IssueTracker

Biometric library version: 1.1.0-alpha01

This bug was reported in Mozilla Fenix, but it's actually a leak in BiometricViewModel.

Mozilla Fenix bug report: https://github.com/mozilla-mobile/fenix/issues/13477

To reproduce, follow the official Android X sample code and add the sample code to a fragment. Display the login prompt, get the fragment destroyed, the fragment is now leaking

As soon as the fragment is destroyed, the leak triggers as the fragment cannot be garbage collected because the AuthenticationCallback implementation is retained.

┬───
│ GC Root: Global variable in native code
│
├─ dalvik.system.PathClassLoader instance
│ Leaking: NO (InternalLeakCanary↓ is not leaking and A ClassLoader is never leaking)
│ ↓ PathClassLoader.runtimeInternalObjects
├─ java.lang.Object[] array
│ Leaking: NO (InternalLeakCanary↓ is not leaking)
│ ↓ Object[].[891]
├─ leakcanary.internal.InternalLeakCanary class
│ Leaking: NO (HomeActivity↓ is not leaking and a class is never leaking)
│ ↓ static InternalLeakCanary.resumedActivity
├─ org.mozilla.fenix.HomeActivity instance
│ Leaking: NO (Activity#mDestroyed is false)
│ ↓ HomeActivity.mViewModelStore
│                ~~~~~~~~~~~~~~~
├─ androidx.lifecycle.ViewModelStore instance
│ Leaking: UNKNOWN
│ ↓ ViewModelStore.mMap
│                  ~~~~
├─ java.util.HashMap instance
│ Leaking: UNKNOWN
│ ↓ HashMap.table
│           ~~~~~
├─ java.util.HashMap$Node[] array
│ Leaking: UNKNOWN
│ ↓ HashMap$Node[].[2]
│                  ~~~
├─ java.util.HashMap$Node instance
│ Leaking: UNKNOWN
│ ↓ HashMap$Node.value
│                ~~~~~
├─ androidx.biometric.BiometricViewModel instance
│ Leaking: UNKNOWN
│ ↓ BiometricViewModel.mClientCallback
│                      ~~~~~~~~~~~~~~~
├─ org.mozilla.fenix.settings.logins.fragment.SavedLoginsAuthFragment$biometricPromptCallback$1 instance
│ Leaking: UNKNOWN
│ Anonymous subclass of androidx.biometric.BiometricPrompt$AuthenticationCallback
│ ↓ SavedLoginsAuthFragment$biometricPromptCallback$1.this$0
│                                                     ~~~~~~
╰→ org.mozilla.fenix.settings.logins.fragment.SavedLoginsAuthFragment instance
​ Leaking: YES (ObjectWatcher was watching this because org.mozilla.fenix.settings.logins.fragment.SavedLoginsAuthFragment received Fragment#onDestroy() callback and Fragment#mFragmentManager is null)
​ key = 77a534a9-73fb-4b7e-9bb1-1c26e0064ca8
​ watchDurationMillis = 9964
​ retainedDurationMillis = 4964

Looking at the Android X BiometricPrompt sources, the callback is set in BiometricPrompt.init() which is called from the BiometricPrompt constructor and never unset.

    private void init(
            @Nullable FragmentActivity activity,
            @Nullable FragmentManager fragmentManager,
            @Nullable Executor executor,
            @NonNull AuthenticationCallback callback) {

        mClientFragmentManager = fragmentManager;

        if (activity != null) {
            final BiometricViewModel viewModel =
                    new ViewModelProvider(activity).get(BiometricViewModel.class);
            if (executor != null) {
                viewModel.setClientExecutor(executor);
            }
            viewModel.setClientCallback(callback);
        }
    }

So, as it stands, there is no way to clear the callback, and BiometricViewModel is always created with an activity lifecycle, so the callback always has the lifecycle of the activity. This means that currently the library will leak any fragment it's used from.