[dnssd] deduplicate service instance updates

When OTBR_MDNS=mDNSResponder is enabled, redundant service browse and
resolve callbacks can occur for already discovered TREL peers. Because
TrelDnssd::OnTrelServiceInstanceAdded unconditionally called
OnTrelServiceInstanceRemoved right at the start of the function, each
duplicate mDNS update triggered peer removal (kDnssdRemoved) followed
by peer re-addition (kReAdded).

This continuous state cycling in OpenThread's TrelPeerTable caused
unnecessary multi-radio link probing and out-of-order fragment delivery
between 15.4 and TREL interfaces, leading to kErrorDuplicated drops on
valid ICMPv6 Echo Reply fragments during integration tests.

Before calling OnTrelServiceInstanceRemoved, check if the peer entry
already exists with identical socket address and TXT data. If unchanged,
update mDiscoverTime and exit without triggering peer removal or
re-addition in OpenThread.

Author: jwhui

src/trel_dnssd/trel_dnssd.cpp

@@ -289,8 +289,7 @@ void TrelDnssd::OnTrelServiceInstanceAdded(const Mdns::Publisher::DiscoveredInst
     Ip6Address         selectedAddress;
     otPlatTrelPeerInfo peerInfo;
 
-    // Remove any existing TREL service instance before adding
-    OnTrelServiceInstanceRemoved(instanceName);
+    memset(&peerInfo, 0, sizeof(peerInfo));
 
     otbrLogDebug("Peer discovered: %s hostname %s addresses %zu port %d priority %d "
                  "weight %d",
@@ -331,6 +330,25 @@ void TrelDnssd::OnTrelServiceInstanceAdded(const Mdns::Publisher::DiscoveredInst
 
         VerifyOrExit(peer.mValid, otbrLogWarning("Peer %s is invalid", aInstanceInfo.mName.c_str()));
 
+        auto it = mPeers.find(instanceName);
+        if (it != mPeers.end())
+        {
+            if (it->second.Matches(peer))
+            {
+                it->second.mDiscoverTime = Clock::now();
+                ExitNow();
+            }
+
+            if (memcmp(&it->second.mExtAddr, &peer.mExtAddr, sizeof(otExtAddress)) != 0)
+            {
+                OnTrelServiceInstanceRemoved(instanceName);
+            }
+            else
+            {
+                mPeers.erase(it);
+            }
+        }
+
         otPlatTrelHandleDiscoveredPeerInfo(mHost.GetInstance(), &peerInfo);
 
         mPeers.emplace(instanceName, peer);
@@ -387,6 +405,8 @@ void TrelDnssd::NotifyRemovePeer(const Peer &aPeer)
 {
     otPlatTrelPeerInfo peerInfo;
 
+    memset(&peerInfo, 0, sizeof(peerInfo));
+
     peerInfo.mRemoved   = true;
     peerInfo.mTxtData   = aPeer.mTxtData.data();
     peerInfo.mTxtLength = aPeer.mTxtData.size();
@@ -466,7 +486,8 @@ uint16_t TrelDnssd::CountDuplicatePeers(const TrelDnssd::Peer &aPeer)
             continue;
         }
 
-        if (!memcmp(&entry.second.mSockAddr, &aPeer.mSockAddr, sizeof(otSockAddr)) &&
+        if ((entry.second.mSockAddr.mPort == aPeer.mSockAddr.mPort) &&
+            !memcmp(&entry.second.mSockAddr.mAddress, &aPeer.mSockAddr.mAddress, sizeof(otIp6Address)) &&
             !memcmp(&entry.second.mExtAddr, &aPeer.mExtAddr, sizeof(otExtAddress)))
         {
             count++;

src/trel_dnssd/trel_dnssd.hpp

@@ -39,6 +39,7 @@
 #if OTBR_ENABLE_TREL_DNSSD
 
 #include <assert.h>
+#include <string.h>
 #include <utility>
 
 #include <openthread/instance.h>
@@ -141,6 +142,13 @@ class TrelDnssd : public Mdns::StateObserver
 
         void ReadExtAddrFromTxtData(void);
 
+        bool Matches(const Peer &aOther) const
+        {
+            return (mSockAddr.mPort == aOther.mSockAddr.mPort) &&
+                   (memcmp(&mSockAddr.mAddress, &aOther.mSockAddr.mAddress, sizeof(otIp6Address)) == 0) &&
+                   (mTxtData == aOther.mTxtData);
+        }
+
         Clock::time_point    mDiscoverTime;
         std::vector<uint8_t> mTxtData;
         otSockAddr           mSockAddr;