Maintain a synchronized DsfUpdate member in DsfSubscription.

Summary:
This diff just trivially locks and moves the
received DSFUpdate into the synchronized member variable.
In stacked diffs, we will use this to synchronize b/w hwUpdate
and stream threads.

Reviewed By: peygar

Differential Revision:
D61231193

Privacy Context Container: L1125642

fbshipit-source-id: aa8176e28085cac2d49529ad855494e6bdf47476

fboss/agent/DsfSubscription.cpp

@@ -211,8 +211,16 @@ void DsfSubscription::handleFsdbUpdate(fsdb::OperSubPathUnit&& operStateUnit) {
           remoteNodeName_);
     }
   }
-  updateWithRollbackProtection(
-      dsfUpdate.switchId2SystemPorts, dsfUpdate.switchId2Intfs);
+  {
+    auto nextDsfUpdateWlock = nextDsfUpdate_.wlock();
+    SCOPE_EXIT {
+      nextDsfUpdateWlock->clear();
+    };
+    *nextDsfUpdateWlock = std::move(dsfUpdate);
+    updateWithRollbackProtection(
+        nextDsfUpdateWlock->switchId2SystemPorts,
+        nextDsfUpdateWlock->switchId2Intfs);
+  }
 }
 
 bool DsfSubscription::isLocal(SwitchID nodeSwitchId) const {

fboss/agent/DsfSubscription.h

@@ -80,6 +80,7 @@ class DsfSubscription {
   folly::IPAddress remoteIp_;
   SwSwitch* sw_;
   DsfSession session_;
+  folly::Synchronized<DsfUpdate> nextDsfUpdate_;
   // Used for tests only
   std::shared_ptr<SwitchState> cachedState_;
   template <typename T>