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[firebase_messaging]: Android: a burst of background messages starts a FlutterEngine per service start, and can lose all of them

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@SelaseKay ci sta già lavorando.

Dal 13/10/2026.

  • #18791 di @SelaseKay — aperta

Valutazione

Difficoltà
1/5
Tempo stimato
Meno di un'ora
Idoneità per principianti
45/100
Tipo di issue
Bug
Chiarezza
Specificata chiaramente
Stato di attività
Attiva
Stack tecnologico
android, dart, flutter
Ambito
backend, mobile

Direzione di ricerca

Read FlutterFirebaseMessagingBackgroundExecutor.java around line 140 and trace how startBackgroundIsolate() assigns the engine before the queued callback runs. Reproduce with a burst of background data messages on Android, then check that only one engine is created and every queued message is handled. The issue reports a one-line fix and a successful 12-message run, though it does not name a test file.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

Needs Attention platform: android plugin: messaging reproduced type: bug
Is there an existing issue for this?
  • I have searched the existing issues.
Which plugins are affected?

Messaging

Which platforms are affected?

Android

Description

When several data messages reach an app that isn't running (a device switched on, or back online, after missing a few), FlutterFirebaseMessagingBackgroundExecutor creates a background FlutterEngine for every FlutterFirebaseMessagingBackgroundService start that lands before the first engine exists, instead of one.

Expected: one background engine, and every message handled.

Actual: one engine per early service start. On a profile build, 12 messages held while the device was off, then boot: 6 × Creating background FlutterEngine instance. and 6 × FlutterFirebaseMessagingBackgroundService started! in one process. On a debug build, 22 held messages: 22 engines, then the low-memory killer, and no notification shown.

Cause: onCreate calls startBackgroundIsolate(), whose guard is if (backgroundFlutterEngine != null) return;. But the engine is only assigned inside the callback that method posts to the main looper after ensureInitializationCompleteAsync. Every service start before then posts another callback. The callback checks only isNotRunning(), which stays true until Dart sends MessagingBackground#initialized, so each one creates a new engine, overwrites backgroundFlutterEngine and backgroundChannel, and starts another isolate. The queued intents drain only in onInitialized. Their work was already completed, so if the process dies first (memory pressure from N engines) they are lost.

Fix (FlutterFirebaseMessagingBackgroundExecutor.java, L140 on 16.7.0 and on main at 90c22c8dd9):

-                if (isNotRunning()) {
+                if (isNotRunning() && backgroundFlutterEngine == null) {

onCreate, the posted runnable and its callback all run on the main looper, so once the first callback assigns the engine, every later one skips. With that change, the same 12-message boot burst makes one engine and shows all 12 notifications.

Reproducing the issue
  1. Register a handler with FirebaseMessaging.onBackgroundMessage.
  2. Power the device off.
  3. Send ≥10 high-priority, data-only messages to it.
  4. Boot the device without opening the app.
  5. Count Creating background FlutterEngine instance. in logcat: one per early service start, not one.

A profile or release build shows the duplicate engines cleanly. On a debug build the first engine's kernel load on the main thread (7–10 s) can also trip a background ANR on the pending FCM broadcast, which is a separate, debug-only effect.

Firebase Core version

4.15.0

Flutter Version

3.47.7

Relevant Log Output
# Profile build, 12 held data messages, device booted without opening the app (one process):
I FLTFireBGExecutor: Creating background FlutterEngine instance.     (×6)
I FLTFireMsgService: FlutterFirebaseMessagingBackgroundService started!     (×6)

# The same burst with the one-line fix:
I FLTFireBGExecutor: Creating background FlutterEngine instance.     (×1)
I FLTFireMsgService: FlutterFirebaseMessagingBackgroundService started!     (×1)
Flutter dependencies

firebase_messaging 16.7.0, firebase_core 4.15.0.

Additional context and comments
  • Versions: firebase_messaging 16.7.0 and main at 90c22c8dd9. Earlier 16.x releases (checked: 16.4.2, 16.4.3, 16.6.0) have the same isNotRunning()-only check in the callback.
  • Tested on an Android 16 emulator.
  • We're carrying the one-line change above as a build-time patch until a fix ships, and we're happy to open a PR with it if that helps.
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