[AIRFLOW-6268] Prevent ajax calls when no dags (apache#6839)

Prevent 4 AJAX calls on empty search result page of DAGs view.

airflow/www/templates/airflow/dags.html

@@ -53,13 +53,13 @@ <h2>DAGs</h2>
                 <th>Owner</th>
                 <th style="padding-left: 5px;">Recent Tasks
                   <span id="statuses_info" class="glyphicon glyphicon-info-sign" aria-hidden="true" title="Status of tasks from all active DAG runs or, if not currently active, from most recent run."></span>
-                  <img id="loading" width="15" src="{{ url_for("static", filename="loading.gif") }}">
+                  <img class="loading-task-stats" width="15" src="{{ url_for("static", filename="loading.gif") }}">
                 </th>
                 <th style="padding-left: 5px;">Last Run <span id="statuses_info" class="glyphicon glyphicon-info-sign" aria-hidden="true" title="Execution Date/Time of Highest Dag Run."></span>
                 </th>
                 <th style="padding-left: 5px;">DAG Runs
                   <span id="statuses_info" class="glyphicon glyphicon-info-sign" aria-hidden="true" title="Status of all previous DAG runs."></span>
-                  <img id="loading" width="15" src="{{ url_for("static", filename="loading.gif") }}">
+                  <img class="loading-dag-stats"  width="15" src="{{ url_for("static", filename="loading.gif") }}">
                 </th>
                 <th class="text-center">Links</th>
             </tr>
@@ -303,7 +303,8 @@ <h2>DAGs</h2>
       circle_margin = 4;
       stroke_width = 2;
       stroke_width_hover = 6;
-      d3.json("{{ url_for('Airflow.blocked') }}", function(error, json) {
+
+      function blockedHandler(error, json) {
         $.each(json, function() {
           $('.label.schedule.' + this.dag_id)
           .attr('title', this.active_dag_run + '/' + this.max_active_runs + ' active dag runs')
@@ -313,8 +314,9 @@ <h2>DAGs</h2>
             .css('background-color', 'red');
           }
         });
-      });
-      d3.json("{{ url_for('Airflow.last_dagruns') }}?dag_ids=" + (encoded_dag_ids.join(',')), function(error, json) {
+      }
+
+      function lastDagRunsHandler(error, json) {
         for(var safe_dag_id in json) {
           dag_id = json[safe_dag_id].dag_id;
           last_run = json[safe_dag_id].last_run;
@@ -328,8 +330,9 @@ <h2>DAGs</h2>
           g.selectAll(".loading-last-run").remove();
         }
         d3.selectAll(".loading-last-run").remove();
-      });
-      d3.json("{{ url_for('Airflow.dag_stats') }}?dag_ids=" + (encoded_dag_ids.join(',')), function(error, json) {
+      }
+
+      function dagStatsHandler(error, json) {
         for(var dag_id in json) {
             states = json[dag_id];
             g = d3.select('svg#dag-run-' + dag_id.replace(/\./g, '__dot__'))
@@ -398,7 +401,7 @@ <h2>DAGs</h2>
               .duration(500)
               .delay(function(d, i){return i*50;})
               .style("opacity", 1);
-            d3.select("#loading").remove();
+            d3.select(".loading-dag-stats").remove();
         }
         $("#pause_header").tooltip();
         $("#statuses_info").tooltip();
@@ -407,8 +410,9 @@ <h2>DAGs</h2>
           html: true,
           container: "body",
         });
-      });
-      d3.json("{{ url_for('Airflow.task_stats') }}?dag_ids=" + (encoded_dag_ids.join(',')), function(error, json) {
+      }
+
+      function taskStatsHandler(error, json) {
         for(var dag_id in json) {
             states = json[dag_id];
             g = d3.select('svg#task-run-' + dag_id.replace(/\./g, '__dot__'))
@@ -477,7 +481,7 @@ <h2>DAGs</h2>
               .duration(500)
               .delay(function(d, i){return i*50;})
               .style("opacity", 1);
-            d3.select("#loading").remove();
+            d3.select(".loading-task-stats").remove();
         }
         $("#pause_header").tooltip();
         $("#statuses_info").tooltip();
@@ -486,6 +490,20 @@ <h2>DAGs</h2>
           html: true,
           container: "body",
         });
-      });
+      }
+
+      if (encoded_dag_ids.length > 0) {
+        // dags on page fetch stats
+        d3.json("{{ url_for('Airflow.blocked') }}", blockedHandler);
+        d3.json("{{ url_for('Airflow.last_dagruns') }}?dag_ids=" + (encoded_dag_ids.join(',')), lastDagRunsHandler);
+        d3.json("{{ url_for('Airflow.dag_stats') }}?dag_ids=" + (encoded_dag_ids.join(',')), dagStatsHandler);
+        d3.json("{{ url_for('Airflow.task_stats') }}?dag_ids=" + (encoded_dag_ids.join(',')), taskStatsHandler);
+      }
+      else {
+        // no dags, hide the loading gifs
+        $(".loading-task-stats").remove();
+        $(".loading-dag-stats").remove();
+      }
+
   </script>
 {% endblock %}