[DF] Also loop over all graph edges for SaveGraph(rootnode)

This fixes ROOT-9977, "[DF] SaveGraph produces an empty graph"

tree/dataframe/inc/ROOT/RDF/RLoopManager.hxx

@@ -179,7 +179,11 @@ public:
    /// For each booked filter, returns either the name or "Unnamed Filter"
    std::vector<std::string> GetFiltersNames();
 
-   /// For all the actions, either booked or run
+   /// Return all graph edges known to RLoopManager
+   /// This includes Filters and Ranges but not Defines.
+   std::vector<RNodeBase *> GetGraphEdges() const;
+
+   /// Return all actions, either booked or already run
    std::vector<RDFInternal::RActionBase *> GetAllActions();
 
    std::vector<RDFInternal::RActionBase *> GetBookedActions() { return fBookedActions; }

tree/dataframe/src/RDFGraphUtils.cxx

@@ -61,13 +61,20 @@ std::string GraphCreatorHelper::RepresentGraph(RLoopManager *loopManager)
 {
 
    auto actions = loopManager->GetAllActions();
-   std::vector<std::shared_ptr<GraphNode>> leaves;
+   std::vector<std::shared_ptr<GraphNode>> nodes;
    for (auto action : actions) {
       // Triggers the graph construction. When action->GetGraph() will return, the node will be linked to all the branch
-      leaves.push_back(action->GetGraph());
+      nodes.emplace_back(action->GetGraph());
+   }
+
+   auto edges = loopManager->GetGraphEdges();
+   std::vector<std::shared_ptr<GraphNode>> leaves;
+   for (auto edge : edges) {
+      // Triggers the graph construction. When action->GetGraph() will return, the node will be linked to all the branch
+      nodes.emplace_back(edge->GetGraph());
    }
 
-   return FromGraphActionsToDot(leaves);
+   return FromGraphActionsToDot(nodes);
 }
 
 std::shared_ptr<GraphNode>

tree/dataframe/src/RLoopManager.cxx

@@ -688,6 +688,14 @@ std::vector<std::string> RLoopManager::GetFiltersNames()
    return filters;
 }
 
+std::vector<RNodeBase *> RLoopManager::GetGraphEdges() const
+{
+   std::vector<RNodeBase *> nodes;
+   nodes.insert(nodes.end(), fBookedFilters.begin(), fBookedFilters.end());
+   nodes.insert(nodes.end(), fBookedRanges.begin(), fBookedRanges.end());
+   return nodes;
+}
+
 std::vector<RDFInternal::RActionBase *> RLoopManager::GetAllActions()
 {
    std::vector<RDFInternal::RActionBase *> actions;