[meshcop] give priority to better matches during join, not channel (#2137)

Previous behaviour of the joiner would result in the joiner joining
the network with the highest channel number, rather than the most
suitable. This commit utilizes the priority specified in thread spec
1.1.1 section 8.4.4.1.2 to choose the most suited network for joining.
This removes the bug where it would be impossible to join the desired
network due to another network on a higher channel allowing all joiners
in the steering data.

src/core/meshcop/joiner.cpp

@@ -67,7 +67,9 @@ Joiner::Joiner(ThreadNetif &aNetif):
     mContext(NULL),
     mCcitt(0),
     mAnsi(0),
+    mJoinerRouterIsSpecific(false),
     mJoinerRouterChannel(0),
+    mJoinerRouterRssi(0),
     mJoinerRouterPanId(0),
     mJoinerUdpPort(0),
     mVendorName(NULL),
@@ -119,6 +121,8 @@ otError Joiner::Start(const char *aPSKd, const char *aProvisioningUrl,
     SuccessOrExit(error);
 
     mJoinerRouterPanId = Mac::kPanIdBroadcast;
+    mJoinerRouterRssi = 0;
+    mJoinerRouterIsSpecific = false;
     SuccessOrExit(error = netif.GetMle().Discover(0, netif.GetMac().GetPanId(), true, false, HandleDiscoverResult,
                                                   this));
 
@@ -197,19 +201,32 @@ void Joiner::HandleDiscoverResult(otActiveScanResult *aResult)
         steeringData.SetLength(aResult->mSteeringData.mLength);
         memcpy(steeringData.GetValue(), aResult->mSteeringData.m8, steeringData.GetLength());
 
-        if (steeringData.DoesAllowAny() ||
-            (steeringData.GetBit(mCcitt % steeringData.GetNumBits()) &&
-             steeringData.GetBit(mAnsi % steeringData.GetNumBits())))
+        if (steeringData.DoesAllowAny())
         {
-            mJoinerUdpPort = aResult->mJoinerUdpPort;
-            mJoinerRouterPanId = aResult->mPanId;
-            mJoinerRouterChannel = aResult->mChannel;
-            memcpy(&mJoinerRouter, &aResult->mExtAddress, sizeof(mJoinerRouter));
+            VerifyOrExit(!mJoinerRouterIsSpecific);
+            VerifyOrExit(aResult->mRssi > mJoinerRouterRssi);
+        }
+        else if (steeringData.GetBit(mCcitt % steeringData.GetNumBits()) &&
+                 steeringData.GetBit(mAnsi % steeringData.GetNumBits()))
+        {
+            if (mJoinerRouterIsSpecific)
+            {
+                VerifyOrExit(aResult->mRssi > mJoinerRouterRssi);
+            }
+
+            mJoinerRouterIsSpecific = true;
         }
         else
         {
             otLogDebgMeshCoP(GetInstance(), "Steering data does not include this device");
+            ExitNow();
         }
+
+        mJoinerUdpPort = aResult->mJoinerUdpPort;
+        mJoinerRouterPanId = aResult->mPanId;
+        mJoinerRouterChannel = aResult->mChannel;
+        mJoinerRouterRssi = aResult->mRssi;
+        memcpy(&mJoinerRouter, &aResult->mExtAddress, sizeof(mJoinerRouter));
     }
     else if (mJoinerRouterPanId != Mac::kPanIdBroadcast)
     {

src/core/meshcop/joiner.hpp

@@ -140,7 +140,9 @@ private:
     uint16_t mCcitt;
     uint16_t mAnsi;
 
+    bool mJoinerRouterIsSpecific;
     uint8_t mJoinerRouterChannel;
+    int8_t mJoinerRouterRssi;
     uint16_t mJoinerRouterPanId;
     uint16_t mJoinerUdpPort;
     Mac::ExtAddress mJoinerRouter;