[Crash] java.util.ConcurrentModificationException at java.util.ArrayDeque$DeqIterator.next(ArrayDeque.java:625)

[sv-ohorvath]

Steps to reproduce

Similar to #20031. Both caught while running an automated test for switching locales. Can also be reproduced manually.

1. Fresh install
2. Go to Settings>Language
3. Select a language (e.g. Romanian)
4. Select French
5. Select "Use system language" or another language.

Expected behavior

Languages get changed

Actual behavior

App crashes.

Device information

• Device vendor/model and Android version: Pixel 2 (Android 9)
• Firefox for Android version: Nightly 91.0a1, 6/14

6c45a4f0-97dc-4e95-a210-205d26fff631
java.util.ConcurrentModificationException

• Sentry: https://sentry.prod.mozaws.net/operations/fenix/?query=df5251e8ed674d918283beba6a1f7143
• Socorro: https://crash-stats.mozilla.org/report/index/bp-6fce0f57-7609-49f0-b607-f38e70210615

---

java.util.ConcurrentModificationException at java.util.ArrayDeque$DeqIterator.next(ArrayDeque.java:625) at androidx.navigation.NavController$1.onStateChanged(NavController.java:2) at androidx.lifecycle.LifecycleRegistry$ObserverWithState.dispatchEvent(LifecycleRegistry.java:3) at androidx.lifecycle.LifecycleRegistry.forwardPass(LifecycleRegistry.java:9) at androidx.lifecycle.LifecycleRegistry.sync(LifecycleRegistry.java:9) at androidx.lifecycle.LifecycleRegistry.moveToState(LifecycleRegistry.java:5) at androidx.lifecycle.LifecycleRegistry.handleLifecycleEvent(LifecycleRegistry.java:2) at androidx.fragment.app.Fragment.performStart(Fragment.java:7) at androidx.fragment.app.FragmentStateManager.start(FragmentStateManager.java:3) at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:28) at androidx.fragment.app.FragmentManager.moveFragmentToExpectedState(FragmentManager.java:4) at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:97) at androidx.fragment.app.FragmentManager.dispatchStateChange(FragmentManager.java:3) at androidx.fragment.app.FragmentManager.dispatchStart(FragmentManager.java:3) at androidx.fragment.app.FragmentController.dispatchStart(FragmentController.java:1) at androidx.fragment.app.FragmentActivity.onStart(FragmentActivity.java:9) at androidx.appcompat.app.AppCompatActivity.onStart(AppCompatActivity.java:1) at org.mozilla.fenix.HomeActivity.access$onStart$s-594849490(HomeActivity.kt:1) at org.mozilla.fenix.HomeActivity$onStart$1.invoke(HomeActivity.kt:1) at androidx.core.app.AppOpsManagerCompat.measureNoInline(AppOpsManagerCompat.java:2) at org.mozilla.fenix.HomeActivity.onStart(HomeActivity.kt:1) at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1391) at android.app.Activity.performStart(Activity.java:7165) at android.app.ActivityThread.handleStartActivity(ActivityThread.java:2975) at android.app.servertransaction.TransactionExecutor.performLifecycleSequence(TransactionExecutor.java:180) at android.app.servertransaction.TransactionExecutor.cycleToPath(TransactionExecutor.java:165) at android.app.servertransaction.TransactionExecutor.executeLifecycleState(TransactionExecutor.java:142) at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:70) at android.app.ClientTransactionHandler.executeTransaction(ClientTransactionHandler.java:55) at android.app.ActivityThread.handleRelaunchActivityLocally(ActivityThread.java:4782) at android.app.ActivityThread.access$3200(ActivityThread.java:200) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1826) at android.os.Handler.dispatchMessage(Handler.java:106) at android.os.Looper.loop(Looper.java:193) at android.app.ActivityThread.main(ActivityThread.java:6718) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)

┆Issue is synchronized with this Jira Task

[mcarare]

@sv-ohorvath Can you reproduce this consistently? I tried on different devices and I cannot reproduce it.

[sv-ohorvath]

@sv-ohorvath Can you reproduce this consistently? I tried on different devices and I cannot reproduce it.

No, not consistently, but after a few tries. I think it's important to clear the app profile if you don't get the crash after switching languages a few times. And also to keep switching languages fast, not just the two, maybe you get a crash after the 3rd-4th switch.

[mcarare]

@sv-ohorvath Thank you for your input, already tried that. Does the crash only happen when you select "Follow device language" or you can reproduce it without ever selecting that option?

[sv-ohorvath]

@sv-ohorvath Thank you for your input, already tried that. Does the crash only happen when you select "Follow device language" or you can reproduce it without ever selecting that option?

Had to double-check that. It crashes when you select another language too.

[sflorean]

No issues found when testing on Nightly 7/9 and following the steps provided above.
Devices:

• Google Pixel 3 (Android 11)
• Google Pixel (Android 10)
• Google Pixel 3 XL (Android 9)