FEATURE: disable service worker for all browsers except for android

Service worker is still quite experimental, only enable on android
where it provides many benefits

app/assets/javascripts/discourse/initializers/register-service-worker.js.es6

@@ -1,12 +1,44 @@
+function deRegister() {
+  if (navigator.serviceWorker.getRegistrations) {
+    navigator.serviceWorker.getRegistrations().then(r => {
+      r.forEach(reg => {
+        if (reg.active && reg.active.scriptURL.indexOf('service-worker.js') > 0) {
+          reg.unregister();
+        }
+      });
+    });
+  }
+}
+
 export default {
   name: 'register-service-worker',
 
-  initialize() {
+  initialize(container) {
+    const siteSettings = container.lookup('site-settings:main');
     const isSecure = (document.location.protocol === 'https:') ||
       (location.hostname === "localhost");
 
     if (isSecure && ('serviceWorker' in navigator)) {
-      navigator.serviceWorker.register(`${Discourse.BaseUri}/service-worker.js`);
+
+      let agents = siteSettings.service_worker_user_agents.split('|');
+
+      let allowed = false;
+
+      if (agents[0].length > 0) {
+        const userAgent = (navigator.userAgent || "unknown").toLowerCase();
+        for(let i=0; i<agents.length; i++) {
+          allowed = userAgent.indexOf(agents[i]) !== -1;
+          if (allowed) { break; }
+        }
+      }
+
+      if (allowed) {
+        // TODO no need to register if we already registered recently
+        // consider using local storage to figure out when last registered
+        navigator.serviceWorker.register(`${Discourse.BaseUri}/service-worker.js`);
+      } else {
+        deRegister();
+      }
     }
   }
 };

config/site_settings.yml

@@ -1268,6 +1268,12 @@ uncategorized:
   summary_percent_filter: 20
   summary_max_results: 100
 
+  service_worker_user_agents:
+    hidden: true
+    default: 'android'
+    type: list
+    client: true
+
   # View heat thresholds
   topic_views_heat_low:
     client: true