && parent->IsStateValidOrRestoring(), error = OT_ERROR_INVALID_STATE);
+    VerifyOrExit(GetParent().IsStateValidOrRestoring(), error = OT_ERROR_INVALID_STATE);
 
     mTimer.Stop();
 
@@ -133,18 +138,20 @@ exit:
 
 otError DataPollSender::GetPollDestinationAddress(Mac::Address &aDest) const
 {
-    otError   error  = OT_ERROR_NONE;
-    Neighbor *parent = Get<Mle::MleRouter>().GetParentCandidate();
+    otError         error  = OT_ERROR_NONE;
+    const Neighbor &parent = GetParent();
 
-    VerifyOrExit((parent != NULL) && parent->IsStateValidOrRestoring(), error = OT_ERROR_ABORT);
+    VerifyOrExit(parent.IsStateValidOrRestoring(), error = OT_ERROR_ABORT);
 
-    if ((Get<Mac::Mac>().GetShortAddress() == Mac::kShortAddrInvalid) || (parent != &Get<Mle::MleRouter>().GetParent()))
+    // Use extended address attaching to a new parent (i.e. parent is the parent candidate).
+    if ((Get<Mac::Mac>().GetShortAddress() == Mac::kShortAddrInvalid) ||
+        (&parent == &Get<Mle::MleRouter>().GetParentCandidate()))
     {
-        aDest.SetExtended(parent->GetExtAddress());
+        aDest.SetExtended(parent.GetExtAddress());
     }
     else
     {
-        aDest.SetShort(parent->GetRloc16());
+        aDest.SetShort(parent.GetRloc16());
     }
 
 exit:
@@ -205,7 +212,7 @@ void DataPollSender::HandlePollSent(Mac::TxFrame &aFrame, otError aError)
         Get<MeshForwarder>().UpdateNeighborOnSentFrame(aFrame, aError, macDest);
     }
 
-    if (Get<Mle::MleRouter>().GetParentCandidate()->IsStateInvalid())
+    if (GetParent().IsStateInvalid())
     {
         StopPolling();
         Get<Mle::MleRouter>().BecomeDetached();

src/core/mac/data_poll_sender.hpp

@@ -40,6 +40,7 @@
 #include "common/locator.hpp"
 #include "common/timer.hpp"
 #include "mac/mac_frame.hpp"
+#include "thread/topology.hpp"
 
 namespace ot {
 
@@ -266,10 +267,11 @@ private:
         kRecalculatePollPeriod,
     };
 
-    void        ScheduleNextPoll(PollPeriodSelector aPollPeriodSelector);
-    uint32_t    CalculatePollPeriod(void) const;
-    static void HandlePollTimer(Timer &aTimer);
-    static void UpdateIfLarger(uint32_t &aPeriod, uint32_t aNewPeriod);
+    void            ScheduleNextPoll(PollPeriodSelector aPollPeriodSelector);
+    uint32_t        CalculatePollPeriod(void) const;
+    const Neighbor &GetParent(void) const;
+    static void     HandlePollTimer(Timer &aTimer);
+    static void     UpdateIfLarger(uint32_t &aPeriod, uint32_t aNewPeriod);
 
     TimeMilli mTimerStartTime;
     uint32_t  mPollPeriod;

src/core/thread/mle.cpp

@@ -3959,22 +3959,6 @@ bool Mle::IsMeshLocalAddress(const Ip6::Address &aAddress) const
     return aAddress.PrefixMatch(GetMeshLocal16()) >= Ip6::Address::kMeshLocalPrefixLength;
 }
 
-Router *Mle::GetParentCandidate(void)
-{
-    Router *rval;
-
-    if (mParentCandidate.IsStateValid())
-    {
-        rval = &mParentCandidate;
-    }
-    else
-    {
-        rval = &mParent;
-    }
-
-    return rval;
-}
-
 otError Mle::CheckReachability(uint16_t aMeshSource, uint16_t aMeshDest, Ip6::Header &aIp6Header)
 {
     otError          error = OT_ERROR_DROP;

src/core/thread/mle.hpp

@@ -755,15 +755,12 @@ public:
     Router &GetParent(void) { return mParent; }
 
     /**
-     * This method returns a pointer to the parent candidate or parent.
+     * This method get the parent candidate.
      *
-     * This method is useful when sending IEEE 802.15.4 Data Request frames while attempting to attach to a new parent.
-     *
-     * If attempting to attach to a new parent, this method returns the parent candidate.
-     * If not attempting to attach, this method returns the parent.
+     * The parent candidate is valid when attempting to attach to a new parent.
      *
      */
-    Router *GetParentCandidate(void);
+    Router &GetParentCandidate(void) { return mParentCandidate; }
 
     /**
      * This method indicates whether or not an IPv6 address is an RLOC.