[dualtor] Fix neighbor miss when mux is not ready (#2696)

What I did
The issue is that MuxOrch::m_syncdNeighbors assumes all cached zero-mac neighbors have tunnel routes installed. The check before adding tunnel routes ignores the following zero-mac neighbor events.
Let's ensure that, if a zero-mac neighbor is present in MuxOrch::m_syncdNeighbors, it has a tunnel route installed.
So let's check the tunnel route install success before adding a neighbor to MuxOrch::m_syncdNeighbors.

Why I did it
To fix #2675
If MuxOrch is not fully initialized, and there is a FAILED neighbor added to kernel, the tunnel route creation will fail. But the subsequent FAILED neighbor events cannot trigger tunnel route creation because MuxOrch::m_syncdNeighbors caches the first event and regard the tunnel as already installed.

How I verified it
UT and verify on testbed.

Signed-off-by: Longxiang Lyu <lolv@microsoft.com>

orchagent/muxorch.cpp

@@ -1087,7 +1087,7 @@ void MuxOrch::updateNeighbor(const NeighborUpdate& update)
         return;
     }
 
-    auto standalone_tunnel_neigh_it = standalone_tunnel_neighbors_.find(update.entry.ip_address);
+    bool is_tunnel_route_installed = isStandaloneTunnelRouteInstalled(update.entry.ip_address);
     // Handling zero MAC neighbor updates
     if (!update.mac)
     {
@@ -1098,7 +1098,7 @@ void MuxOrch::updateNeighbor(const NeighborUpdate& update)
 
         if (update.add)
         {
-            if (standalone_tunnel_neigh_it == standalone_tunnel_neighbors_.end())
+            if (!is_tunnel_route_installed)
             {
                 createStandaloneTunnelRoute(update.entry.ip_address);
             }
@@ -1113,7 +1113,7 @@ void MuxOrch::updateNeighbor(const NeighborUpdate& update)
      * make sure to remove any existing tunnel routes to prevent conflicts.
      * This block also covers the case of neighbor deletion.
      */
-    if (standalone_tunnel_neigh_it != standalone_tunnel_neighbors_.end())
+    if (is_tunnel_route_installed)
     {
         removeStandaloneTunnelRoute(update.entry.ip_address);
     }
@@ -1446,6 +1446,11 @@ void MuxOrch::removeStandaloneTunnelRoute(IpAddress neighborIp)
     standalone_tunnel_neighbors_.erase(neighborIp);
 }
 
+bool MuxOrch::isStandaloneTunnelRouteInstalled(const IpAddress& neighborIp)
+{
+    return standalone_tunnel_neighbors_.find(neighborIp) != standalone_tunnel_neighbors_.end();
+}
+
 MuxCableOrch::MuxCableOrch(DBConnector *db, DBConnector *sdb, const std::string& tableName):
               Orch2(db, tableName, request_),
               app_tunnel_route_table_(db, APP_TUNNEL_ROUTE_TABLE_NAME),

orchagent/muxorch.h

@@ -194,6 +194,8 @@ class MuxOrch : public Orch2, public Observer, public Subject
     bool removeNextHopTunnel(std::string tunnelKey, IpAddress& ipAddr);
     sai_object_id_t getNextHopTunnelId(std::string tunnelKey, IpAddress& ipAddr);
 
+    bool isStandaloneTunnelRouteInstalled(const IpAddress& neighborIp);
+
 private:
     virtual bool addOperation(const Request& request);
     virtual bool delOperation(const Request& request);

orchagent/neighorch.cpp

@@ -731,17 +731,33 @@ void NeighOrch::doTask(Consumer &consumer)
                     mac_address = MacAddress(fvValue(*i));
             }
 
