firebase-ios-sdk: Firestore networking fails intermittently

[REQUIRED] Step 1: Describe your environment

• Xcode version: 12.5.1
• Firebase SDK version: 8.5.0
• Installation method: Swift Package Manager
• Firebase Component: Firestore

[REQUIRED] Step 2: Describe the problem

Steps to reproduce:

Occasionally (feels like a quarter of the time) when I launch my app it fails to retrieve data from the Firestore server. The snapshot listener is called with cached local data, but nothing from the server. Then, a few minutes later, data arrives successfully.

My app waits for non-cached server data before proceeding, so this is really noticeable. It has been happening all year, but I assumed it was a hardware issue (an old second-hand iPhone 7). But it happens just as often in Xcode simulators.

Note that most of the time Firestore works fine, which makes this bug difficult to reproduce. And yes, I have a very reliable internet connection.

To test the hypothesis that the main thread was blocked, I tried adding a function that calls itself asynchronously every few seconds and prints a message. But it ran fine.

Relevant Code:

  private func onUserChanged(uid: String) {
    if let listener = userSnapshotListener {
      listener.remove()
    }
    requireServerData = true
    userSnapshotListener = Firestore.firestore()
      .collection("user")
      .document(uid)
      .addSnapshotListener(
        includeMetadataChanges: true
      ) { documentSnapshot, err in
        if let err = err {
          os_log("Query user: %s",
                 type: .error, err.localizedDescription)
        } else if let snapshot = documentSnapshot {
          self.processUserSnapshot(uid: uid, document: snapshot)
        }
      }
  }
  
  // Processes a recently-fetched update to the current user's
  // Firestore data.
  private func processUserSnapshot(
    uid: String,
    document: DocumentSnapshot
  ) {
    // Ensure the first result is from the server, not the cache.
    if requireServerData {
      if document.metadata.isFromCache {
        print("rejecting cached result")
        return
      }
      requireServerData = false
    }
    print("got result from the server")
  }

Here’s a typical log for the app (called Metropolis). It uses Firebase Messaging as well, although that’s probably not relevant. And I’ve seen Firestore take a lot longer than this to reach the server.

2021-08-06 17:34:18.502040+1000 Metropolis[82323:3195908] 8.5.0 - [Firebase/Messaging][I-FCM001000] FIRMessaging Remote Notifications proxy enabled, will swizzle remote notification receiver handlers. If you'd prefer to manually integrate Firebase Messaging, add "FirebaseAppDelegateProxyEnabled" to your Info.plist, and set it to NO. Follow the instructions at:
https://firebase.google.com/docs/cloud-messaging/ios/client#method_swizzling_in_firebase_messaging
to ensure proper integration.
rejecting cached result
2021-08-06 17:34:18.743205+1000 Metropolis[82323:3195909] 8.5.0 - [Firebase/Messaging][I-FCM012002] Error in application:didFailToRegisterForRemoteNotificationsWithError: remote notifications are not supported in the simulator
2021-08-06 17:34:18.894094+1000 Metropolis[82323:3195909] [] nw_protocol_get_quic_image_block_invoke dlopen libquic failed
2021-08-06 17:34:19.506400+1000 Metropolis[82323:3195909] Product request failed: UNKNOWN_ERROR
2021-08-06 17:34:29.728918+1000 Metropolis[82323:3196063] 8.5.0 - [Firebase/Firestore][I-FST000001] Could not reach Cloud Firestore backend. Backend didn't respond within 10 seconds.
 This typically indicates that your device does not have a healthy Internet connection at the moment. The client will operate in offline mode until it is able to successfully connect to the backend.
2021-08-06 17:34:48.921787+1000 Metropolis[82323:3196267] 8.5.0 - [Firebase/Firestore][I-FST000001] WatchStream (600002acaa18) Stream error: 'Unavailable: failed to connect to all addresses'
2021-08-06 17:34:48.938634+1000 Metropolis[82323:3196267] 8.5.0 - [Firebase/Firestore][I-FST000001] WatchStream (600002acaa18) Stream error: 'Unavailable: failed to connect to all addresses'
2021-08-06 17:34:49.869870+1000 Metropolis[82323:3196265] 8.5.0 - [Firebase/Firestore][I-FST000001] WatchStream (600002acaa18) Stream error: 'Unavailable: failed to connect to all addresses'
got result from the server