[offline] Cache inspector reports sequentially, not in parallel

CHANGELOG.md

@@ -59,6 +59,7 @@
         - Reduce database queries on shortlist page.
         - Provide ResultSet fallback translation in lookup.
         - Mark non-Open311 updates as processed by daemon. #4552
+        - Inspector/planned offline report caching now fetches URLs sequentially, not in parallel.
     - Changes:
         - Switch to OpenStreetMap for reverse geocoding. #4444
         - Convert all uploaded images to JPEGs.

web/cobrands/fixmystreet/offline.js

@@ -245,10 +245,12 @@ fixmystreet.cachet = (function(){
                 imagesToGet.push(img.src);
                 imagesToGet.push(img.src.replace('.jpeg', '.fp.jpeg'));
             });
-            var imagePromises = imagesToGet.map(function(url) {
-                return cacheURL(url);
-            });
-            return Promise.all(imagePromises).finally(function() {
+            // Fetch URLs sequentially rather than in parallel
+            return imagesToGet.reduce(function(p, url) {
+                return p.then(function() {
+                    return cacheURL(url);
+                });
+            }, Promise.resolve()).finally(function() {
                 fixmystreet.offlineBanner.progress();
                 fixmystreet.offlineData.add(item.url, item.lastupdate);
             });
@@ -259,10 +261,12 @@ fixmystreet.cachet = (function(){
     // This fetches the HTML and any img elements in that HTML
     function cacheReports(items) {
         fixmystreet.offlineBanner.startProgress(items.length);
-        var promises = items.map(function(item) {
-            return cacheReport(item);
-        });
-        return Promise.all(promises);
+        // We want to cache these one at a time to avoid hammering the server
+        return items.reduce(function(p, item) {
+            return p.then(function() {
+                return cacheReport(item);
+            });
+        }, Promise.resolve());
     }
 
     return {