[coap] clear cached coap requests with rloc source when rloc changes (#4485)

This commit helps to reduce unnecessary CoAP retransmission when rloc
changes. When comes to SRV_DATA.ntf, it also helps to avoid invalid
network data registration. Take the issue reported in #4472 as example,
in a Thread Network with packet loss, the child server may switch parents
frequently, there might be cached SRV_DATA.ntf including old rloc16,
both the cached one and new one might be sent after switched, causing
Leader improperly contain two server entries in the Network Data.

src/core/coap/coap.cpp

@@ -79,6 +79,24 @@ void CoapBase::ClearRequestsAndResponses(void)
     mResponsesQueue.DequeueAllResponses();
 }
 
+void CoapBase::ClearRequests(const Ip6::Address &aAddress)
+{
+    Message *nextMessage;
+
+    // Remove pending messages with the specified source.
+    for (Message *message = static_cast<Message *>(mPendingRequests.GetHead()); message != NULL; message = nextMessage)
+    {
+        CoapMetadata coapMetadata;
+        nextMessage = static_cast<Message *>(message->GetNext());
+        coapMetadata.ReadFrom(*message);
+
+        if (coapMetadata.mSourceAddress == aAddress)
+        {
+            FinalizeCoapTransaction(*message, coapMetadata, NULL, NULL, OT_ERROR_ABORT);
+        }
+    }
+}
+
 otError CoapBase::AddResource(Resource &aResource)
 {
     return mResources.Add(aResource);

src/core/coap/coap.hpp

@@ -453,6 +453,14 @@ public:
      */
     void ClearRequestsAndResponses(void);
 
+    /**
+     * This method clears requests with specified source address used by this CoAP agent.
+     *
+     * @param[in]  aAddress A reference to the specified address.
+     *
+     */
+    void ClearRequests(const Ip6::Address &aAddress);
+
     /**
      * This method adds a resource to the CoAP server.
      *

src/core/thread/mle.cpp

@@ -978,6 +978,12 @@ void Mle::SetRloc16(uint16_t aRloc16)
     if (aRloc16 != oldRloc16)
     {
         otLogNoteMle("RLOC16 %04x -> %04x", oldRloc16, aRloc16);
+
+        // Clear cached CoAP with old RLOC source
+        if (oldRloc16 != Mac::kShortAddrInvalid)
+        {
+            Get<Coap::Coap>().ClearRequests(mMeshLocal16.GetAddress());
+        }
     }
 
     Get<ThreadNetif>().RemoveUnicastAddress(mMeshLocal16);