Bug 1621308 - Give up client sync early during shutdown r=glasserc

A significant amount of clients report OFFLINE_ERROR during shutdown.

Instead of adding a new uptake status to be reported when detecting offline during shutdown (eg. INTERRUPTED, not *_ERROR), we do nothing and give up immediately.

Differential Revision: https://phabricator.services.mozilla.com/D66271

--HG--
extra : moz-landing-system : lando

services/settings/RemoteSettingsClient.jsm

@@ -426,6 +426,14 @@ class RemoteSettingsClient extends EventEmitter {
       console.warn(`${this.identifier} sync already running`);
       return;
     }
+
+    // Prevent network requests and IndexedDB calls to be initiated
+    // during shutdown.
+    if (Services.startup.shuttingDown) {
+      console.warn(`${this.identifier} sync interrputed by shutdown`);
+      return;
+    }
+
     this._syncRunning = true;
 
     let importedFromDump = [];