[syncd/FlexCounter]:Fix last remove bug (sonic-net#679)

- The bug description
When the last object in FlexCounter was removed, the FlexCounter is also removed from FlexCounterManager. At the next time, when a new observed object is added and a new FlexCounter will be created. But the new FlexCounter cannot get the configuration. Because the configuration message from FlexCounterDB has been consumed at the first time.

- How to fix it
Add a flag to indicate whether the configuration also be removed from the FlexCounterDB. If not, don't delete the FlexCounter from FlexCounterManager even no observed objects in the FlexCounter.

Signed-off-by: Ze Gan <ganze718@gmail.com>

syncd/FlexCounter.cpp

@@ -26,6 +26,7 @@ FlexCounter::FlexCounter(
     SWSS_LOG_ENTER();
 
     m_enable = false;
+    m_isDiscarded = false;
 
     startFlexCounterThread();
 }
@@ -819,6 +820,8 @@ void FlexCounter::removeCounterPlugins()
     m_rifPlugins.clear();
     m_priorityGroupPlugins.clear();
     m_bufferPoolPlugins.clear();
+
+    m_isDiscarded = true;
 }
 
 void FlexCounter::addCounterPlugin(
@@ -828,6 +831,8 @@ void FlexCounter::addCounterPlugin(
 
     SWSS_LOG_ENTER();
 
+    m_isDiscarded = false;
+
     for (auto& fvt: values)
     {
         auto& field = fvField(fvt);
@@ -901,6 +906,11 @@ bool FlexCounter::isEmpty()
     return allIdsEmpty() && allPluginsEmpty();
 }
 
+bool FlexCounter::isDiscarded()
+{
+    return isEmpty() && m_isDiscarded;
+}
+
 bool FlexCounter::allIdsEmpty() const
 {
     SWSS_LOG_ENTER();

syncd/FlexCounter.h

@@ -48,6 +48,8 @@ namespace syncd
 
             bool isEmpty();
 
+            bool isDiscarded();
+
         private:
 
             void setPollInterval(
@@ -405,5 +407,7 @@ namespace syncd
             std::shared_ptr<sairedis::SaiInterface> m_vendorSai;
 
             std::string m_dbCounters;
+
+            bool m_isDiscarded;
     };
 }

syncd/FlexCounterManager.cpp

@@ -62,7 +62,7 @@ void FlexCounterManager::removeCounterPlugins(
 
     fc->removeCounterPlugins();
 
-    if (fc->isEmpty())
+    if (fc->isDiscarded())
     {
         removeInstance(instanceId);
     }
@@ -91,7 +91,7 @@ void FlexCounterManager::addCounter(
 
     fc->addCounter(vid, rid, values);
 
-    if (fc->isEmpty())
+    if (fc->isDiscarded())
     {
         removeInstance(instanceId);
     }
@@ -107,7 +107,7 @@ void FlexCounterManager::removeCounter(
 
     fc->removeCounter(vid);
 
-    if (fc->isEmpty())
+    if (fc->isDiscarded())
     {
         removeInstance(instanceId);
     }