Make silent task retries more obvious

appengine/swarming/ui/res/imp/common/common-behavior.html

 <!--
   Copyright 2016 The LUCI Authors. All rights reserved.
   Use of this source code is governed under the Apache License, Version 2.0
   that can be found in the LICENSE file.
 
   It contains the definition of the following Behaviors:
 
     SwarmingBehaviors.CommonBehavior
 
   To use it, include
@@ skipping 84 matching lines @@
             console.log("ignoring failure because a second request happened.");
             this.set(busy, false);
             return;
           }
           this.set(bindTo, false);
           this.set(busy, false);
           return Promise.reject(reason);
         }.bind(this));
       },
 
+      // _getJsonAsyncArr is the array analog to _getJsonAsync.  For example,
+      // if there is an array that will contain multiple results rendered with
+      // a dom-repeat, _getJsonAsyncArr will make an XHR GET request and put
+      // the results into the given array at the index specified by idx.
+      //
+      // Prior to making this call, it is assumed that arr is initialized to be
+      // an array of empty (or filler) objects. busyArr be initialized in the
+      // same way.  Due to how Polymer deals with arrays, arrays of primitives
+      // are not supported, thus clients observing busyArr should iterate over
+      // all objects in busyArr and test to see if busyArr[i].status is true.
+      //
+      // To avoid multiple requests clobering one another, an object _jsonAsync
+      // is created on "this" to debounce requests - the most recent request
+      // will win out.
+      _getJsonAsyncArr: function(idx, arr, url, busyArr, headers, params) {
+        if (!arr || !url || !busyArr) {
+          console.log("Need at least a polymer array to bind to, a busy element, and a url");
+          return;
+        }
+        var key = arr + ":" + idx;
+        this.splice(busyArr, idx, 1, {status:true});
+        var now = new Date();
+        this._jsonAsync = this._jsonAsync || {};
+        this._jsonAsync[key] = now;
+        if (params) {
+          url = url + "?" + sk.query.fromParamSet(params);
+        }
+        return sk.request("GET", url, "", headers).then(JSON.parse).then(function(json){
+          if (this._jsonAsync[key] !== now) {
+            console.log("ignoring result because a second request happened for ", key);
+            this.splice(busyArr, idx, 1, {status:false});
+            return;
+          }
+          this.splice(arr, idx, 1, json);
+          this.splice(busyArr, idx, 1, {status:false});
+        }.bind(this)).catch(function(reason){
+          console.log("Reason for failure of request to " + url, reason);
+
+          if (this._jsonAsync[key] !== now) {
+            console.log("ignoring failure because a second request happened.");
+            this.splice(busyArr, idx, 1, {status:false});
+            return;
+          }
+          this.splice(busyArr, idx, 1, {status:false});
+          return Promise.reject(reason);
+        }.bind(this));
+      },
+
       _humanDuration: function(timeInSecs) {
         return sk.human.strDuration(timeInSecs) || "0s";
       },
 
       _not: function(a) {
         return !a;
       },
 
       _or: function() {
         var result = false;
@@ skipping 40 matching lines @@
         }
         return this._humanDuration((second.getTime() - first.getTime())/1000);
       },
 
       _truthy: function(a){
         return !!a;
       }
     };
   })();
 </script>