[active-standby] reset mux probing backoff factor in link flaps (#245)

Description of PR
Summary:
Fixes # (issue)

This PR is to address consistency in PXE boot scenario. In PXE boot, there is no ICMP heartbeats. During booting up, standby might switch to active. Mux probing is now the only way to find out mux toggles. But it's also expected to see mux probing fail when link is down, backoff factor can already be maximum when link is up.

If we don't reset backoff factor, it might take ~40s for peer to correct the inconsistency.

sign-off: Jing Zhang [email protected]

src/link_manager/LinkManagerStateMachineActiveStandby.cpp

@@ -567,6 +567,9 @@ void ActiveStandbyStateMachine::handleStateChange(LinkStateEvent &event, link_st
             // There is a problem with this approach as it will hide link flaps that result in lost heart-beats.
             initLinkProberState(nextState, true);
 //            enterMuxWaitState(nextState);
+
+            // reset mux probing backoff factor when link goes up
+            mMuxUnknownBackoffFactor = 1;
         } else if (ls(mCompositeState) == link_state::LinkState::Up &&
                    ls(nextState) == link_state::LinkState::Down &&
                    ms(mCompositeState) != mux_state::MuxState::Label::Standby) {
@@ -575,6 +578,10 @@ void ActiveStandbyStateMachine::handleStateChange(LinkStateEvent &event, link_st
 
             mActiveUnknownUpCount = 0;
             mStandbyUnknownUpCount = 0;
+
+            mWaitActiveUpCount = 0;
+            mWaitStandbyUpBackoffFactor = 1;
+            mUnknownActiveUpBackoffFactor = 1;
         } else {
             mStateTransitionHandler[ps(nextState)][ms(nextState)][ls(nextState)](nextState);
         }