-            if (m_syncdNeighbors.find(neighbor_entry) == m_syncdNeighbors.end()
-                    || m_syncdNeighbors[neighbor_entry].mac != mac_address)
+            bool nbr_not_found = (m_syncdNeighbors.find(neighbor_entry) == m_syncdNeighbors.end());
+            if (nbr_not_found || m_syncdNeighbors[neighbor_entry].mac != mac_address)
             {
-                // only for unresolvable neighbors that are new
-                if (!mac_address) 
+                if (!mac_address)
                 {
-                    if (m_syncdNeighbors.find(neighbor_entry) == m_syncdNeighbors.end())
+                    if (nbr_not_found)
                     {
-                        addZeroMacTunnelRoute(neighbor_entry, mac_address);
+                        // only for unresolvable neighbors that are new
+                        if (addZeroMacTunnelRoute(neighbor_entry, mac_address))
+                        {
+                            it = consumer.m_toSync.erase(it);
+                        }
+                        else
+                        {
+                            it++;
+                            continue;
+                        }
+                    }
+                    else
+                    {
+                        /*
+                         * For neighbors that were previously resolvable but are now unresolvable,
+                         * we expect such neighbor entries to be deleted prior to a zero MAC update
+                         * arriving for that same neighbor.
+                         */
+                        it = consumer.m_toSync.erase(it);
                     }
-                    it = consumer.m_toSync.erase(it);
                 }
                 else if (addNeighbor(neighbor_entry, mac_address))
                 {
@@ -1744,12 +1760,18 @@ void NeighOrch::updateSrv6Nexthop(const NextHopKey &nh, const sai_object_id_t &n
         m_syncdNextHops.erase(nh);
     }
 }
-void NeighOrch::addZeroMacTunnelRoute(const NeighborEntry& entry, const MacAddress& mac)
+
+bool NeighOrch::addZeroMacTunnelRoute(const NeighborEntry& entry, const MacAddress& mac)
 {
     SWSS_LOG_INFO("Creating tunnel route for neighbor %s", entry.ip_address.to_string().c_str());
     MuxOrch* mux_orch = gDirectory.get<MuxOrch*>();
     NeighborUpdate update = {entry, mac, true};
     mux_orch->update(SUBJECT_TYPE_NEIGH_CHANGE, static_cast<void *>(&update));
-    m_syncdNeighbors[entry] = { mac, false };
-}
+    if (mux_orch->isStandaloneTunnelRouteInstalled(entry.ip_address))
+    {
+        m_syncdNeighbors[entry] = { mac, false };
+        return true;
+    }
 
+    return false;
+}

orchagent/neighorch.h

@@ -116,7 +116,7 @@ class NeighOrch : public Orch, public Subject, public Observer
     bool resolveNeighborEntry(const NeighborEntry &, const MacAddress &);
     void clearResolvedNeighborEntry(const NeighborEntry &);
 
-    void addZeroMacTunnelRoute(const NeighborEntry &, const MacAddress &);
+    bool addZeroMacTunnelRoute(const NeighborEntry &, const MacAddress &);
 };
 
 #endif /* SWSS_NEIGHORCH_H */

tests/test_mux.py

@@ -1093,6 +1093,37 @@ def test_neighbor_miss(
                 expected_mac=mac if exp_result[REAL_MAC] else '00:00:00:00:00:00'
             )
 
+    def test_neighbor_miss_no_mux(
+            self, dvs, dvs_route, setup_vlan, setup_tunnel, setup,
+            setup_peer_switch, neighbor_cleanup, testlog
+    ):
+        config_db = dvs.get_config_db()
+        appdb = swsscommon.DBConnector(swsscommon.APPL_DB, dvs.redis_sock, 0)
+
+        test_ip = self.SERV1_SOC_IPV4
+        self.ping_ip(dvs, test_ip)
+
+        # no mux present, no standalone tunnel route installed
+        self.check_neighbor_state(dvs, dvs_route, test_ip, expect_route=False)
+
+        # setup the mux
+        config_db = dvs.get_config_db()
+        self.create_mux_cable(config_db)
+        # tunnel route should be installed immediately after mux setup
+        self.check_neighbor_state(dvs, dvs_route, test_ip, expect_route=True)
+
+        # set port state as standby
+        self.set_mux_state(appdb, "Ethernet0", "standby")
+        self.check_neighbor_state(dvs, dvs_route, test_ip, expect_route=True)
+
+        # set port state as active
+        self.set_mux_state(appdb, "Ethernet0", "active")
+        self.check_neighbor_state(dvs, dvs_route, test_ip, expect_route=True)
+
+        # clear the FAILED neighbor
+        self.clear_neighbors(dvs)
+        self.check_neighbor_state(dvs, dvs_route, test_ip, expect_route=False)
+
     def test_neighbor_miss_no_peer(
             self, dvs, dvs_route, setup_vlan, setup_mux_cable, setup_tunnel,
             remove_peer_switch, neighbor_cleanup, testlog