Keep topology nav visible if selected

If the scope-app API unexpectedly restarts it has no report
at hand until it receives one from the probe and sends the
node count 0 for all topologies to the frontend. Once the report
arrives, it will send the proper count.

What happened was the frontend did hide Processes for a short
time till the node count recovered. This moved the topology
selection to the always visible Containers (hide_if_empty == false)
while keeping the graph as is.

Once the node count recovers, Processes comes back but the
selection is still at Containers.

This fixes to keep the selected topology visible at all time
even if the API returns a node count of 0. This allows to
keep looking at the graph. The topology will disappear once
the user selected a different topology while a backend response
is received.

Fixes #2646

client/app/scripts/reducers/__tests__/root-test.js

@@ -280,6 +280,25 @@ describe('RootReducer', () => {
     }]
   };
 
+  const ReceiveTopologiesHiddenAction = {
+    type: ActionTypes.RECEIVE_TOPOLOGIES,
+    topologies: [{
+      url: '/topo1',
+      name: 'Topo1',
+      stats: {
+        node_count: 1
+      }
+    }, {
+      hide_if_empty: true,
+      url: '/topo2',
+      name: 'Topo2',
+      stats: {
+        node_count: 0,
+        filtered_nodes: 0
+      }
+    }]
+  };
+
   const RouteAction = {
     type: ActionTypes.ROUTE_TOPOLOGY,
     state: {}
@@ -567,6 +586,21 @@ describe('RootReducer', () => {
     expect(isTopologyNodeCountZero(nextState)).toBeFalsy();
   });
 
+  it('keeps hidden topology visible if selected', () => {
+    let nextState = initialState;
+    nextState = reducer(nextState, ClickTopology2Action);
+    nextState = reducer(nextState, ReceiveTopologiesHiddenAction);
+    expect(nextState.get('currentTopologyId')).toEqual('topo2');
+    expect(nextState.get('topologies').toJS().length).toEqual(2);
+  });
+
+  it('keeps hidden topology hidden if not selected', () => {
+    let nextState = initialState;
+    nextState = reducer(nextState, ClickTopologyAction);
+    nextState = reducer(nextState, ReceiveTopologiesHiddenAction);
+    expect(nextState.get('topologies').toJS().length).toEqual(1);
+  });
+
   // selection of relatives
 
   it('keeps relatives as a stack', () => {

client/app/scripts/reducers/root.js

@@ -110,17 +110,17 @@ function calcSelectType(topology) {
 // adds ID field to topology (based on last part of URL path) and save urls in
 // map for easy lookup
 function processTopologies(state, nextTopologies) {
+  // add IDs to topology objects in-place
+  const topologiesWithId = updateTopologyIds(nextTopologies);
   // filter out hidden topos
-  const visibleTopologies = filterHiddenTopologies(nextTopologies);
+  const visibleTopologies = filterHiddenTopologies(topologiesWithId, state.get('currentTopologyId'));
   // set `selectType` field for topology and sub_topologies options (recursive).
   const topologiesWithSelectType = visibleTopologies.map(calcSelectType);
-  // add IDs to topology objects in-place
-  const topologiesWithId = updateTopologyIds(topologiesWithSelectType);
   // cache URLs by ID
   state = state.set('topologyUrlsById',
-    setTopologyUrlsById(state.get('topologyUrlsById'), topologiesWithId));
+    setTopologyUrlsById(state.get('topologyUrlsById'), topologiesWithSelectType));
 
-  const topologiesWithFullnames = addTopologyFullname(topologiesWithId);
+  const topologiesWithFullnames = addTopologyFullname(topologiesWithSelectType);
   const immNextTopologies = fromJS(topologiesWithFullnames).sortBy(topologySorter);
   return state.set('topologies', immNextTopologies);
 }

client/app/scripts/utils/topology-utils.js

@@ -126,9 +126,9 @@ export function setTopologyUrlsById(topologyUrlsById, topologies) {
   return urlMap;
 }
 
-export function filterHiddenTopologies(topologies) {
+export function filterHiddenTopologies(topologies, currentTopologyId) {
   return topologies.filter(t => (!t.hide_if_empty || t.stats.node_count > 0 ||
-                               t.stats.filtered_nodes > 0));
+                               t.stats.filtered_nodes > 0 || t.id === currentTopologyId));
 }
 
 export function getCurrentTopologyOptions(state) {