[routeorch]: Fix observer detaching procedure for next hop observers (sonic-net#985)

- erase the correct observer in the observer list
- remove the NextHopObserverEntry once no observer is observing the entry
- refine some logs and comments

Signed-off-by: Shu0T1an ChenG <[email protected]>

Author: stcheng

orchagent/routeorch.cpp

@@ -160,6 +160,7 @@ void RouteOrch::attach(Observer *observer, const IpAddress& dstAddr)
             dstAddr.to_string().c_str());
 
     // Trigger next hop change for the first time the observer is attached
+    // Note that rbegin() is pointing to the entry with longest prefix match
     auto route = observerEntry->second.routeTable.rbegin();
     if (route != observerEntry->second.routeTable.rend())
     {
@@ -171,19 +172,34 @@ void RouteOrch::attach(Observer *observer, const IpAddress& dstAddr)
 void RouteOrch::detach(Observer *observer, const IpAddress& dstAddr)
 {
     SWSS_LOG_ENTER();
+
     auto observerEntry = m_nextHopObservers.find(dstAddr);
 
     if (observerEntry == m_nextHopObservers.end())
     {
-        SWSS_LOG_ERROR("Failed to detach observer for %s. Entry not found.\n", dstAddr.to_string().c_str());
+        SWSS_LOG_ERROR("Failed to locate observer for destination IP %s",
+                dstAddr.to_string().c_str());
         assert(false);
+        return;
     }
 
-    for (auto iter = observerEntry->second.observers.begin(); iter != observerEntry->second.observers.end(); ++iter)
+    // Find the observer
+    for (auto iter = observerEntry->second.observers.begin();
+            iter != observerEntry->second.observers.end(); ++iter)
     {
         if (observer == *iter)
         {
-            m_observers.erase(iter);
+            observerEntry->second.observers.erase(iter);
+
+            SWSS_LOG_NOTICE("Detached next hop observer for destination IP %s",
+                    dstAddr.to_string().c_str());
+
+            // Remove NextHopObserverEntry if no observer is tracking this
+            // destination IP.
+            if (observerEntry->second.observers.empty())
+            {
+                m_nextHopObservers.erase(observerEntry);
+            }
             break;
         }
     }