[netif] change SubscribeMulticast() to return void (#4942)

src/core/backbone_router/local.cpp

@@ -224,7 +224,7 @@ void Local::SetState(BackboneRouterState aState)
     {
         // Subscribe All Network Backbone Routers Multicast Address for both Secondary and Primary state.
         mAllNetworkBackboneRouters.GetAddress().SetMulticastNetworkPrefix(Get<Mle::MleRouter>().GetMeshLocalPrefix());
-        IgnoreError(Get<ThreadNetif>().SubscribeMulticast(mAllNetworkBackboneRouters));
+        Get<ThreadNetif>().SubscribeMulticast(mAllNetworkBackboneRouters);
     }
     else if (aState == OT_BACKBONE_ROUTER_STATE_DISABLED)
     {
@@ -353,7 +353,7 @@ void Local::ApplyMeshLocalPrefix(void)
 
     IgnoreError(Get<ThreadNetif>().UnsubscribeMulticast(mAllNetworkBackboneRouters));
     mAllNetworkBackboneRouters.GetAddress().SetMulticastNetworkPrefix(Get<Mle::MleRouter>().GetMeshLocalPrefix());
-    IgnoreError(Get<ThreadNetif>().SubscribeMulticast(mAllNetworkBackboneRouters));
+    Get<ThreadNetif>().SubscribeMulticast(mAllNetworkBackboneRouters);
 
     if (IsPrimary())
     {
@@ -382,7 +382,7 @@ void Local::UpdateAllDomainBackboneRouters(Leader::DomainPrefixState aState)
     if (aState == Leader::kDomainPrefixAdded || aState == Leader::kDomainPrefixRefreshed)
     {
         mAllDomainBackboneRouters.GetAddress().SetMulticastNetworkPrefix(*Get<Leader>().GetDomainPrefix());
-        IgnoreError(Get<ThreadNetif>().SubscribeMulticast(mAllDomainBackboneRouters));
+        Get<ThreadNetif>().SubscribeMulticast(mAllDomainBackboneRouters);
     }
 
 exit:

src/core/net/netif.cpp

@@ -303,11 +303,9 @@ exit:
     return error;
 }
 
-otError Netif::SubscribeMulticast(NetifMulticastAddress &aAddress)
+void Netif::SubscribeMulticast(NetifMulticastAddress &aAddress)
 {
-    otError error;
-
-    SuccessOrExit(error = mMulticastAddresses.Add(aAddress));
+    SuccessOrExit(mMulticastAddresses.Add(aAddress));
 
     Get<Notifier>().Signal(OT_CHANGED_IP6_MULTICAST_SUBSCRIBED);
 
@@ -315,7 +313,7 @@ otError Netif::SubscribeMulticast(NetifMulticastAddress &aAddress)
     mAddressCallback(&aAddress.mAddress, kMulticastPrefixLength, /* IsAdded */ true, mAddressCallbackContext);
 
 exit:
-    return error;
+    return;
 }
 
 otError Netif::UnsubscribeMulticast(const NetifMulticastAddress &aAddress)

src/core/net/netif.hpp

@@ -309,11 +309,8 @@ public:
      *
      * @param[in]  aAddress  A reference to the multicast address.
      *
-     * @retval OT_ERROR_NONE     Successfully subscribed to @p aAddress.
-     * @retval OT_ERROR_ALREADY  The multicast address is already subscribed.
-     *
      */
-    otError SubscribeMulticast(NetifMulticastAddress &aAddress);
+    void SubscribeMulticast(NetifMulticastAddress &aAddress);
 
     /**
      * This method unsubscribes the network interface to a multicast address.

src/core/thread/mle.cpp

@@ -940,8 +940,8 @@ void Mle::ApplyMeshLocalPrefix(void)
 
     // Add the addresses back into the table.
     Get<ThreadNetif>().AddUnicastAddress(mMeshLocal64);
-    IgnoreError(Get<ThreadNetif>().SubscribeMulticast(mLinkLocalAllThreadNodes));
-    IgnoreError(Get<ThreadNetif>().SubscribeMulticast(mRealmLocalAllThreadNodes));
+    Get<ThreadNetif>().SubscribeMulticast(mLinkLocalAllThreadNodes);
+    Get<ThreadNetif>().SubscribeMulticast(mRealmLocalAllThreadNodes);
 
     if (IsAttached())
     {

tests/unit/test_netif.cpp

@@ -136,11 +136,11 @@ void TestNetifMulticastAddresses(void)
                  "UnsubscribeAllRoutersMulticast() did not fail when not subscribed");
 
     IgnoreError(netifAddress.GetAddress().FromString(kTestAddress1));
-    SuccessOrQuit(netif.SubscribeMulticast(netifAddress), "SubscribeMulticast() failed");
+    netif.SubscribeMulticast(netifAddress);
     VerifyMulticastAddressList(netif, &addresses[2], 4);
 
-    VerifyOrQuit(netif.SubscribeMulticast(netifAddress) == OT_ERROR_ALREADY,
-                 "SubscribeMulticast() did not fail when address was already subscribed");
+    netif.SubscribeMulticast(netifAddress);
+    VerifyMulticastAddressList(netif, &addresses[2], 4);
 
     SuccessOrQuit(netif.UnsubscribeAllNodesMulticast(), "UnsubscribeAllNodesMulticast() failed");
     VerifyMulticastAddressList(netif, &addresses[5], 1);