Do not re-evaluate backrefs for ksync flow entries

The backref entries are updated in DB and ksync tasks, which are mutually
exclusive. The flow event task was accessing the same and it doesnt actually
require. Avoiding backref reevaluation.

Change-Id: I8cb9b92196626ea219f7f9e1055e2579c62c7c86
closes-bug: #1726709

src/ksync/ksync_entry.h

@@ -121,6 +121,7 @@ class KSyncEntry {
     // Get an unresolved reference. 
     // This entry will be added into resolveq_ of unresolved-entry
     virtual KSyncEntry *UnresolvedReference() = 0;
+    virtual bool ShouldReEvalBackReference() const { return true; }
 
     // Returns true if entry is resolved and referring entry can be written
     bool IsResolved();

src/ksync/ksync_object.cc

@@ -1397,7 +1397,8 @@ void KSyncObject::NotifyEvent(KSyncEntry *entry, KSyncEntry::KSyncEvent event) {
     }
 
     entry->SetState(state);
-    if (dep_reval == true && entry->IsResolved()) {
+    if (dep_reval == true && entry->IsResolved() &&
+        entry->ShouldReEvalBackReference()) {
         BackRefReEval(entry);
     }
 

src/vnsw/agent/vrouter/ksync/flowtable_ksync.h

@@ -77,6 +77,11 @@ class FlowTableKSyncEntry : public KSyncNetlinkEntry {
     virtual uint32_t GetTableIndex() const;
     virtual bool Sync();
     virtual KSyncEntry *UnresolvedReference();
+    virtual bool ShouldReEvalBackReference() const {
+        // no need to re-eval unresolved reference for flow entries, as they
+        // do not depend on anything
+        return false;
+    }
     bool AllowDeleteStateComp() {return false;}
     virtual void ErrorHandler(int, uint32_t, KSyncEvent) const;
     virtual std::string VrouterError(uint32_t error) const;