BLOCKING_EXECUTOR in ExecutorsRegistrar uses unbounded CachedThreadPool — causes 68+ threads on low-memory devices

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Valutazione

Difficoltà
4/5
Tempo stimato
3-5 giorni
Idoneità per principianti
52/100
Tipo di issue
Bug
Chiarezza
Abbastanza chiara
Stato di attività
Tranquilla
Stack tecnologico
android, java
Ambito
mobile, performance

Direzione di ricerca

Inizia in firebase-common/.../concurrent/ExecutorsRegistrar.java e verifica come viene creato e utilizzato BLOCKING_EXECUTOR. Confronta le opzioni ThreadPoolExecutor con limiti e LinkedBlockingQueue descritte nella issue, quindi riproduci lo scenario di memoria insufficiente oppure esamina i thread dump. Il lavoro è completato quando la creazione dei thread bloccanti è limitata, mentre il recupero dei thread inattivi e il comportamento dell’executor rimangono appropriati.

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

Descrizione

api: core api: messaging

Description

ExecutorsRegistrar.BLOCKING_EXECUTOR uses Executors.newCachedThreadPool() with no upper bound on thread creation:

// firebase-common/.../concurrent/ExecutorsRegistrar.java
static final Lazy<ScheduledExecutorService> BLOCKING_EXECUTOR =
    new Lazy<>(
        () ->
            scheduled(
                Executors.newCachedThreadPool(
                    factory("Firebase Blocking", ...))));

On low-end Android devices (2GB RAM, 2 CPU cores), this creates 68+ "Firebase Blocking Thread" instances during normal app usage. Each thread consumes ~1MB of stack space, contributing to memory pressure that triggers the low memory killer (lmkd).

Environment

  • Firebase product: firebase-messaging 25.0.1 (only Firebase product in use; BLOCKING_EXECUTOR comes from the transitive firebase-common dependency)
  • Device: 2GB RAM / 2-core CPU (emulator configured to simulate low-end Android Go devices)
  • Android version: API 34

Evidence

Thread dump captured via Thread.getAllStackTraces() shows 68 threads named "Firebase Blocking Thread #0" through "Firebase Blocking Thread #67", all in TIMED_WAITING state:

"Firebase Blocking Thread #0"  TIMED_WAITING
"Firebase Blocking Thread #1"  TIMED_WAITING
...
"Firebase Blocking Thread #67" TIMED_WAITING

Shortly after, the process was killed by lmkd:

lowmemorykiller: reason: low watermark is breached and swap is low (276kB < 151960kB)
ActivityManager: Process has died: fg  TOP

The app was the foreground TOP process and was still killed due to system-wide memory exhaustion.

Comparison

On a higher-spec emulator (4+ cores, more RAM), the same executor creates ~37 "Firebase Blocking Thread" instances. The slower CPU on the 2GB device means tasks take longer to complete, so more threads pile up before they can be reused — the exact scenario where unbounded pools are most dangerous.

Suggested fix

Replace Executors.newCachedThreadPool() with a bounded ThreadPoolExecutor in ExecutorsRegistrar.BLOCKING_EXECUTOR:

static final Lazy<ScheduledExecutorService> BLOCKING_EXECUTOR =
    new Lazy<>(
        () ->
            scheduled(
                new ThreadPoolExecutor(
                    0,
                    Math.max(4, Runtime.getRuntime().availableProcessors()),
                    60L, TimeUnit.SECONDS,
                    new SynchronousQueue<>(),
                    factory("Firebase Blocking", ...))));

This caps thread creation while preserving the same idle-thread reaping behavior. A LinkedBlockingQueue alternative would allow tasks to queue safely when all threads are busy.

Lingua principale
Java
Stelle
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Fork
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Merge medio
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PR unite (30g)
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