[data-poll-sender] add logic to select poll destination (#4023)

This commit changes the interaction of `Mac` and `DataPollSender` such
that the selection of a data poll destination MAC address is delegated
to `DataPollSender` (related code is moved into `DataPollSender` from
`Mac`).

src/core/mac/data_poll_sender.cpp

@@ -131,6 +131,26 @@ exit:
     return error;
 }
 
+otError DataPollSender::GetPollDestinationAddress(Mac::Address &aDest) const
+{
+    otError   error  = OT_ERROR_NONE;
+    Neighbor *parent = Get<Mle::MleRouter>().GetParentCandidate();
+
+    VerifyOrExit((parent != NULL) && parent->IsStateValidOrRestoring(), error = OT_ERROR_ABORT);
+
+    if ((Get<Mac::Mac>().GetShortAddress() == Mac::kShortAddrInvalid) || (parent != Get<Mle::MleRouter>().GetParent()))
+    {
+        aDest.SetExtended(parent->GetExtAddress());
+    }
+    else
+    {
+        aDest.SetShort(parent->GetRloc16());
+    }
+
+exit:
+    return error;
+}
+
 otError DataPollSender::SetExternalPollPeriod(uint32_t aPeriod)
 {
     otError error = OT_ERROR_NONE;

src/core/mac/data_poll_sender.hpp

@@ -131,6 +131,17 @@ public:
      */
     uint32_t GetExternalPollPeriod(void) const { return mExternalPollPeriod; }
 
+    /**
+     * This method gets the destination MAC address for a data poll frame.
+     *
+     * @param[out] aDest       Reference to a `MAC::Address` to output the poll destination address (on success).
+     *
+     * @retval OT_ERROR_NONE   @p aDest was updated successfully.
+     * @retval OT_ERROR_ABORT  Abort the data poll transmission (not currently attached to any parent).
+     *
+     */
+    otError GetPollDestinationAddress(Mac::Address &aDest) const;
+
     /**
      * This method informs the data poll sender of success/error status of a previously requested poll frame
      * transmission.

src/core/mac/mac.cpp

@@ -791,41 +791,31 @@ void Mac::GenerateNonce(const ExtAddress &aAddress, uint32_t aFrameCounter, uint
 
 otError Mac::PrepareDataRequest(Frame &aFrame)
 {
-    otError   error  = OT_ERROR_NONE;
+    otError  error = OT_ERROR_NONE;
-    Neighbor *parent = Get<Mle::MleRouter>().GetParentCandidate();
+    Address  src, dst;
-    uint16_t  fcf;
+    uint16_t fcf;
-    bool      useExtendedAddr;
 
-    VerifyOrExit((parent != NULL) && parent->IsStateValidOrRestoring(), error = OT_ERROR_ABORT);
+    SuccessOrExit(error = Get<DataPollSender>().GetPollDestinationAddress(dst));
+    VerifyOrExit(!dst.IsNone(), error = OT_ERROR_ABORT);
 
     fcf = Frame::kFcfFrameMacCmd | Frame::kFcfPanidCompression | Frame::kFcfFrameVersion2006 | Frame::kFcfAckRequest |
           Frame::kFcfSecurityEnabled;
 
-    useExtendedAddr = (GetShortAddress() == kShortAddrInvalid) || (parent != Get<Mle::MleRouter>().GetParent());
+    if (dst.IsExtended())
-
-    if (useExtendedAddr)
     {
         fcf |= Frame::kFcfDstAddrExt | Frame::kFcfSrcAddrExt;
+        src.SetExtended(GetExtAddress());
     }
     else
     {
         fcf |= Frame::kFcfDstAddrShort | Frame::kFcfSrcAddrShort;
+        src.SetShort(GetShortAddress());
     }
 
     aFrame.InitMacHeader(fcf, Frame::kKeyIdMode1 | Frame::kSecEncMic32);
     aFrame.SetDstPanId(GetPanId());
-
+    aFrame.SetSrcAddr(src);
-    if (useExtendedAddr)
+    aFrame.SetDstAddr(dst);
-    {
-        aFrame.SetSrcAddr(GetExtAddress());
-        aFrame.SetDstAddr(parent->GetExtAddress());
-    }
-    else
-    {
-        aFrame.SetSrcAddr(GetShortAddress());
-        aFrame.SetDstAddr(parent->GetRloc16());
-    }
-
     aFrame.SetCommandId(Frame::kMacCmdDataRequest);
 
 exit: