core: fail fast if sync connection fails

this adds reslience to the sync logic in cases where network
isn't responding or is very slow to respond. Previously there were
cases where the sync would get stuck in Connecting state.
This will no longer happen.

packages/core/api/sync/index.js

@@ -405,13 +405,30 @@ class Sync {
   async checkConnection() {
     try {
       if (this.connection.state !== signalr.HubConnectionState.Connected) {
-        if (this.connection.state !== signalr.HubConnectionState.Disconnected)
+        if (this.connection.state !== signalr.HubConnectionState.Disconnected) {
           await this.connection.stop();
+        }
 
-        await this.connection.start();
+        await promiseTimeout(15000, this.connection.start());
       }
     } catch (e) {
+      this.connection.stop();
       this.logger.warn(e.message);
+      throw new Error(
+        "Could not connect to the Sync server. Please try again."
+      );
     }
   }
 }
+
+function promiseTimeout(ms, promise) {
+  // Create a promise that rejects in <ms> milliseconds
+  let timeout = new Promise((resolve, reject) => {
+    let id = setTimeout(() => {
+      clearTimeout(id);
+      reject(new Error("Sync timed out in " + ms + "ms."));
+    }, ms);
+  });
+  // Returns a race between our timeout and the passed in promise
+  return Promise.race([promise, timeout]);
+}